diff --git a/compiler/rustc_borrowck/src/lib.rs b/compiler/rustc_borrowck/src/lib.rs
index 4a4887f19702f..163170a1d1aa0 100644
--- a/compiler/rustc_borrowck/src/lib.rs
+++ b/compiler/rustc_borrowck/src/lib.rs
@@ -19,7 +19,7 @@ extern crate tracing;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::graph::dominators::Dominators;
 use rustc_data_structures::vec_map::VecMap;
-use rustc_errors::{Diagnostic, DiagnosticBuilder, ErrorGuaranteed};
+use rustc_errors::{Diagnostic, DiagnosticBuilder};
 use rustc_hir as hir;
 use rustc_hir::def_id::LocalDefId;
 use rustc_index::bit_set::ChunkedBitSet;
@@ -192,13 +192,13 @@ fn do_mir_borrowck<'tcx>(
         }
     }
 
-    let mut errors = error::BorrowckErrors::new();
+    let mut errors = error::BorrowckErrors::new(infcx.tcx);
 
     // Gather the upvars of a closure, if any.
     let tables = tcx.typeck_opt_const_arg(def);
-    if let Some(ErrorGuaranteed { .. }) = tables.tainted_by_errors {
-        infcx.set_tainted_by_errors();
-        errors.set_tainted_by_errors();
+    if let Some(e) = tables.tainted_by_errors {
+        infcx.set_tainted_by_errors(e);
+        errors.set_tainted_by_errors(e);
     }
     let upvars: Vec<_> = tables
         .closure_min_captures_flattened(def.did)
@@ -2260,6 +2260,7 @@ mod error {
     use super::*;
 
     pub struct BorrowckErrors<'tcx> {
+        tcx: TyCtxt<'tcx>,
         /// This field keeps track of move errors that are to be reported for given move indices.
         ///
         /// There are situations where many errors can be reported for a single move out (see #53807)
@@ -2282,19 +2283,24 @@ mod error {
         tainted_by_errors: Option<ErrorGuaranteed>,
     }
 
-    impl BorrowckErrors<'_> {
-        pub fn new() -> Self {
+    impl<'tcx> BorrowckErrors<'tcx> {
+        pub fn new(tcx: TyCtxt<'tcx>) -> Self {
             BorrowckErrors {
+                tcx,
                 buffered_move_errors: BTreeMap::new(),
                 buffered: Default::default(),
                 tainted_by_errors: None,
             }
         }
 
-        // FIXME(eddyb) this is a suboptimal API because `tainted_by_errors` is
-        // set before any emission actually happens (weakening the guarantee).
         pub fn buffer_error(&mut self, t: DiagnosticBuilder<'_, ErrorGuaranteed>) {
-            self.tainted_by_errors = Some(ErrorGuaranteed::unchecked_claim_error_was_emitted());
+            if let None = self.tainted_by_errors {
+                self.tainted_by_errors = Some(
+                    self.tcx
+                        .sess
+                        .delay_span_bug(t.span.clone(), "diagnostic buffered but not emitted"),
+                )
+            }
             t.buffer(&mut self.buffered);
         }
 
@@ -2302,8 +2308,8 @@ mod error {
             t.buffer(&mut self.buffered);
         }
 
-        pub fn set_tainted_by_errors(&mut self) {
-            self.tainted_by_errors = Some(ErrorGuaranteed::unchecked_claim_error_was_emitted());
+        pub fn set_tainted_by_errors(&mut self, e: ErrorGuaranteed) {
+            self.tainted_by_errors = Some(e);
         }
     }
 
diff --git a/compiler/rustc_borrowck/src/nll.rs b/compiler/rustc_borrowck/src/nll.rs
index f8856b56d140b..4a12e1b1b92e0 100644
--- a/compiler/rustc_borrowck/src/nll.rs
+++ b/compiler/rustc_borrowck/src/nll.rs
@@ -303,7 +303,10 @@ pub(crate) fn compute_regions<'cx, 'tcx>(
 
     if !nll_errors.is_empty() {
         // Suppress unhelpful extra errors in `infer_opaque_types`.
-        infcx.set_tainted_by_errors();
+        infcx.set_tainted_by_errors(infcx.tcx.sess.delay_span_bug(
+            body.span,
+            "`compute_regions` tainted `infcx` with errors but did not emit any errors",
+        ));
     }
 
     let remapped_opaque_tys = regioncx.infer_opaque_types(&infcx, opaque_type_values);
diff --git a/compiler/rustc_borrowck/src/region_infer/opaque_types.rs b/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
index b2db77944fd1e..b9885952a893c 100644
--- a/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
+++ b/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
@@ -219,8 +219,8 @@ impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
         instantiated_ty: OpaqueHiddenType<'tcx>,
         origin: OpaqueTyOrigin,
     ) -> Ty<'tcx> {
-        if self.is_tainted_by_errors() {
-            return self.tcx.ty_error();
+        if let Some(e) = self.tainted_by_errors() {
+            return self.tcx.ty_error_with_guaranteed(e);
         }
 
         let definition_ty = instantiated_ty
diff --git a/compiler/rustc_codegen_cranelift/src/abi/mod.rs b/compiler/rustc_codegen_cranelift/src/abi/mod.rs
index 99059e788a074..1e22537c2ba42 100644
--- a/compiler/rustc_codegen_cranelift/src/abi/mod.rs
+++ b/compiler/rustc_codegen_cranelift/src/abi/mod.rs
@@ -22,7 +22,19 @@ fn clif_sig_from_fn_abi<'tcx>(
     default_call_conv: CallConv,
     fn_abi: &FnAbi<'tcx, Ty<'tcx>>,
 ) -> Signature {
-    let call_conv = match fn_abi.conv {
+    let call_conv = conv_to_call_conv(fn_abi.conv, default_call_conv);
+
+    let inputs = fn_abi.args.iter().map(|arg_abi| arg_abi.get_abi_param(tcx).into_iter()).flatten();
+
+    let (return_ptr, returns) = fn_abi.ret.get_abi_return(tcx);
+    // Sometimes the first param is an pointer to the place where the return value needs to be stored.
+    let params: Vec<_> = return_ptr.into_iter().chain(inputs).collect();
+
+    Signature { params, returns, call_conv }
+}
+
+pub(crate) fn conv_to_call_conv(c: Conv, default_call_conv: CallConv) -> CallConv {
+    match c {
         Conv::Rust | Conv::C => default_call_conv,
         Conv::RustCold => CallConv::Cold,
         Conv::X86_64SysV => CallConv::SystemV,
@@ -38,15 +50,8 @@ fn clif_sig_from_fn_abi<'tcx>(
         | Conv::X86VectorCall
         | Conv::AmdGpuKernel
         | Conv::AvrInterrupt
-        | Conv::AvrNonBlockingInterrupt => todo!("{:?}", fn_abi.conv),
-    };
-    let inputs = fn_abi.args.iter().map(|arg_abi| arg_abi.get_abi_param(tcx).into_iter()).flatten();
-
-    let (return_ptr, returns) = fn_abi.ret.get_abi_return(tcx);
-    // Sometimes the first param is an pointer to the place where the return value needs to be stored.
-    let params: Vec<_> = return_ptr.into_iter().chain(inputs).collect();
-
-    Signature { params, returns, call_conv }
+        | Conv::AvrNonBlockingInterrupt => todo!("{:?}", c),
+    }
 }
 
 pub(crate) fn get_function_sig<'tcx>(
diff --git a/compiler/rustc_codegen_cranelift/src/main_shim.rs b/compiler/rustc_codegen_cranelift/src/main_shim.rs
index cae6312a60735..f7434633ea442 100644
--- a/compiler/rustc_codegen_cranelift/src/main_shim.rs
+++ b/compiler/rustc_codegen_cranelift/src/main_shim.rs
@@ -63,10 +63,14 @@ pub(crate) fn maybe_create_entry_wrapper(
                 AbiParam::new(m.target_config().pointer_type()),
             ],
             returns: vec![AbiParam::new(m.target_config().pointer_type() /*isize*/)],
-            call_conv: CallConv::triple_default(m.isa().triple()),
+            call_conv: crate::conv_to_call_conv(
+                tcx.sess.target.options.entry_abi,
+                CallConv::triple_default(m.isa().triple()),
+            ),
         };
 
-        let cmain_func_id = m.declare_function("main", Linkage::Export, &cmain_sig).unwrap();
+        let entry_name = tcx.sess.target.options.entry_name.as_ref();
+        let cmain_func_id = m.declare_function(entry_name, Linkage::Export, &cmain_sig).unwrap();
 
         let instance = Instance::mono(tcx, rust_main_def_id).polymorphize(tcx);
 
diff --git a/compiler/rustc_codegen_gcc/src/context.rs b/compiler/rustc_codegen_gcc/src/context.rs
index 62a61eb8548da..2e71c3665daa7 100644
--- a/compiler/rustc_codegen_gcc/src/context.rs
+++ b/compiler/rustc_codegen_gcc/src/context.rs
@@ -425,8 +425,9 @@ impl<'gcc, 'tcx> MiscMethods<'tcx> for CodegenCx<'gcc, 'tcx> {
     }
 
     fn declare_c_main(&self, fn_type: Self::Type) -> Option<Self::Function> {
-        if self.get_declared_value("main").is_none() {
-            Some(self.declare_cfn("main", fn_type))
+        let entry_name = self.sess().target.entry_name.as_ref();
+        if self.get_declared_value(entry_name).is_none() {
+            Some(self.declare_entry_fn(entry_name, fn_type, ()))
         }
         else {
             // If the symbol already exists, it is an error: for example, the user wrote
diff --git a/compiler/rustc_codegen_gcc/src/declare.rs b/compiler/rustc_codegen_gcc/src/declare.rs
index a619e2f771252..eae77508c973a 100644
--- a/compiler/rustc_codegen_gcc/src/declare.rs
+++ b/compiler/rustc_codegen_gcc/src/declare.rs
@@ -65,13 +65,13 @@ impl<'gcc, 'tcx> CodegenCx<'gcc, 'tcx> {
         global
     }
 
-    pub fn declare_cfn(&self, name: &str, _fn_type: Type<'gcc>) -> RValue<'gcc> {
+    pub fn declare_entry_fn(&self, name: &str, _fn_type: Type<'gcc>, callconv: () /*llvm::CCallConv*/) -> RValue<'gcc> {
         // TODO(antoyo): use the fn_type parameter.
         let const_string = self.context.new_type::<u8>().make_pointer().make_pointer();
         let return_type = self.type_i32();
         let variadic = false;
         self.linkage.set(FunctionType::Exported);
-        let func = declare_raw_fn(self, name, () /*llvm::CCallConv*/, return_type, &[self.type_i32(), const_string], variadic);
+        let func = declare_raw_fn(self, name, callconv, return_type, &[self.type_i32(), const_string], variadic);
         // NOTE: it is needed to set the current_func here as well, because get_fn() is not called
         // for the main function.
         *self.current_func.borrow_mut() = Some(func);
diff --git a/compiler/rustc_codegen_llvm/src/abi.rs b/compiler/rustc_codegen_llvm/src/abi.rs
index d478efc863a9e..a6fd2a7de6bd0 100644
--- a/compiler/rustc_codegen_llvm/src/abi.rs
+++ b/compiler/rustc_codegen_llvm/src/abi.rs
@@ -398,23 +398,7 @@ impl<'ll, 'tcx> FnAbiLlvmExt<'ll, 'tcx> for FnAbi<'tcx, Ty<'tcx>> {
     }
 
     fn llvm_cconv(&self) -> llvm::CallConv {
-        match self.conv {
-            Conv::C | Conv::Rust | Conv::CCmseNonSecureCall => llvm::CCallConv,
-            Conv::RustCold => llvm::ColdCallConv,
-            Conv::AmdGpuKernel => llvm::AmdGpuKernel,
-            Conv::AvrInterrupt => llvm::AvrInterrupt,
-            Conv::AvrNonBlockingInterrupt => llvm::AvrNonBlockingInterrupt,
-            Conv::ArmAapcs => llvm::ArmAapcsCallConv,
-            Conv::Msp430Intr => llvm::Msp430Intr,
-            Conv::PtxKernel => llvm::PtxKernel,
-            Conv::X86Fastcall => llvm::X86FastcallCallConv,
-            Conv::X86Intr => llvm::X86_Intr,
-            Conv::X86Stdcall => llvm::X86StdcallCallConv,
-            Conv::X86ThisCall => llvm::X86_ThisCall,
-            Conv::X86VectorCall => llvm::X86_VectorCall,
-            Conv::X86_64SysV => llvm::X86_64_SysV,
-            Conv::X86_64Win64 => llvm::X86_64_Win64,
-        }
+        self.conv.into()
     }
 
     fn apply_attrs_llfn(&self, cx: &CodegenCx<'ll, 'tcx>, llfn: &'ll Value) {
@@ -596,3 +580,25 @@ impl<'tcx> AbiBuilderMethods<'tcx> for Builder<'_, '_, 'tcx> {
         llvm::get_param(self.llfn(), index as c_uint)
     }
 }
+
+impl From<Conv> for llvm::CallConv {
+    fn from(conv: Conv) -> Self {
+        match conv {
+            Conv::C | Conv::Rust | Conv::CCmseNonSecureCall => llvm::CCallConv,
+            Conv::RustCold => llvm::ColdCallConv,
+            Conv::AmdGpuKernel => llvm::AmdGpuKernel,
+            Conv::AvrInterrupt => llvm::AvrInterrupt,
+            Conv::AvrNonBlockingInterrupt => llvm::AvrNonBlockingInterrupt,
+            Conv::ArmAapcs => llvm::ArmAapcsCallConv,
+            Conv::Msp430Intr => llvm::Msp430Intr,
+            Conv::PtxKernel => llvm::PtxKernel,
+            Conv::X86Fastcall => llvm::X86FastcallCallConv,
+            Conv::X86Intr => llvm::X86_Intr,
+            Conv::X86Stdcall => llvm::X86StdcallCallConv,
+            Conv::X86ThisCall => llvm::X86_ThisCall,
+            Conv::X86VectorCall => llvm::X86_VectorCall,
+            Conv::X86_64SysV => llvm::X86_64_SysV,
+            Conv::X86_64Win64 => llvm::X86_64_Win64,
+        }
+    }
+}
diff --git a/compiler/rustc_codegen_llvm/src/context.rs b/compiler/rustc_codegen_llvm/src/context.rs
index eaa2ccfc835c5..4dcc7cd54477d 100644
--- a/compiler/rustc_codegen_llvm/src/context.rs
+++ b/compiler/rustc_codegen_llvm/src/context.rs
@@ -576,8 +576,14 @@ impl<'ll, 'tcx> MiscMethods<'tcx> for CodegenCx<'ll, 'tcx> {
     }
 
     fn declare_c_main(&self, fn_type: Self::Type) -> Option<Self::Function> {
-        if self.get_declared_value("main").is_none() {
-            Some(self.declare_cfn("main", llvm::UnnamedAddr::Global, fn_type))
+        let entry_name = self.sess().target.entry_name.as_ref();
+        if self.get_declared_value(entry_name).is_none() {
+            Some(self.declare_entry_fn(
+                entry_name,
+                self.sess().target.entry_abi.into(),
+                llvm::UnnamedAddr::Global,
+                fn_type,
+            ))
         } else {
             // If the symbol already exists, it is an error: for example, the user wrote
             // #[no_mangle] extern "C" fn main(..) {..}
diff --git a/compiler/rustc_codegen_llvm/src/declare.rs b/compiler/rustc_codegen_llvm/src/declare.rs
index f79ef11720df9..dc21a02cec44a 100644
--- a/compiler/rustc_codegen_llvm/src/declare.rs
+++ b/compiler/rustc_codegen_llvm/src/declare.rs
@@ -90,6 +90,28 @@ impl<'ll, 'tcx> CodegenCx<'ll, 'tcx> {
         declare_raw_fn(self, name, llvm::CCallConv, unnamed, visibility, fn_type)
     }
 
+    /// Declare an entry Function
+    ///
+    /// The ABI of this function can change depending on the target (although for now the same as
+    /// `declare_cfn`)
+    ///
+    /// If there’s a value with the same name already declared, the function will
+    /// update the declaration and return existing Value instead.
+    pub fn declare_entry_fn(
+        &self,
+        name: &str,
+        callconv: llvm::CallConv,
+        unnamed: llvm::UnnamedAddr,
+        fn_type: &'ll Type,
+    ) -> &'ll Value {
+        let visibility = if self.tcx.sess.target.default_hidden_visibility {
+            llvm::Visibility::Hidden
+        } else {
+            llvm::Visibility::Default
+        };
+        declare_raw_fn(self, name, callconv, unnamed, visibility, fn_type)
+    }
+
     /// Declare a Rust function.
     ///
     /// If there’s a value with the same name already declared, the function will
diff --git a/compiler/rustc_codegen_ssa/src/back/symbol_export.rs b/compiler/rustc_codegen_ssa/src/back/symbol_export.rs
index 752f6b1ef40c9..22f534d909ab6 100644
--- a/compiler/rustc_codegen_ssa/src/back/symbol_export.rs
+++ b/compiler/rustc_codegen_ssa/src/back/symbol_export.rs
@@ -180,7 +180,8 @@ fn exported_symbols_provider_local<'tcx>(
         .collect();
 
     if tcx.entry_fn(()).is_some() {
-        let exported_symbol = ExportedSymbol::NoDefId(SymbolName::new(tcx, "main"));
+        let exported_symbol =
+            ExportedSymbol::NoDefId(SymbolName::new(tcx, tcx.sess.target.entry_name.as_ref()));
 
         symbols.push((
             exported_symbol,
diff --git a/compiler/rustc_const_eval/src/interpret/eval_context.rs b/compiler/rustc_const_eval/src/interpret/eval_context.rs
index ed69d8554d2cc..f59f34b874c19 100644
--- a/compiler/rustc_const_eval/src/interpret/eval_context.rs
+++ b/compiler/rustc_const_eval/src/interpret/eval_context.rs
@@ -23,7 +23,7 @@ use super::{
     MemPlaceMeta, Memory, MemoryKind, Operand, Place, PlaceTy, PointerArithmetic, Provenance,
     Scalar, StackPopJump,
 };
-use crate::transform::validate::equal_up_to_regions;
+use crate::util;
 
 pub struct InterpCx<'mir, 'tcx, M: Machine<'mir, 'tcx>> {
     /// Stores the `Machine` instance.
@@ -354,8 +354,8 @@ pub(super) fn mir_assign_valid_types<'tcx>(
     // Type-changing assignments can happen when subtyping is used. While
     // all normal lifetimes are erased, higher-ranked types with their
     // late-bound lifetimes are still around and can lead to type
-    // differences. So we compare ignoring lifetimes.
-    if equal_up_to_regions(tcx, param_env, src.ty, dest.ty) {
+    // differences.
+    if util::is_subtype(tcx, param_env, src.ty, dest.ty) {
         // Make sure the layout is equal, too -- just to be safe. Miri really
         // needs layout equality. For performance reason we skip this check when
         // the types are equal. Equal types *can* have different layouts when
diff --git a/compiler/rustc_const_eval/src/transform/validate.rs b/compiler/rustc_const_eval/src/transform/validate.rs
index 81b82a21fa1a7..860dee5898057 100644
--- a/compiler/rustc_const_eval/src/transform/validate.rs
+++ b/compiler/rustc_const_eval/src/transform/validate.rs
@@ -2,7 +2,6 @@
 
 use rustc_data_structures::fx::FxHashSet;
 use rustc_index::bit_set::BitSet;
-use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::mir::interpret::Scalar;
 use rustc_middle::mir::visit::NonUseContext::VarDebugInfo;
 use rustc_middle::mir::visit::{PlaceContext, Visitor};
@@ -12,8 +11,7 @@ use rustc_middle::mir::{
     ProjectionElem, RuntimePhase, Rvalue, SourceScope, Statement, StatementKind, Terminator,
     TerminatorKind, UnOp, START_BLOCK,
 };
-use rustc_middle::ty::fold::BottomUpFolder;
-use rustc_middle::ty::{self, InstanceDef, ParamEnv, Ty, TyCtxt, TypeFoldable, TypeVisitable};
+use rustc_middle::ty::{self, InstanceDef, ParamEnv, Ty, TyCtxt, TypeVisitable};
 use rustc_mir_dataflow::impls::MaybeStorageLive;
 use rustc_mir_dataflow::storage::always_storage_live_locals;
 use rustc_mir_dataflow::{Analysis, ResultsCursor};
@@ -70,44 +68,6 @@ impl<'tcx> MirPass<'tcx> for Validator {
     }
 }
 
-/// Returns whether the two types are equal up to lifetimes.
-/// All lifetimes, including higher-ranked ones, get ignored for this comparison.
-/// (This is unlike the `erasing_regions` methods, which keep higher-ranked lifetimes for soundness reasons.)
-///
-/// The point of this function is to approximate "equal up to subtyping".  However,
-/// the approximation is incorrect as variance is ignored.
-pub fn equal_up_to_regions<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    param_env: ParamEnv<'tcx>,
-    src: Ty<'tcx>,
-    dest: Ty<'tcx>,
-) -> bool {
-    // Fast path.
-    if src == dest {
-        return true;
-    }
-
-    // Normalize lifetimes away on both sides, then compare.
-    let normalize = |ty: Ty<'tcx>| {
-        tcx.try_normalize_erasing_regions(param_env, ty).unwrap_or(ty).fold_with(
-            &mut BottomUpFolder {
-                tcx,
-                // FIXME: We erase all late-bound lifetimes, but this is not fully correct.
-                // If you have a type like `<for<'a> fn(&'a u32) as SomeTrait>::Assoc`,
-                // this is not necessarily equivalent to `<fn(&'static u32) as SomeTrait>::Assoc`,
-                // since one may have an `impl SomeTrait for fn(&32)` and
-                // `impl SomeTrait for fn(&'static u32)` at the same time which
-                // specify distinct values for Assoc. (See also #56105)
-                lt_op: |_| tcx.lifetimes.re_erased,
-                // Leave consts and types unchanged.
-                ct_op: |ct| ct,
-                ty_op: |ty| ty,
-            },
-        )
-    };
-    tcx.infer_ctxt().build().can_eq(param_env, normalize(src), normalize(dest)).is_ok()
-}
-
 struct TypeChecker<'a, 'tcx> {
     when: &'a str,
     body: &'a Body<'tcx>,
@@ -183,22 +143,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             return true;
         }
 
-        // Normalize projections and things like that.
-        // Type-changing assignments can happen when subtyping is used. While
-        // all normal lifetimes are erased, higher-ranked types with their
-        // late-bound lifetimes are still around and can lead to type
-        // differences. So we compare ignoring lifetimes.
-
-        // First, try with reveal_all. This might not work in some cases, as the predicates
-        // can be cleared in reveal_all mode. We try the reveal first anyways as it is used
-        // by some other passes like inlining as well.
-        let param_env = self.param_env.with_reveal_all_normalized(self.tcx);
-        if equal_up_to_regions(self.tcx, param_env, src, dest) {
-            return true;
-        }
-
-        // If this fails, we can try it without the reveal.
-        equal_up_to_regions(self.tcx, self.param_env, src, dest)
+        crate::util::is_subtype(self.tcx, self.param_env, src, dest)
     }
 }
 
diff --git a/compiler/rustc_const_eval/src/util/compare_types.rs b/compiler/rustc_const_eval/src/util/compare_types.rs
new file mode 100644
index 0000000000000..a9cb191cc593f
--- /dev/null
+++ b/compiler/rustc_const_eval/src/util/compare_types.rs
@@ -0,0 +1,63 @@
+//! Routines to check for relations between fully inferred types.
+//!
+//! FIXME: Move this to a more general place. The utility of this extends to
+//! other areas of the compiler as well.
+
+use rustc_infer::infer::{DefiningAnchor, TyCtxtInferExt};
+use rustc_infer::traits::ObligationCause;
+use rustc_middle::ty::{ParamEnv, Ty, TyCtxt};
+use rustc_trait_selection::traits::ObligationCtxt;
+
+/// Returns whether the two types are equal up to subtyping.
+///
+/// This is used in case we don't know the expected subtyping direction
+/// and still want to check whether anything is broken.
+pub fn is_equal_up_to_subtyping<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    param_env: ParamEnv<'tcx>,
+    src: Ty<'tcx>,
+    dest: Ty<'tcx>,
+) -> bool {
+    // Fast path.
+    if src == dest {
+        return true;
+    }
+
+    // Check for subtyping in either direction.
+    is_subtype(tcx, param_env, src, dest) || is_subtype(tcx, param_env, dest, src)
+}
+
+/// Returns whether `src` is a subtype of `dest`, i.e. `src <: dest`.
+///
+/// This mostly ignores opaque types as it can be used in constraining contexts
+/// while still computing the final underlying type.
+pub fn is_subtype<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    param_env: ParamEnv<'tcx>,
+    src: Ty<'tcx>,
+    dest: Ty<'tcx>,
+) -> bool {
+    if src == dest {
+        return true;
+    }
+
+    let mut builder =
+        tcx.infer_ctxt().ignoring_regions().with_opaque_type_inference(DefiningAnchor::Bubble);
+    let infcx = builder.build();
+    let ocx = ObligationCtxt::new(&infcx);
+    let cause = ObligationCause::dummy();
+    let src = ocx.normalize(cause.clone(), param_env, src);
+    let dest = ocx.normalize(cause.clone(), param_env, dest);
+    match ocx.sub(&cause, param_env, src, dest) {
+        Ok(()) => {}
+        Err(_) => return false,
+    };
+    let errors = ocx.select_all_or_error();
+    // With `Reveal::All`, opaque types get normalized away, with `Reveal::UserFacing`
+    // we would get unification errors because we're unable to look into opaque types,
+    // even if they're constrained in our current function.
+    //
+    // It seems very unlikely that this hides any bugs.
+    let _ = infcx.inner.borrow_mut().opaque_type_storage.take_opaque_types();
+    errors.is_empty()
+}
diff --git a/compiler/rustc_const_eval/src/util/mod.rs b/compiler/rustc_const_eval/src/util/mod.rs
index 4d0f81a406008..76ea5a24e69ed 100644
--- a/compiler/rustc_const_eval/src/util/mod.rs
+++ b/compiler/rustc_const_eval/src/util/mod.rs
@@ -2,6 +2,7 @@ pub mod aggregate;
 mod alignment;
 mod call_kind;
 pub mod collect_writes;
+mod compare_types;
 mod find_self_call;
 mod might_permit_raw_init;
 mod type_name;
@@ -9,6 +10,7 @@ mod type_name;
 pub use self::aggregate::expand_aggregate;
 pub use self::alignment::is_disaligned;
 pub use self::call_kind::{call_kind, CallDesugaringKind, CallKind};
+pub use self::compare_types::{is_equal_up_to_subtyping, is_subtype};
 pub use self::find_self_call::find_self_call;
 pub use self::might_permit_raw_init::might_permit_raw_init;
 pub use self::type_name::type_name;
diff --git a/compiler/rustc_expand/src/base.rs b/compiler/rustc_expand/src/base.rs
index 95fff929d46fe..04fe6c4007e94 100644
--- a/compiler/rustc_expand/src/base.rs
+++ b/compiler/rustc_expand/src/base.rs
@@ -1436,7 +1436,7 @@ fn pretty_printing_compatibility_hack(item: &Item, sess: &ParseSess) -> bool {
                             let crate_matches = if c.starts_with("allsorts-rental") {
                                 true
                             } else {
-                                let mut version = c.trim_start_matches("rental-").split(".");
+                                let mut version = c.trim_start_matches("rental-").split('.');
                                 version.next() == Some("0")
                                     && version.next() == Some("5")
                                     && version
diff --git a/compiler/rustc_hir_analysis/src/astconv/mod.rs b/compiler/rustc_hir_analysis/src/astconv/mod.rs
index 16c40cf1299cb..83b95fe0e911e 100644
--- a/compiler/rustc_hir_analysis/src/astconv/mod.rs
+++ b/compiler/rustc_hir_analysis/src/astconv/mod.rs
@@ -115,7 +115,7 @@ pub trait AstConv<'tcx> {
     /// (e.g., resolve) that is translated into a ty-error. This is
     /// used to help suppress derived errors typeck might otherwise
     /// report.
-    fn set_tainted_by_errors(&self);
+    fn set_tainted_by_errors(&self, e: ErrorGuaranteed);
 
     fn record_ty(&self, hir_id: hir::HirId, ty: Ty<'tcx>, span: Span);
 }
@@ -2620,8 +2620,12 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 }
             }
             Res::Err => {
-                self.set_tainted_by_errors();
-                self.tcx().ty_error()
+                let e = self
+                    .tcx()
+                    .sess
+                    .delay_span_bug(path.span, "path with `Res:Err` but no error emitted");
+                self.set_tainted_by_errors(e);
+                self.tcx().ty_error_with_guaranteed(e)
             }
             _ => span_bug!(span, "unexpected resolution: {:?}", path.res),
         }
diff --git a/compiler/rustc_hir_analysis/src/check/check.rs b/compiler/rustc_hir_analysis/src/check/check.rs
index f76b282fa76d5..b4805de961889 100644
--- a/compiler/rustc_hir_analysis/src/check/check.rs
+++ b/compiler/rustc_hir_analysis/src/check/check.rs
@@ -449,8 +449,8 @@ fn check_opaque_meets_bounds<'tcx>(
 
     let misc_cause = traits::ObligationCause::misc(span, hir_id);
 
-    match infcx.at(&misc_cause, param_env).eq(opaque_ty, hidden_ty) {
-        Ok(infer_ok) => ocx.register_infer_ok_obligations(infer_ok),
+    match ocx.eq(&misc_cause, param_env, opaque_ty, hidden_ty) {
+        Ok(()) => {}
         Err(ty_err) => {
             tcx.sess.delay_span_bug(
                 span,
diff --git a/compiler/rustc_hir_analysis/src/check/compare_method.rs b/compiler/rustc_hir_analysis/src/check/compare_method.rs
index 5a222031c5610..0b9209771cd32 100644
--- a/compiler/rustc_hir_analysis/src/check/compare_method.rs
+++ b/compiler/rustc_hir_analysis/src/check/compare_method.rs
@@ -402,10 +402,8 @@ pub fn collect_trait_impl_trait_tys<'tcx>(
         unnormalized_trait_sig.inputs_and_output.iter().chain(trait_sig.inputs_and_output.iter()),
     );
 
-    match infcx.at(&cause, param_env).eq(trait_return_ty, impl_return_ty) {
-        Ok(infer::InferOk { value: (), obligations }) => {
-            ocx.register_obligations(obligations);
-        }
+    match ocx.eq(&cause, param_env, trait_return_ty, impl_return_ty) {
+        Ok(()) => {}
         Err(terr) => {
             let mut diag = struct_span_err!(
                 tcx.sess,
@@ -442,10 +440,8 @@ pub fn collect_trait_impl_trait_tys<'tcx>(
     // the lifetimes of the return type, but do this after unifying just the
     // return types, since we want to avoid duplicating errors from
     // `compare_predicate_entailment`.
-    match infcx.at(&cause, param_env).eq(trait_fty, impl_fty) {
-        Ok(infer::InferOk { value: (), obligations }) => {
-            ocx.register_obligations(obligations);
-        }
+    match ocx.eq(&cause, param_env, trait_fty, impl_fty) {
+        Ok(()) => {}
         Err(terr) => {
             // This function gets called during `compare_predicate_entailment` when normalizing a
             // signature that contains RPITIT. When the method signatures don't match, we have to
diff --git a/compiler/rustc_hir_analysis/src/collect.rs b/compiler/rustc_hir_analysis/src/collect.rs
index 2f64a88f03afe..a738ee4a14887 100644
--- a/compiler/rustc_hir_analysis/src/collect.rs
+++ b/compiler/rustc_hir_analysis/src/collect.rs
@@ -518,7 +518,7 @@ impl<'tcx> AstConv<'tcx> for ItemCtxt<'tcx> {
         ty
     }
 
-    fn set_tainted_by_errors(&self) {
+    fn set_tainted_by_errors(&self, _: ErrorGuaranteed) {
         // There's no obvious place to track this, so just let it go.
     }
 
diff --git a/compiler/rustc_hir_typeck/src/coercion.rs b/compiler/rustc_hir_typeck/src/coercion.rs
index c1e4ab600f34f..af0b7f62ae319 100644
--- a/compiler/rustc_hir_typeck/src/coercion.rs
+++ b/compiler/rustc_hir_typeck/src/coercion.rs
@@ -1561,7 +1561,9 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
                 // Mark that we've failed to coerce the types here to suppress
                 // any superfluous errors we might encounter while trying to
                 // emit or provide suggestions on how to fix the initial error.
-                fcx.set_tainted_by_errors();
+                fcx.set_tainted_by_errors(
+                    fcx.tcx.sess.delay_span_bug(cause.span, "coercion error but no error emitted"),
+                );
                 let (expected, found) = if label_expression_as_expected {
                     // In the case where this is a "forced unit", like
                     // `break`, we want to call the `()` "expected"
diff --git a/compiler/rustc_hir_typeck/src/demand.rs b/compiler/rustc_hir_typeck/src/demand.rs
index 9ca7730daa68d..5a34ab401749f 100644
--- a/compiler/rustc_hir_typeck/src/demand.rs
+++ b/compiler/rustc_hir_typeck/src/demand.rs
@@ -154,7 +154,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             Err(e) => e,
         };
 
-        self.set_tainted_by_errors();
+        self.set_tainted_by_errors(self.tcx.sess.delay_span_bug(
+            expr.span,
+            "`TypeError` when attempting coercion but no error emitted",
+        ));
         let expr = expr.peel_drop_temps();
         let cause = self.misc(expr.span);
         let expr_ty = self.resolve_vars_with_obligations(checked_ty);
diff --git a/compiler/rustc_hir_typeck/src/expr.rs b/compiler/rustc_hir_typeck/src/expr.rs
index 13a03b33de815..752d2e0ff7858 100644
--- a/compiler/rustc_hir_typeck/src/expr.rs
+++ b/compiler/rustc_hir_typeck/src/expr.rs
@@ -527,12 +527,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             self.resolve_ty_and_res_fully_qualified_call(qpath, expr.hir_id, expr.span);
         let ty = match res {
             Res::Err => {
-                self.set_tainted_by_errors();
-                tcx.ty_error()
+                let e =
+                    self.tcx.sess.delay_span_bug(qpath.span(), "`Res::Err` but no error emitted");
+                self.set_tainted_by_errors(e);
+                tcx.ty_error_with_guaranteed(e)
             }
             Res::Def(DefKind::Ctor(_, CtorKind::Fictive), _) => {
-                report_unexpected_variant_res(tcx, res, qpath, expr.span);
-                tcx.ty_error()
+                let e = report_unexpected_variant_res(tcx, res, qpath, expr.span);
+                tcx.ty_error_with_guaranteed(e)
             }
             _ => self.instantiate_value_path(segs, opt_ty, res, expr.span, expr.hir_id).0,
         };
@@ -1962,7 +1964,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expr_span: Span,
     ) {
         if variant.is_recovered() {
-            self.set_tainted_by_errors();
+            self.set_tainted_by_errors(
+                self.tcx
+                    .sess
+                    .delay_span_bug(expr_span, "parser recovered but no error was emitted"),
+            );
             return;
         }
         let mut err = self.err_ctxt().type_error_struct_with_diag(
diff --git a/compiler/rustc_hir_typeck/src/fallback.rs b/compiler/rustc_hir_typeck/src/fallback.rs
index 5d44092a5f68e..ac6b0924ab572 100644
--- a/compiler/rustc_hir_typeck/src/fallback.rs
+++ b/compiler/rustc_hir_typeck/src/fallback.rs
@@ -104,7 +104,7 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
         // type, `?T` is not considered unsolved, but `?I` is. The
         // same is true for float variables.)
         let fallback = match ty.kind() {
-            _ if self.is_tainted_by_errors() => self.tcx.ty_error(),
+            _ if let Some(e) = self.tainted_by_errors() => self.tcx.ty_error_with_guaranteed(e),
             ty::Infer(ty::IntVar(_)) => self.tcx.types.i32,
             ty::Infer(ty::FloatVar(_)) => self.tcx.types.f64,
             _ => match diverging_fallback.get(&ty) {
diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs
index c826a886ca641..b85a23257286b 100644
--- a/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs
+++ b/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs
@@ -140,8 +140,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         debug!("write_ty({:?}, {:?}) in fcx {}", id, self.resolve_vars_if_possible(ty), self.tag());
         self.typeck_results.borrow_mut().node_types_mut().insert(id, ty);
 
-        if ty.references_error() {
-            self.set_tainted_by_errors();
+        if let Err(e) = ty.error_reported() {
+            self.set_tainted_by_errors(e);
         }
     }
 
@@ -528,7 +528,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     pub fn node_ty(&self, id: hir::HirId) -> Ty<'tcx> {
         match self.typeck_results.borrow().node_types().get(id) {
             Some(&t) => t,
-            None if self.is_tainted_by_errors() => self.tcx.ty_error(),
+            None if let Some(e) = self.tainted_by_errors() => self.tcx.ty_error_with_guaranteed(e),
             None => {
                 bug!(
                     "no type for node {}: {} in fcx {}",
@@ -543,7 +543,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     pub fn node_ty_opt(&self, id: hir::HirId) -> Option<Ty<'tcx>> {
         match self.typeck_results.borrow().node_types().get(id) {
             Some(&t) => Some(t),
-            None if self.is_tainted_by_errors() => Some(self.tcx.ty_error()),
+            None if let Some(e) = self.tainted_by_errors() => Some(self.tcx.ty_error_with_guaranteed(e)),
             None => None,
         }
     }
@@ -1148,9 +1148,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 explicit_late_bound = ExplicitLateBound::Yes;
             }
 
-            if let Err(GenericArgCountMismatch { reported: Some(_), .. }) = arg_count.correct {
+            if let Err(GenericArgCountMismatch { reported: Some(e), .. }) = arg_count.correct {
                 infer_args_for_err.insert(index);
-                self.set_tainted_by_errors(); // See issue #53251.
+                self.set_tainted_by_errors(e); // See issue #53251.
             }
         }
 
@@ -1440,12 +1440,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         if !ty.is_ty_var() {
             ty
         } else {
-            if !self.is_tainted_by_errors() {
+            let e = self.tainted_by_errors().unwrap_or_else(|| {
                 self.err_ctxt()
                     .emit_inference_failure_err((**self).body_id, sp, ty.into(), E0282, true)
-                    .emit();
-            }
-            let err = self.tcx.ty_error();
+                    .emit()
+            });
+            let err = self.tcx.ty_error_with_guaranteed(e);
             self.demand_suptype(sp, err, ty);
             err
         }
diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs
index c3833b4872d99..a31ab9c8b23b8 100644
--- a/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs
+++ b/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs
@@ -511,8 +511,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             tys.into_iter().any(|ty| ty.references_error() || ty.is_ty_var())
         }
 
-        self.set_tainted_by_errors();
         let tcx = self.tcx;
+        // FIXME: taint after emitting errors and pass through an `ErrorGuaranteed`
+        self.set_tainted_by_errors(
+            tcx.sess.delay_span_bug(call_span, "no errors reported for args"),
+        );
 
         // Get the argument span in the context of the call span so that
         // suggestions and labels are (more) correct when an arg is a
@@ -1207,7 +1210,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let (def, ty) = self.finish_resolving_struct_path(qpath, path_span, hir_id);
         let variant = match def {
             Res::Err => {
-                self.set_tainted_by_errors();
+                self.set_tainted_by_errors(
+                    self.tcx.sess.delay_span_bug(path_span, "`Res::Err` but no error emitted"),
+                );
                 return None;
             }
             Res::Def(DefKind::Variant, _) => match ty.kind() {
diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs
index d5e4b6de581c3..177d521d2804c 100644
--- a/compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs
+++ b/compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs
@@ -4,6 +4,7 @@ mod checks;
 mod suggestions;
 
 pub use _impl::*;
+use rustc_errors::ErrorGuaranteed;
 pub use suggestions::*;
 
 use crate::coercion::DynamicCoerceMany;
@@ -289,8 +290,8 @@ impl<'a, 'tcx> AstConv<'tcx> for FnCtxt<'a, 'tcx> {
         }
     }
 
-    fn set_tainted_by_errors(&self) {
-        self.infcx.set_tainted_by_errors()
+    fn set_tainted_by_errors(&self, e: ErrorGuaranteed) {
+        self.infcx.set_tainted_by_errors(e)
     }
 
     fn record_ty(&self, hir_id: hir::HirId, ty: Ty<'tcx>, _span: Span) {
diff --git a/compiler/rustc_hir_typeck/src/lib.rs b/compiler/rustc_hir_typeck/src/lib.rs
index 6fd609aeaa060..334d6d0aa6c20 100644
--- a/compiler/rustc_hir_typeck/src/lib.rs
+++ b/compiler/rustc_hir_typeck/src/lib.rs
@@ -53,7 +53,7 @@ use crate::check::check_fn;
 use crate::coercion::DynamicCoerceMany;
 use crate::gather_locals::GatherLocalsVisitor;
 use rustc_data_structures::unord::UnordSet;
-use rustc_errors::{struct_span_err, MultiSpan};
+use rustc_errors::{struct_span_err, ErrorGuaranteed, MultiSpan};
 use rustc_hir as hir;
 use rustc_hir::def::Res;
 use rustc_hir::intravisit::Visitor;
@@ -344,7 +344,7 @@ fn typeck_with_fallback<'tcx>(
 
         fcx.select_all_obligations_or_error();
 
-        if !fcx.infcx.is_tainted_by_errors() {
+        if let None = fcx.infcx.tainted_by_errors() {
             fcx.check_transmutes();
         }
 
@@ -428,7 +428,12 @@ impl<'tcx> EnclosingBreakables<'tcx> {
     }
 }
 
-fn report_unexpected_variant_res(tcx: TyCtxt<'_>, res: Res, qpath: &hir::QPath<'_>, span: Span) {
+fn report_unexpected_variant_res(
+    tcx: TyCtxt<'_>,
+    res: Res,
+    qpath: &hir::QPath<'_>,
+    span: Span,
+) -> ErrorGuaranteed {
     struct_span_err!(
         tcx.sess,
         span,
@@ -437,7 +442,7 @@ fn report_unexpected_variant_res(tcx: TyCtxt<'_>, res: Res, qpath: &hir::QPath<'
         res.descr(),
         rustc_hir_pretty::qpath_to_string(qpath),
     )
-    .emit();
+    .emit()
 }
 
 /// Controls whether the arguments are tupled. This is used for the call
diff --git a/compiler/rustc_hir_typeck/src/mem_categorization.rs b/compiler/rustc_hir_typeck/src/mem_categorization.rs
index 362f1c3430041..0b5dc946c1def 100644
--- a/compiler/rustc_hir_typeck/src/mem_categorization.rs
+++ b/compiler/rustc_hir_typeck/src/mem_categorization.rs
@@ -133,7 +133,7 @@ impl<'a, 'tcx> MemCategorizationContext<'a, 'tcx> {
     }
 
     fn is_tainted_by_errors(&self) -> bool {
-        self.infcx.is_tainted_by_errors()
+        self.infcx.tainted_by_errors().is_some()
     }
 
     fn resolve_type_vars_or_error(
diff --git a/compiler/rustc_hir_typeck/src/method/suggest.rs b/compiler/rustc_hir_typeck/src/method/suggest.rs
index e2c5edd0e8833..4ac4914bd4479 100644
--- a/compiler/rustc_hir_typeck/src/method/suggest.rs
+++ b/compiler/rustc_hir_typeck/src/method/suggest.rs
@@ -114,7 +114,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let report_candidates = |span: Span,
                                  err: &mut Diagnostic,
                                  sources: &mut Vec<CandidateSource>,
-                                 sugg_span: Span| {
+                                 sugg_span: Option<Span>| {
             sources.sort();
             sources.dedup();
             // Dynamic limit to avoid hiding just one candidate, which is silly.
@@ -175,7 +175,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         } else {
                             err.note(&note_str);
                         }
-                        if let Some(trait_ref) = self.tcx.impl_trait_ref(impl_did) {
+                        if let Some(sugg_span) = sugg_span
+                            && let Some(trait_ref) = self.tcx.impl_trait_ref(impl_did) {
                             let path = self.tcx.def_path_str(trait_ref.def_id);
 
                             let ty = match item.kind {
@@ -224,20 +225,22 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             err.span_note(item_span, msg);
                             None
                         };
-                        let path = self.tcx.def_path_str(trait_did);
-                        print_disambiguation_help(
-                            item_name,
-                            args,
-                            err,
-                            path,
-                            rcvr_ty,
-                            item.kind,
-                            item.def_id,
-                            sugg_span,
-                            idx,
-                            self.tcx.sess.source_map(),
-                            item.fn_has_self_parameter,
-                        );
+                        if let Some(sugg_span) = sugg_span {
+                            let path = self.tcx.def_path_str(trait_did);
+                            print_disambiguation_help(
+                                item_name,
+                                args,
+                                err,
+                                path,
+                                rcvr_ty,
+                                item.kind,
+                                item.def_id,
+                                sugg_span,
+                                idx,
+                                self.tcx.sess.source_map(),
+                                item.fn_has_self_parameter,
+                            );
+                        }
                     }
                 }
             }
@@ -407,9 +410,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         sugg_span,
                     );
 
-                    report_candidates(span, &mut err, &mut static_candidates, sugg_span);
+                    report_candidates(span, &mut err, &mut static_candidates, None);
                 } else if static_candidates.len() > 1 {
-                    report_candidates(span, &mut err, &mut static_candidates, sugg_span);
+                    report_candidates(span, &mut err, &mut static_candidates, Some(sugg_span));
                 }
 
                 let mut bound_spans = vec![];
@@ -1015,7 +1018,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 );
                 err.span_label(item_name.span, format!("multiple `{}` found", item_name));
 
-                report_candidates(span, &mut err, &mut sources, sugg_span);
+                report_candidates(span, &mut err, &mut sources, Some(sugg_span));
                 err.emit();
             }
 
diff --git a/compiler/rustc_hir_typeck/src/pat.rs b/compiler/rustc_hir_typeck/src/pat.rs
index eb10f3e2c107f..a62d43561302b 100644
--- a/compiler/rustc_hir_typeck/src/pat.rs
+++ b/compiler/rustc_hir_typeck/src/pat.rs
@@ -839,12 +839,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let (res, opt_ty, segments) = path_resolution;
         match res {
             Res::Err => {
-                self.set_tainted_by_errors();
-                return tcx.ty_error();
+                let e = tcx.sess.delay_span_bug(qpath.span(), "`Res::Err` but no error emitted");
+                self.set_tainted_by_errors(e);
+                return tcx.ty_error_with_guaranteed(e);
             }
             Res::Def(DefKind::AssocFn | DefKind::Ctor(_, CtorKind::Fictive | CtorKind::Fn), _) => {
-                report_unexpected_variant_res(tcx, res, qpath, pat.span);
-                return tcx.ty_error();
+                let e = report_unexpected_variant_res(tcx, res, qpath, pat.span);
+                return tcx.ty_error_with_guaranteed(e);
             }
             Res::SelfCtor(..)
             | Res::Def(
@@ -985,9 +986,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         ti: TopInfo<'tcx>,
     ) -> Ty<'tcx> {
         let tcx = self.tcx;
-        let on_error = || {
+        let on_error = |e| {
             for pat in subpats {
-                self.check_pat(pat, tcx.ty_error(), def_bm, ti);
+                self.check_pat(pat, tcx.ty_error_with_guaranteed(e), def_bm, ti);
             }
         };
         let report_unexpected_res = |res: Res| {
@@ -1014,36 +1015,39 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     err.span_label(pat.span, "not a tuple variant or struct");
                 }
             }
-            err.emit();
-            on_error();
+            let e = err.emit();
+            on_error(e);
+            e
         };
 
         // Resolve the path and check the definition for errors.
         let (res, opt_ty, segments) =
             self.resolve_ty_and_res_fully_qualified_call(qpath, pat.hir_id, pat.span);
         if res == Res::Err {
-            self.set_tainted_by_errors();
-            on_error();
-            return self.tcx.ty_error();
+            let e = tcx.sess.delay_span_bug(pat.span, "`Res:Err` but no error emitted");
+            self.set_tainted_by_errors(e);
+            on_error(e);
+            return tcx.ty_error_with_guaranteed(e);
         }
 
         // Type-check the path.
         let (pat_ty, res) =
             self.instantiate_value_path(segments, opt_ty, res, pat.span, pat.hir_id);
         if !pat_ty.is_fn() {
-            report_unexpected_res(res);
-            return tcx.ty_error();
+            let e = report_unexpected_res(res);
+            return tcx.ty_error_with_guaranteed(e);
         }
 
         let variant = match res {
             Res::Err => {
-                self.set_tainted_by_errors();
-                on_error();
-                return tcx.ty_error();
+                let e = tcx.sess.delay_span_bug(pat.span, "`Res::Err` but no error emitted");
+                self.set_tainted_by_errors(e);
+                on_error(e);
+                return tcx.ty_error_with_guaranteed(e);
             }
             Res::Def(DefKind::AssocConst | DefKind::AssocFn, _) => {
-                report_unexpected_res(res);
-                return tcx.ty_error();
+                let e = report_unexpected_res(res);
+                return tcx.ty_error_with_guaranteed(e);
             }
             Res::Def(DefKind::Ctor(_, CtorKind::Fn), _) => tcx.expect_variant_res(res),
             _ => bug!("unexpected pattern resolution: {:?}", res),
@@ -1082,9 +1086,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
         } else {
             // Pattern has wrong number of fields.
-            self.e0023(pat.span, res, qpath, subpats, &variant.fields, expected, had_err);
-            on_error();
-            return tcx.ty_error();
+            let e = self.e0023(pat.span, res, qpath, subpats, &variant.fields, expected, had_err);
+            on_error(e);
+            return tcx.ty_error_with_guaranteed(e);
         }
         pat_ty
     }
@@ -1098,7 +1102,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         fields: &'tcx [ty::FieldDef],
         expected: Ty<'tcx>,
         had_err: bool,
-    ) {
+    ) -> ErrorGuaranteed {
         let subpats_ending = pluralize!(subpats.len());
         let fields_ending = pluralize!(fields.len());
 
@@ -1245,7 +1249,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
         }
 
-        err.emit();
+        err.emit()
     }
 
     fn check_pat_tuple(
diff --git a/compiler/rustc_hir_typeck/src/writeback.rs b/compiler/rustc_hir_typeck/src/writeback.rs
index 2eca40d678a84..6c2ee35fa50da 100644
--- a/compiler/rustc_hir_typeck/src/writeback.rs
+++ b/compiler/rustc_hir_typeck/src/writeback.rs
@@ -83,10 +83,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         wbcx.typeck_results.treat_byte_string_as_slice =
             mem::take(&mut self.typeck_results.borrow_mut().treat_byte_string_as_slice);
 
-        if self.is_tainted_by_errors() {
-            // FIXME(eddyb) keep track of `ErrorGuaranteed` from where the error was emitted.
-            wbcx.typeck_results.tainted_by_errors =
-                Some(ErrorGuaranteed::unchecked_claim_error_was_emitted());
+        if let Some(e) = self.tainted_by_errors() {
+            wbcx.typeck_results.tainted_by_errors = Some(e);
         }
 
         debug!("writeback: typeck results for {:?} are {:#?}", item_def_id, wbcx.typeck_results);
@@ -674,10 +672,8 @@ impl<'cx, 'tcx> WritebackCx<'cx, 'tcx> {
         // We may have introduced e.g. `ty::Error`, if inference failed, make sure
         // to mark the `TypeckResults` as tainted in that case, so that downstream
         // users of the typeck results don't produce extra errors, or worse, ICEs.
-        if resolver.replaced_with_error {
-            // FIXME(eddyb) keep track of `ErrorGuaranteed` from where the error was emitted.
-            self.typeck_results.tainted_by_errors =
-                Some(ErrorGuaranteed::unchecked_claim_error_was_emitted());
+        if let Some(e) = resolver.replaced_with_error {
+            self.typeck_results.tainted_by_errors = Some(e);
         }
 
         x
@@ -708,8 +704,8 @@ struct Resolver<'cx, 'tcx> {
     span: &'cx dyn Locatable,
     body: &'tcx hir::Body<'tcx>,
 
-    /// Set to `true` if any `Ty` or `ty::Const` had to be replaced with an `Error`.
-    replaced_with_error: bool,
+    /// Set to `Some` if any `Ty` or `ty::Const` had to be replaced with an `Error`.
+    replaced_with_error: Option<ErrorGuaranteed>,
 }
 
 impl<'cx, 'tcx> Resolver<'cx, 'tcx> {
@@ -718,12 +714,14 @@ impl<'cx, 'tcx> Resolver<'cx, 'tcx> {
         span: &'cx dyn Locatable,
         body: &'tcx hir::Body<'tcx>,
     ) -> Resolver<'cx, 'tcx> {
-        Resolver { tcx: fcx.tcx, infcx: fcx, span, body, replaced_with_error: false }
+        Resolver { tcx: fcx.tcx, infcx: fcx, span, body, replaced_with_error: None }
     }
 
-    fn report_error(&self, p: impl Into<ty::GenericArg<'tcx>>) {
-        if !self.tcx.sess.has_errors().is_some() {
-            self.infcx
+    fn report_error(&self, p: impl Into<ty::GenericArg<'tcx>>) -> ErrorGuaranteed {
+        match self.tcx.sess.has_errors() {
+            Some(e) => e,
+            None => self
+                .infcx
                 .err_ctxt()
                 .emit_inference_failure_err(
                     Some(self.body.id()),
@@ -732,7 +730,7 @@ impl<'cx, 'tcx> Resolver<'cx, 'tcx> {
                     E0282,
                     false,
                 )
-                .emit();
+                .emit(),
         }
     }
 }
@@ -773,9 +771,9 @@ impl<'cx, 'tcx> TypeFolder<'tcx> for Resolver<'cx, 'tcx> {
             }
             Err(_) => {
                 debug!("Resolver::fold_ty: input type `{:?}` not fully resolvable", t);
-                self.report_error(t);
-                self.replaced_with_error = true;
-                self.tcx().ty_error()
+                let e = self.report_error(t);
+                self.replaced_with_error = Some(e);
+                self.tcx().ty_error_with_guaranteed(e)
             }
         }
     }
@@ -790,9 +788,9 @@ impl<'cx, 'tcx> TypeFolder<'tcx> for Resolver<'cx, 'tcx> {
             Ok(ct) => self.tcx.erase_regions(ct),
             Err(_) => {
                 debug!("Resolver::fold_const: input const `{:?}` not fully resolvable", ct);
-                self.report_error(ct);
-                self.replaced_with_error = true;
-                self.tcx().const_error(ct.ty())
+                let e = self.report_error(ct);
+                self.replaced_with_error = Some(e);
+                self.tcx().const_error_with_guaranteed(ct.ty(), e)
             }
         }
     }
diff --git a/compiler/rustc_infer/src/infer/mod.rs b/compiler/rustc_infer/src/infer/mod.rs
index fd3b3e4d59fa6..b9ed6b28c220d 100644
--- a/compiler/rustc_infer/src/infer/mod.rs
+++ b/compiler/rustc_infer/src/infer/mod.rs
@@ -34,7 +34,7 @@ pub use rustc_middle::ty::IntVarValue;
 use rustc_middle::ty::{self, GenericParamDefKind, InferConst, Ty, TyCtxt};
 use rustc_middle::ty::{ConstVid, FloatVid, IntVid, TyVid};
 use rustc_span::symbol::Symbol;
-use rustc_span::{Span, DUMMY_SP};
+use rustc_span::Span;
 
 use std::cell::{Cell, RefCell};
 use std::fmt;
@@ -1208,7 +1208,8 @@ impl<'tcx> InferCtxt<'tcx> {
     /// reporting errors that often occur as a result of earlier
     /// errors, but where it's hard to be 100% sure (e.g., unresolved
     /// inference variables, regionck errors).
-    pub fn is_tainted_by_errors(&self) -> bool {
+    #[must_use = "this method does not have any side effects"]
+    pub fn tainted_by_errors(&self) -> Option<ErrorGuaranteed> {
         debug!(
             "is_tainted_by_errors(err_count={}, err_count_on_creation={}, \
              tainted_by_errors={})",
@@ -1217,19 +1218,25 @@ impl<'tcx> InferCtxt<'tcx> {
             self.tainted_by_errors.get().is_some()
         );
 
+        if let Some(e) = self.tainted_by_errors.get() {
+            return Some(e);
+        }
+
         if self.tcx.sess.err_count() > self.err_count_on_creation {
-            return true; // errors reported since this infcx was made
+            // errors reported since this infcx was made
+            let e = self.tcx.sess.has_errors().unwrap();
+            self.set_tainted_by_errors(e);
+            return Some(e);
         }
-        self.tainted_by_errors.get().is_some()
+
+        None
     }
 
     /// Set the "tainted by errors" flag to true. We call this when we
     /// observe an error from a prior pass.
-    pub fn set_tainted_by_errors(&self) {
-        debug!("set_tainted_by_errors()");
-        self.tainted_by_errors.set(Some(
-            self.tcx.sess.delay_span_bug(DUMMY_SP, "`InferCtxt` incorrectly tainted by errors"),
-        ));
+    pub fn set_tainted_by_errors(&self, e: ErrorGuaranteed) {
+        debug!("set_tainted_by_errors(ErrorGuaranteed)");
+        self.tainted_by_errors.set(Some(e));
     }
 
     pub fn skip_region_resolution(&self) {
@@ -1270,7 +1277,7 @@ impl<'tcx> InferCtxt<'tcx> {
             let mut inner = self.inner.borrow_mut();
             let inner = &mut *inner;
             assert!(
-                self.is_tainted_by_errors() || inner.region_obligations.is_empty(),
+                self.tainted_by_errors().is_some() || inner.region_obligations.is_empty(),
                 "region_obligations not empty: {:#?}",
                 inner.region_obligations
             );
@@ -1707,7 +1714,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     ) {
         let errors = self.resolve_regions(outlives_env);
 
-        if !self.is_tainted_by_errors() {
+        if let None = self.tainted_by_errors() {
             // As a heuristic, just skip reporting region errors
             // altogether if other errors have been reported while
             // this infcx was in use.  This is totally hokey but
diff --git a/compiler/rustc_infer/src/infer/nll_relate/mod.rs b/compiler/rustc_infer/src/infer/nll_relate/mod.rs
index 600f94f095eac..167a82d4499a1 100644
--- a/compiler/rustc_infer/src/infer/nll_relate/mod.rs
+++ b/compiler/rustc_infer/src/infer/nll_relate/mod.rs
@@ -556,8 +556,9 @@ where
         self.ambient_variance_info = self.ambient_variance_info.xform(info);
 
         debug!(?self.ambient_variance);
-
-        let r = self.relate(a, b)?;
+        // In a bivariant context this always succeeds.
+        let r =
+            if self.ambient_variance == ty::Variance::Bivariant { a } else { self.relate(a, b)? };
 
         self.ambient_variance = old_ambient_variance;
 
diff --git a/compiler/rustc_infer/src/infer/sub.rs b/compiler/rustc_infer/src/infer/sub.rs
index 8c8445a4d9eb8..bd3c5780b891b 100644
--- a/compiler/rustc_infer/src/infer/sub.rs
+++ b/compiler/rustc_infer/src/infer/sub.rs
@@ -116,9 +116,9 @@ impl<'tcx> TypeRelation<'tcx> for Sub<'_, '_, 'tcx> {
                 Ok(a)
             }
 
-            (&ty::Error(_), _) | (_, &ty::Error(_)) => {
-                infcx.set_tainted_by_errors();
-                Ok(self.tcx().ty_error())
+            (&ty::Error(e), _) | (_, &ty::Error(e)) => {
+                infcx.set_tainted_by_errors(e);
+                Ok(self.tcx().ty_error_with_guaranteed(e))
             }
 
             (&ty::Opaque(a_def_id, _), &ty::Opaque(b_def_id, _)) if a_def_id == b_def_id => {
diff --git a/compiler/rustc_lint/src/unused.rs b/compiler/rustc_lint/src/unused.rs
index 50f9cb0b56f6d..cdf279313a672 100644
--- a/compiler/rustc_lint/src/unused.rs
+++ b/compiler/rustc_lint/src/unused.rs
@@ -582,7 +582,7 @@ trait UnusedDelimLint {
                 let sm = cx.sess().source_map();
                 let lo_replace =
                     if keep_space.0 &&
-                        let Ok(snip) = sm.span_to_prev_source(lo) && !snip.ends_with(" ") {
+                        let Ok(snip) = sm.span_to_prev_source(lo) && !snip.ends_with(' ') {
                         " ".to_string()
                         } else {
                             "".to_string()
@@ -590,7 +590,7 @@ trait UnusedDelimLint {
 
                 let hi_replace =
                     if keep_space.1 &&
-                        let Ok(snip) = sm.span_to_next_source(hi) && !snip.starts_with(" ") {
+                        let Ok(snip) = sm.span_to_next_source(hi) && !snip.starts_with(' ') {
                         " ".to_string()
                         } else {
                             "".to_string()
diff --git a/compiler/rustc_macros/src/diagnostics/utils.rs b/compiler/rustc_macros/src/diagnostics/utils.rs
index ba06f61299f37..dff088b9bdfa6 100644
--- a/compiler/rustc_macros/src/diagnostics/utils.rs
+++ b/compiler/rustc_macros/src/diagnostics/utils.rs
@@ -830,5 +830,5 @@ pub(super) fn should_generate_set_arg(field: &Field) -> bool {
 }
 
 pub(super) fn is_doc_comment(attr: &Attribute) -> bool {
-    attr.path.segments.last().unwrap().ident.to_string() == "doc"
+    attr.path.segments.last().unwrap().ident == "doc"
 }
diff --git a/compiler/rustc_middle/src/ty/print/pretty.rs b/compiler/rustc_middle/src/ty/print/pretty.rs
index 48e803597b02e..93d6850560d7c 100644
--- a/compiler/rustc_middle/src/ty/print/pretty.rs
+++ b/compiler/rustc_middle/src/ty/print/pretty.rs
@@ -2194,11 +2194,9 @@ impl<'tcx> FmtPrinter<'_, 'tcx> {
 
         define_scoped_cx!(self);
 
-        let possible_names =
-            ('a'..='z').rev().map(|s| Symbol::intern(&format!("'{s}"))).collect::<Vec<_>>();
+        let possible_names = ('a'..='z').rev().map(|s| Symbol::intern(&format!("'{s}")));
 
         let mut available_names = possible_names
-            .into_iter()
             .filter(|name| !self.used_region_names.contains(&name))
             .collect::<Vec<_>>();
         debug!(?available_names);
diff --git a/compiler/rustc_mir_dataflow/src/value_analysis.rs b/compiler/rustc_mir_dataflow/src/value_analysis.rs
index db4b0a3deda9d..cc69a1bb02db1 100644
--- a/compiler/rustc_mir_dataflow/src/value_analysis.rs
+++ b/compiler/rustc_mir_dataflow/src/value_analysis.rs
@@ -899,7 +899,7 @@ fn debug_with_context_rec<V: Debug + Eq>(
         let info_elem = map.places[child].proj_elem.unwrap();
         let child_place_str = match info_elem {
             TrackElem::Field(field) => {
-                if place_str.starts_with("*") {
+                if place_str.starts_with('*') {
                     format!("({}).{}", place_str, field.index())
                 } else {
                     format!("{}.{}", place_str, field.index())
diff --git a/compiler/rustc_mir_transform/src/inline.rs b/compiler/rustc_mir_transform/src/inline.rs
index 780b91d9215d5..d7dd5fc852845 100644
--- a/compiler/rustc_mir_transform/src/inline.rs
+++ b/compiler/rustc_mir_transform/src/inline.rs
@@ -1,7 +1,6 @@
 //! Inlining pass for MIR functions
 use crate::deref_separator::deref_finder;
 use rustc_attr::InlineAttr;
-use rustc_const_eval::transform::validate::equal_up_to_regions;
 use rustc_index::bit_set::BitSet;
 use rustc_index::vec::Idx;
 use rustc_middle::middle::codegen_fn_attrs::{CodegenFnAttrFlags, CodegenFnAttrs};
@@ -14,7 +13,8 @@ use rustc_span::{hygiene::ExpnKind, ExpnData, LocalExpnId, Span};
 use rustc_target::abi::VariantIdx;
 use rustc_target::spec::abi::Abi;
 
-use super::simplify::{remove_dead_blocks, CfgSimplifier};
+use crate::simplify::{remove_dead_blocks, CfgSimplifier};
+use crate::util;
 use crate::MirPass;
 use std::iter;
 use std::ops::{Range, RangeFrom};
@@ -180,7 +180,7 @@ impl<'tcx> Inliner<'tcx> {
         let TerminatorKind::Call { args, destination, .. } = &terminator.kind else { bug!() };
         let destination_ty = destination.ty(&caller_body.local_decls, self.tcx).ty;
         let output_type = callee_body.return_ty();
-        if !equal_up_to_regions(self.tcx, self.param_env, output_type, destination_ty) {
+        if !util::is_subtype(self.tcx, self.param_env, output_type, destination_ty) {
             trace!(?output_type, ?destination_ty);
             return Err("failed to normalize return type");
         }
@@ -200,7 +200,7 @@ impl<'tcx> Inliner<'tcx> {
                 arg_tuple_tys.iter().zip(callee_body.args_iter().skip(skipped_args))
             {
                 let input_type = callee_body.local_decls[input].ty;
-                if !equal_up_to_regions(self.tcx, self.param_env, arg_ty, input_type) {
+                if !util::is_subtype(self.tcx, self.param_env, input_type, arg_ty) {
                     trace!(?arg_ty, ?input_type);
                     return Err("failed to normalize tuple argument type");
                 }
@@ -209,7 +209,7 @@ impl<'tcx> Inliner<'tcx> {
             for (arg, input) in args.iter().zip(callee_body.args_iter()) {
                 let input_type = callee_body.local_decls[input].ty;
                 let arg_ty = arg.ty(&caller_body.local_decls, self.tcx);
-                if !equal_up_to_regions(self.tcx, self.param_env, arg_ty, input_type) {
+                if !util::is_subtype(self.tcx, self.param_env, input_type, arg_ty) {
                     trace!(?arg_ty, ?input_type);
                     return Err("failed to normalize argument type");
                 }
@@ -847,7 +847,7 @@ impl<'tcx> Visitor<'tcx> for CostChecker<'_, 'tcx> {
             let parent = Place { local, projection: self.tcx.intern_place_elems(proj_base) };
             let parent_ty = parent.ty(&self.callee_body.local_decls, self.tcx);
             let check_equal = |this: &mut Self, f_ty| {
-                if !equal_up_to_regions(this.tcx, this.param_env, ty, f_ty) {
+                if !util::is_equal_up_to_subtyping(this.tcx, this.param_env, ty, f_ty) {
                     trace!(?ty, ?f_ty);
                     this.validation = Err("failed to normalize projection type");
                     return;
diff --git a/compiler/rustc_parse/src/parser/diagnostics.rs b/compiler/rustc_parse/src/parser/diagnostics.rs
index 350b270cc3dfe..2558ac801addd 100644
--- a/compiler/rustc_parse/src/parser/diagnostics.rs
+++ b/compiler/rustc_parse/src/parser/diagnostics.rs
@@ -1660,14 +1660,14 @@ impl<'a> Parser<'a> {
                 let left = begin_par_sp;
                 let right = self.prev_token.span;
                 let left_snippet = if let Ok(snip) = sm.span_to_prev_source(left) &&
-                        !snip.ends_with(" ") {
+                        !snip.ends_with(' ') {
                                 " ".to_string()
                             } else {
                                 "".to_string()
                             };
 
                 let right_snippet = if let Ok(snip) = sm.span_to_next_source(right) &&
-                        !snip.starts_with(" ") {
+                        !snip.starts_with(' ') {
                                 " ".to_string()
                             } else {
                                 "".to_string()
diff --git a/compiler/rustc_passes/src/check_attr.rs b/compiler/rustc_passes/src/check_attr.rs
index 6b8cd0713732d..acb9bd8e78a4a 100644
--- a/compiler/rustc_passes/src/check_attr.rs
+++ b/compiler/rustc_passes/src/check_attr.rs
@@ -353,7 +353,7 @@ impl CheckAttrVisitor<'_> {
                 attr.span,
                 OnlyHasEffectOn {
                     attr_name: attr.name_or_empty(),
-                    target_name: allowed_target.name().replace(" ", "_"),
+                    target_name: allowed_target.name().replace(' ', "_"),
                 },
             );
         }
diff --git a/compiler/rustc_passes/src/dead.rs b/compiler/rustc_passes/src/dead.rs
index d4722234a8f1e..5d0224c35f364 100644
--- a/compiler/rustc_passes/src/dead.rs
+++ b/compiler/rustc_passes/src/dead.rs
@@ -772,7 +772,7 @@ impl<'tcx> DeadVisitor<'tcx> {
         self.tcx.emit_spanned_lint(
             lint,
             tcx.hir().local_def_id_to_hir_id(first_id),
-            MultiSpan::from_spans(spans.clone()),
+            MultiSpan::from_spans(spans),
             diag,
         );
     }
diff --git a/compiler/rustc_session/src/session.rs b/compiler/rustc_session/src/session.rs
index 103521983578b..d602acec53e32 100644
--- a/compiler/rustc_session/src/session.rs
+++ b/compiler/rustc_session/src/session.rs
@@ -562,7 +562,10 @@ impl Session {
         if self.err_count() == old_count {
             Ok(result)
         } else {
-            Err(ErrorGuaranteed::unchecked_claim_error_was_emitted())
+            Err(self.delay_span_bug(
+                rustc_span::DUMMY_SP,
+                "`self.err_count()` changed but an error was not emitted",
+            ))
         }
     }
     #[allow(rustc::untranslatable_diagnostic)]
diff --git a/compiler/rustc_target/src/abi/call/mod.rs b/compiler/rustc_target/src/abi/call/mod.rs
index c5a6f9893b6f9..0c559ec04a413 100644
--- a/compiler/rustc_target/src/abi/call/mod.rs
+++ b/compiler/rustc_target/src/abi/call/mod.rs
@@ -3,6 +3,7 @@ use crate::abi::{HasDataLayout, TyAbiInterface, TyAndLayout};
 use crate::spec::{self, HasTargetSpec};
 use rustc_span::Symbol;
 use std::fmt;
+use std::str::FromStr;
 
 mod aarch64;
 mod amdgpu;
@@ -737,6 +738,33 @@ impl<'a, Ty> FnAbi<'a, Ty> {
     }
 }
 
+impl FromStr for Conv {
+    type Err = String;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        match s {
+            "C" => Ok(Conv::C),
+            "Rust" => Ok(Conv::Rust),
+            "RustCold" => Ok(Conv::Rust),
+            "ArmAapcs" => Ok(Conv::ArmAapcs),
+            "CCmseNonSecureCall" => Ok(Conv::CCmseNonSecureCall),
+            "Msp430Intr" => Ok(Conv::Msp430Intr),
+            "PtxKernel" => Ok(Conv::PtxKernel),
+            "X86Fastcall" => Ok(Conv::X86Fastcall),
+            "X86Intr" => Ok(Conv::X86Intr),
+            "X86Stdcall" => Ok(Conv::X86Stdcall),
+            "X86ThisCall" => Ok(Conv::X86ThisCall),
+            "X86VectorCall" => Ok(Conv::X86VectorCall),
+            "X86_64SysV" => Ok(Conv::X86_64SysV),
+            "X86_64Win64" => Ok(Conv::X86_64Win64),
+            "AmdGpuKernel" => Ok(Conv::AmdGpuKernel),
+            "AvrInterrupt" => Ok(Conv::AvrInterrupt),
+            "AvrNonBlockingInterrupt" => Ok(Conv::AvrNonBlockingInterrupt),
+            _ => Err(format!("'{}' is not a valid value for entry function call convetion.", s)),
+        }
+    }
+}
+
 // Some types are used a lot. Make sure they don't unintentionally get bigger.
 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
 mod size_asserts {
diff --git a/compiler/rustc_target/src/json.rs b/compiler/rustc_target/src/json.rs
index b5d9263521228..75bb76a9de087 100644
--- a/compiler/rustc_target/src/json.rs
+++ b/compiler/rustc_target/src/json.rs
@@ -89,3 +89,28 @@ impl<A: ToJson> ToJson for Option<A> {
         }
     }
 }
+
+impl ToJson for crate::abi::call::Conv {
+    fn to_json(&self) -> Json {
+        let s = match self {
+            Self::C => "C",
+            Self::Rust => "Rust",
+            Self::RustCold => "RustCold",
+            Self::ArmAapcs => "ArmAapcs",
+            Self::CCmseNonSecureCall => "CCmseNonSecureCall",
+            Self::Msp430Intr => "Msp430Intr",
+            Self::PtxKernel => "PtxKernel",
+            Self::X86Fastcall => "X86Fastcall",
+            Self::X86Intr => "X86Intr",
+            Self::X86Stdcall => "X86Stdcall",
+            Self::X86ThisCall => "X86ThisCall",
+            Self::X86VectorCall => "X86VectorCall",
+            Self::X86_64SysV => "X86_64SysV",
+            Self::X86_64Win64 => "X86_64Win64",
+            Self::AmdGpuKernel => "AmdGpuKernel",
+            Self::AvrInterrupt => "AvrInterrupt",
+            Self::AvrNonBlockingInterrupt => "AvrNonBlockingInterrupt",
+        };
+        Json::String(s.to_owned())
+    }
+}
diff --git a/compiler/rustc_target/src/spec/mod.rs b/compiler/rustc_target/src/spec/mod.rs
index 664592b02a124..6d936d2cb9ff0 100644
--- a/compiler/rustc_target/src/spec/mod.rs
+++ b/compiler/rustc_target/src/spec/mod.rs
@@ -34,6 +34,7 @@
 //! the target's settings, though `target-feature` and `link-args` will *add*
 //! to the list specified by the target, rather than replace.
 
+use crate::abi::call::Conv;
 use crate::abi::Endian;
 use crate::json::{Json, ToJson};
 use crate::spec::abi::{lookup as lookup_abi, Abi};
@@ -1668,6 +1669,14 @@ pub struct TargetOptions {
     /// Whether the target supports stack canary checks. `true` by default,
     /// since this is most common among tier 1 and tier 2 targets.
     pub supports_stack_protector: bool,
+
+    // The name of entry function.
+    // Default value is "main"
+    pub entry_name: StaticCow<str>,
+
+    // The ABI of entry function.
+    // Default value is `Conv::C`, i.e. C call convention
+    pub entry_abi: Conv,
 }
 
 /// Add arguments for the given flavor and also for its "twin" flavors
@@ -1884,6 +1893,8 @@ impl Default for TargetOptions {
             c_enum_min_bits: 32,
             generate_arange_section: true,
             supports_stack_protector: true,
+            entry_name: "main".into(),
+            entry_abi: Conv::C,
         }
     }
 }
@@ -2404,6 +2415,18 @@ impl Target {
                     }
                 }
             } );
+            ($key_name:ident, Conv) => ( {
+                let name = (stringify!($key_name)).replace("_", "-");
+                obj.remove(&name).and_then(|o| o.as_str().and_then(|s| {
+                    match Conv::from_str(s) {
+                        Ok(c) => {
+                            base.$key_name = c;
+                            Some(Ok(()))
+                        }
+                        Err(e) => Some(Err(e))
+                    }
+                })).unwrap_or(Ok(()))
+            } );
         }
 
         if let Some(j) = obj.remove("target-endian") {
@@ -2523,6 +2546,8 @@ impl Target {
         key!(c_enum_min_bits, u64);
         key!(generate_arange_section, bool);
         key!(supports_stack_protector, bool);
+        key!(entry_name);
+        key!(entry_abi, Conv)?;
 
         if base.is_builtin {
             // This can cause unfortunate ICEs later down the line.
@@ -2773,6 +2798,8 @@ impl ToJson for Target {
         target_option_val!(c_enum_min_bits);
         target_option_val!(generate_arange_section);
         target_option_val!(supports_stack_protector);
+        target_option_val!(entry_name);
+        target_option_val!(entry_abi);
 
         if let Some(abi) = self.default_adjusted_cabi {
             d.insert("default-adjusted-cabi".into(), Abi::name(abi).to_json());
diff --git a/compiler/rustc_trait_selection/src/traits/engine.rs b/compiler/rustc_trait_selection/src/traits/engine.rs
index ae29c9f561791..f8346e515d70c 100644
--- a/compiler/rustc_trait_selection/src/traits/engine.rs
+++ b/compiler/rustc_trait_selection/src/traits/engine.rs
@@ -125,6 +125,21 @@ impl<'a, 'tcx> ObligationCtxt<'a, 'tcx> {
             .map(|infer_ok| self.register_infer_ok_obligations(infer_ok))
     }
 
+    /// Checks whether `expected` is a subtype of `actual`: `expected <: actual`.
+    pub fn sub<T: ToTrace<'tcx>>(
+        &self,
+        cause: &ObligationCause<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        expected: T,
+        actual: T,
+    ) -> Result<(), TypeError<'tcx>> {
+        self.infcx
+            .at(cause, param_env)
+            .sup(expected, actual)
+            .map(|infer_ok| self.register_infer_ok_obligations(infer_ok))
+    }
+
+    /// Checks whether `expected` is a supertype of `actual`: `expected :> actual`.
     pub fn sup<T: ToTrace<'tcx>>(
         &self,
         cause: &ObligationCause<'tcx>,
@@ -132,13 +147,10 @@ impl<'a, 'tcx> ObligationCtxt<'a, 'tcx> {
         expected: T,
         actual: T,
     ) -> Result<(), TypeError<'tcx>> {
-        match self.infcx.at(cause, param_env).sup(expected, actual) {
-            Ok(InferOk { obligations, value: () }) => {
-                self.register_obligations(obligations);
-                Ok(())
-            }
-            Err(e) => Err(e),
-        }
+        self.infcx
+            .at(cause, param_env)
+            .sup(expected, actual)
+            .map(|infer_ok| self.register_infer_ok_obligations(infer_ok))
     }
 
     pub fn select_where_possible(&self) -> Vec<FulfillmentError<'tcx>> {
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index f087afa20baca..ad0785d3817e1 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -532,9 +532,12 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         root_obligation: &PredicateObligation<'tcx>,
         error: &SelectionError<'tcx>,
     ) {
-        self.set_tainted_by_errors();
         let tcx = self.tcx;
         let mut span = obligation.cause.span;
+        // FIXME: statically guarantee this by tainting after the diagnostic is emitted
+        self.set_tainted_by_errors(
+            tcx.sess.delay_span_bug(span, "`report_selection_error` did not emit an error"),
+        );
 
         let mut err = match *error {
             SelectionError::Unimplemented => {
@@ -2060,7 +2063,7 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 // check upstream for type errors and don't add the obligations to
                 // begin with in those cases.
                 if self.tcx.lang_items().sized_trait() == Some(trait_ref.def_id()) {
-                    if !self.is_tainted_by_errors() {
+                    if let None = self.tainted_by_errors() {
                         self.emit_inference_failure_err(
                             body_id,
                             span,
@@ -2115,16 +2118,16 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                         if impls.len() > 1 && impls.len() < 5 && has_non_region_infer {
                             self.annotate_source_of_ambiguity(&mut err, &impls, predicate);
                         } else {
-                            if self.is_tainted_by_errors() {
-                                err.delay_as_bug();
+                            if self.tainted_by_errors().is_some() {
+                                err.cancel();
                                 return;
                             }
                             err.note(&format!("cannot satisfy `{}`", predicate));
                         }
                     }
                     _ => {
-                        if self.is_tainted_by_errors() {
-                            err.delay_as_bug();
+                        if self.tainted_by_errors().is_some() {
+                            err.cancel();
                             return;
                         }
                         err.note(&format!("cannot satisfy `{}`", predicate));
@@ -2226,7 +2229,7 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                         ] = path.segments
                         && data.trait_ref.def_id == *trait_id
                         && self.tcx.trait_of_item(*item_id) == Some(*trait_id)
-                        && !self.is_tainted_by_errors()
+                        && let None = self.tainted_by_errors()
                     {
                         let (verb, noun) = match self.tcx.associated_item(item_id).kind {
                             ty::AssocKind::Const => ("refer to the", "constant"),
@@ -2295,7 +2298,7 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 // with error messages.
                 if arg.references_error()
                     || self.tcx.sess.has_errors().is_some()
-                    || self.is_tainted_by_errors()
+                    || self.tainted_by_errors().is_some()
                 {
                     return;
                 }
@@ -2306,7 +2309,7 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             ty::PredicateKind::Subtype(data) => {
                 if data.references_error()
                     || self.tcx.sess.has_errors().is_some()
-                    || self.is_tainted_by_errors()
+                    || self.tainted_by_errors().is_some()
                 {
                     // no need to overload user in such cases
                     return;
@@ -2317,7 +2320,7 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 self.emit_inference_failure_err(body_id, span, a.into(), ErrorCode::E0282, true)
             }
             ty::PredicateKind::Projection(data) => {
-                if predicate.references_error() || self.is_tainted_by_errors() {
+                if predicate.references_error() || self.tainted_by_errors().is_some() {
                     return;
                 }
                 let subst = data
@@ -2351,7 +2354,7 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             }
 
             ty::PredicateKind::ConstEvaluatable(data) => {
-                if predicate.references_error() || self.is_tainted_by_errors() {
+                if predicate.references_error() || self.tainted_by_errors().is_some() {
                     return;
                 }
                 let subst = data.walk().find(|g| g.is_non_region_infer());
@@ -2378,7 +2381,7 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 }
             }
             _ => {
-                if self.tcx.sess.has_errors().is_some() || self.is_tainted_by_errors() {
+                if self.tcx.sess.has_errors().is_some() || self.tainted_by_errors().is_some() {
                     return;
                 }
                 let mut err = struct_span_err!(
@@ -2422,7 +2425,7 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         post.sort();
         post.dedup();
 
-        if self.is_tainted_by_errors()
+        if self.tainted_by_errors().is_some()
             && (crate_names.len() == 1
                 && spans.len() == 0
                 && ["`core`", "`alloc`", "`std`"].contains(&crate_names[0].as_str())
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index c369c5de52bb1..b05942353a343 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -33,7 +33,7 @@ use crate::traits::ProjectionCacheKey;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::fx::{FxHashSet, FxIndexSet};
 use rustc_data_structures::stack::ensure_sufficient_stack;
-use rustc_errors::{Diagnostic, ErrorGuaranteed};
+use rustc_errors::Diagnostic;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_infer::infer::LateBoundRegionConversionTime;
@@ -1089,10 +1089,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         if !self.infcx.tcx.recursion_limit().value_within_limit(depth) {
             match self.query_mode {
                 TraitQueryMode::Standard => {
-                    if self.infcx.is_tainted_by_errors() {
-                        return Err(OverflowError::Error(
-                            ErrorGuaranteed::unchecked_claim_error_was_emitted(),
-                        ));
+                    if let Some(e) = self.infcx.tainted_by_errors() {
+                        return Err(OverflowError::Error(e));
                     }
                     self.infcx.err_ctxt().report_overflow_error(error_obligation, true);
                 }
diff --git a/library/core/src/cell/lazy.rs b/library/core/src/cell/lazy.rs
index 7844be5f783a9..b355d94ce4976 100644
--- a/library/core/src/cell/lazy.rs
+++ b/library/core/src/cell/lazy.rs
@@ -4,6 +4,10 @@ use crate::ops::Deref;
 
 /// A value which is initialized on the first access.
 ///
+/// For a thread-safe version of this struct, see [`std::sync::LazyLock`].
+///
+/// [`std::sync::LazyLock`]: ../../std/sync/struct.LazyLock.html
+///
 /// # Examples
 ///
 /// ```
diff --git a/library/core/src/cell/once.rs b/library/core/src/cell/once.rs
index 3c39394dd8c8e..8c01643c7ac17 100644
--- a/library/core/src/cell/once.rs
+++ b/library/core/src/cell/once.rs
@@ -4,8 +4,14 @@ use crate::mem;
 
 /// A cell which can be written to only once.
 ///
-/// Unlike `RefCell`, a `OnceCell` only provides shared `&T` references to its value.
-/// Unlike `Cell`, a `OnceCell` doesn't require copying or replacing the value to access it.
+/// Unlike [`RefCell`], a `OnceCell` only provides shared `&T` references to its value.
+/// Unlike [`Cell`], a `OnceCell` doesn't require copying or replacing the value to access it.
+///
+/// For a thread-safe version of this struct, see [`std::sync::OnceLock`].
+///
+/// [`RefCell`]: crate::cell::RefCell
+/// [`Cell`]: crate::cell::Cell
+/// [`std::sync::OnceLock`]: ../../std/sync/struct.OnceLock.html
 ///
 /// # Examples
 ///
diff --git a/library/std/src/f32.rs b/library/std/src/f32.rs
index 4127c4056f24e..4e30076246314 100644
--- a/library/std/src/f32.rs
+++ b/library/std/src/f32.rs
@@ -880,7 +880,9 @@ impl f32 {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn asinh(self) -> f32 {
-        (self.abs() + ((self * self) + 1.0).sqrt()).ln().copysign(self)
+        let ax = self.abs();
+        let ix = 1.0 / ax;
+        (ax + (ax / (Self::hypot(1.0, ix) + ix))).ln_1p().copysign(self)
     }
 
     /// Inverse hyperbolic cosine function.
@@ -900,7 +902,11 @@ impl f32 {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn acosh(self) -> f32 {
-        if self < 1.0 { Self::NAN } else { (self + ((self * self) - 1.0).sqrt()).ln() }
+        if self < 1.0 {
+            Self::NAN
+        } else {
+            (self + ((self - 1.0).sqrt() * (self + 1.0).sqrt())).ln()
+        }
     }
 
     /// Inverse hyperbolic tangent function.
diff --git a/library/std/src/f32/tests.rs b/library/std/src/f32/tests.rs
index 4ec16c84aa916..6ee295de6163f 100644
--- a/library/std/src/f32/tests.rs
+++ b/library/std/src/f32/tests.rs
@@ -587,6 +587,11 @@ fn test_asinh() {
     assert_approx_eq!((-2.0f32).asinh(), -1.443635475178810342493276740273105f32);
     // regression test for the catastrophic cancellation fixed in 72486
     assert_approx_eq!((-3000.0f32).asinh(), -8.699514775987968673236893537700647f32);
+
+    // test for low accuracy from issue 104548
+    assert_approx_eq!(60.0f32, 60.0f32.sinh().asinh());
+    // mul needed for approximate comparison to be meaningful
+    assert_approx_eq!(1.0f32, 1e-15f32.sinh().asinh() * 1e15f32);
 }
 
 #[test]
@@ -602,6 +607,9 @@ fn test_acosh() {
     assert!(nan.acosh().is_nan());
     assert_approx_eq!(2.0f32.acosh(), 1.31695789692481670862504634730796844f32);
     assert_approx_eq!(3.0f32.acosh(), 1.76274717403908605046521864995958461f32);
+
+    // test for low accuracy from issue 104548
+    assert_approx_eq!(60.0f32, 60.0f32.cosh().acosh());
 }
 
 #[test]
diff --git a/library/std/src/f64.rs b/library/std/src/f64.rs
index cc64258da60d1..ec67fdad4f726 100644
--- a/library/std/src/f64.rs
+++ b/library/std/src/f64.rs
@@ -882,7 +882,9 @@ impl f64 {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn asinh(self) -> f64 {
-        (self.abs() + ((self * self) + 1.0).sqrt()).ln().copysign(self)
+        let ax = self.abs();
+        let ix = 1.0 / ax;
+        (ax + (ax / (Self::hypot(1.0, ix) + ix))).ln_1p().copysign(self)
     }
 
     /// Inverse hyperbolic cosine function.
@@ -902,7 +904,11 @@ impl f64 {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn acosh(self) -> f64 {
-        if self < 1.0 { Self::NAN } else { (self + ((self * self) - 1.0).sqrt()).ln() }
+        if self < 1.0 {
+            Self::NAN
+        } else {
+            (self + ((self - 1.0).sqrt() * (self + 1.0).sqrt())).ln()
+        }
     }
 
     /// Inverse hyperbolic tangent function.
diff --git a/library/std/src/f64/tests.rs b/library/std/src/f64/tests.rs
index 12baa68f49b76..5b039d445ce14 100644
--- a/library/std/src/f64/tests.rs
+++ b/library/std/src/f64/tests.rs
@@ -575,6 +575,11 @@ fn test_asinh() {
     assert_approx_eq!((-2.0f64).asinh(), -1.443635475178810342493276740273105f64);
     // regression test for the catastrophic cancellation fixed in 72486
     assert_approx_eq!((-67452098.07139316f64).asinh(), -18.72007542627454439398548429400083);
+
+    // test for low accuracy from issue 104548
+    assert_approx_eq!(60.0f64, 60.0f64.sinh().asinh());
+    // mul needed for approximate comparison to be meaningful
+    assert_approx_eq!(1.0f64, 1e-15f64.sinh().asinh() * 1e15f64);
 }
 
 #[test]
@@ -590,6 +595,9 @@ fn test_acosh() {
     assert!(nan.acosh().is_nan());
     assert_approx_eq!(2.0f64.acosh(), 1.31695789692481670862504634730796844f64);
     assert_approx_eq!(3.0f64.acosh(), 1.76274717403908605046521864995958461f64);
+
+    // test for low accuracy from issue 104548
+    assert_approx_eq!(60.0f64, 60.0f64.cosh().acosh());
 }
 
 #[test]
diff --git a/library/std/src/sync/lazy_lock.rs b/library/std/src/sync/lazy_lock.rs
index 535cc1c42fcfd..c8d3289ca4a06 100644
--- a/library/std/src/sync/lazy_lock.rs
+++ b/library/std/src/sync/lazy_lock.rs
@@ -6,7 +6,9 @@ use crate::sync::OnceLock;
 
 /// A value which is initialized on the first access.
 ///
-/// This type is a thread-safe `Lazy`, and can be used in statics.
+/// This type is a thread-safe [`LazyCell`], and can be used in statics.
+///
+/// [`LazyCell`]: crate::cell::LazyCell
 ///
 /// # Examples
 ///
diff --git a/library/std/src/sync/once_lock.rs b/library/std/src/sync/once_lock.rs
index 37413ec62a717..16d1fd2a576b9 100644
--- a/library/std/src/sync/once_lock.rs
+++ b/library/std/src/sync/once_lock.rs
@@ -7,7 +7,9 @@ use crate::sync::Once;
 
 /// A synchronization primitive which can be written to only once.
 ///
-/// This type is a thread-safe `OnceCell`.
+/// This type is a thread-safe [`OnceCell`], and can be used in statics.
+///
+/// [`OnceCell`]: crate::cell::OnceCell
 ///
 /// # Examples
 ///
@@ -33,7 +35,7 @@ use crate::sync::Once;
 #[unstable(feature = "once_cell", issue = "74465")]
 pub struct OnceLock<T> {
     once: Once,
-    // Whether or not the value is initialized is tracked by `state_and_queue`.
+    // Whether or not the value is initialized is tracked by `once.is_completed()`.
     value: UnsafeCell<MaybeUninit<T>>,
     /// `PhantomData` to make sure dropck understands we're dropping T in our Drop impl.
     ///
diff --git a/src/librustdoc/config.rs b/src/librustdoc/config.rs
index 789dd398be50d..e0cdb86d9d1dc 100644
--- a/src/librustdoc/config.rs
+++ b/src/librustdoc/config.rs
@@ -418,7 +418,7 @@ impl Options {
             ) {
                 Ok(p) => p,
                 Err(e) => {
-                    diag.struct_err(&e.to_string()).emit();
+                    diag.struct_err(e).emit();
                     return Err(1);
                 }
             };
@@ -561,7 +561,7 @@ impl Options {
             ) {
                 Ok(p) => p,
                 Err(e) => {
-                    diag.struct_err(&e.to_string()).emit();
+                    diag.struct_err(e).emit();
                     return Err(1);
                 }
             };
diff --git a/src/librustdoc/html/static_files.rs b/src/librustdoc/html/static_files.rs
index b4d4150cddbb8..1f87f95563ad5 100644
--- a/src/librustdoc/html/static_files.rs
+++ b/src/librustdoc/html/static_files.rs
@@ -58,7 +58,7 @@ pub(crate) fn suffix_path(filename: &str, suffix: &str) -> PathBuf {
 }
 
 pub(crate) fn static_filename(filename: &str, contents: &[u8]) -> PathBuf {
-    let filename = filename.rsplit("/").next().unwrap();
+    let filename = filename.rsplit('/').next().unwrap();
     suffix_path(filename, &static_suffix(contents))
 }
 
diff --git a/src/test/ui/coherence/issue-100191-2.rs b/src/test/ui/coherence/issue-100191-2.rs
deleted file mode 100644
index 1c8316f87fa07..0000000000000
--- a/src/test/ui/coherence/issue-100191-2.rs
+++ /dev/null
@@ -1,12 +0,0 @@
-//~ ERROR overflow evaluating the requirement `T: Trait<_>`
-
-#![feature(specialization, with_negative_coherence)]
-#![allow(incomplete_features)]
-
-pub trait Trait<T> {}
-
-default impl<T, U> Trait<T> for U {}
-
-impl<T> Trait<<T as Iterator>::Item> for T {}
-
-fn main() {}
diff --git a/src/test/ui/coherence/issue-100191.rs b/src/test/ui/coherence/issue-100191.rs
deleted file mode 100644
index e8597fde54d3b..0000000000000
--- a/src/test/ui/coherence/issue-100191.rs
+++ /dev/null
@@ -1,21 +0,0 @@
-#![crate_type = "lib"]
-#![feature(specialization, with_negative_coherence)]
-#![allow(incomplete_features)]
-
-trait X {}
-trait Y: X {}
-trait Z {
-    type Assoc: Y;
-}
-struct A<T>(T);
-
-impl<T> Y for T where T: X {}
-impl<T: X> Z for A<T> {
-    type Assoc = T;
-}
-
-// this impl is invalid, but causes an ICE anyway
-impl<T> From<<A<T> as Z>::Assoc> for T {}
-//~^ ERROR type parameter `T` must be used as the type parameter for some local type (e.g., `MyStruct<T>`)
-
-fn main() {}
diff --git a/src/test/ui/issues/issue-52262.rs b/src/test/ui/issues/issue-52262.rs
index 2195b89555791..547643f0d6e20 100644
--- a/src/test/ui/issues/issue-52262.rs
+++ b/src/test/ui/issues/issue-52262.rs
@@ -1,4 +1,3 @@
-// compile-flags:-Ztreat-err-as-bug=5
 #[derive(Debug)]
 enum MyError {
     NotFound { key: Vec<u8> },
diff --git a/src/test/ui/issues/issue-52262.stderr b/src/test/ui/issues/issue-52262.stderr
index c0bde4b2321f1..ef41f078b8037 100644
--- a/src/test/ui/issues/issue-52262.stderr
+++ b/src/test/ui/issues/issue-52262.stderr
@@ -1,5 +1,5 @@
 error[E0507]: cannot move out of `*key` which is behind a shared reference
-  --> $DIR/issue-52262.rs:16:35
+  --> $DIR/issue-52262.rs:15:35
    |
 LL |                 String::from_utf8(*key).unwrap()
    |                                   ^^^^ move occurs because `*key` has type `Vec<u8>`, which does not implement the `Copy` trait
diff --git a/src/test/ui/mir/important-higher-ranked-regions.rs b/src/test/ui/mir/important-higher-ranked-regions.rs
new file mode 100644
index 0000000000000..cadfb3b66f297
--- /dev/null
+++ b/src/test/ui/mir/important-higher-ranked-regions.rs
@@ -0,0 +1,26 @@
+// check-pass
+// compile-flags: -Zvalidate-mir
+
+// This test checks that bivariant parameters are handled correctly
+// in the mir.
+#![allow(coherence_leak_check)]
+trait Trait {
+    type Assoc;
+}
+
+struct Foo<T, U>(T)
+where
+    T: Trait<Assoc = U>;
+
+impl Trait for for<'a> fn(&'a ()) {
+    type Assoc = u32;
+}
+impl Trait for fn(&'static ()) {
+    type Assoc = String;
+}
+
+fn foo(x: Foo<for<'a> fn(&'a ()), u32>) -> Foo<fn(&'static ()), String> {
+    x
+}
+
+fn main() {}
diff --git a/src/test/ui/specialization/issue-43037.stderr b/src/test/ui/specialization/issue-43037.current.stderr
similarity index 94%
rename from src/test/ui/specialization/issue-43037.stderr
rename to src/test/ui/specialization/issue-43037.current.stderr
index 4249cd8947716..26db9d7c99749 100644
--- a/src/test/ui/specialization/issue-43037.stderr
+++ b/src/test/ui/specialization/issue-43037.current.stderr
@@ -1,5 +1,5 @@
 error[E0210]: type parameter `T` must be used as the type parameter for some local type (e.g., `MyStruct<T>`)
-  --> $DIR/issue-43037.rs:17:6
+  --> $DIR/issue-43037.rs:19:6
    |
 LL | impl<T> From<<A<T> as Z>::Assoc> for T {}
    |      ^ type parameter `T` must be used as the type parameter for some local type
diff --git a/src/test/ui/coherence/issue-100191.stderr b/src/test/ui/specialization/issue-43037.negative.stderr
similarity index 94%
rename from src/test/ui/coherence/issue-100191.stderr
rename to src/test/ui/specialization/issue-43037.negative.stderr
index 1adb0f1e4fa7b..26db9d7c99749 100644
--- a/src/test/ui/coherence/issue-100191.stderr
+++ b/src/test/ui/specialization/issue-43037.negative.stderr
@@ -1,5 +1,5 @@
 error[E0210]: type parameter `T` must be used as the type parameter for some local type (e.g., `MyStruct<T>`)
-  --> $DIR/issue-100191.rs:18:6
+  --> $DIR/issue-43037.rs:19:6
    |
 LL | impl<T> From<<A<T> as Z>::Assoc> for T {}
    |      ^ type parameter `T` must be used as the type parameter for some local type
diff --git a/src/test/ui/specialization/issue-43037.rs b/src/test/ui/specialization/issue-43037.rs
index c49119f9c095b..a1e3f998b2370 100644
--- a/src/test/ui/specialization/issue-43037.rs
+++ b/src/test/ui/specialization/issue-43037.rs
@@ -1,4 +1,6 @@
+// revisions: current negative
 #![feature(specialization)]
+#![cfg_attr(negative, feature(with_negative_coherence))]
 #![allow(incomplete_features)]
 
 trait X {}
diff --git a/src/test/ui/specialization/issue-45814.stderr b/src/test/ui/specialization/issue-45814.current.stderr
similarity index 94%
rename from src/test/ui/specialization/issue-45814.stderr
rename to src/test/ui/specialization/issue-45814.current.stderr
index 419345addc2fc..5013559b80ea9 100644
--- a/src/test/ui/specialization/issue-45814.stderr
+++ b/src/test/ui/specialization/issue-45814.current.stderr
@@ -2,7 +2,7 @@ error[E0275]: overflow evaluating the requirement `T: Trait<_>`
    |
    = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`issue_45814`)
 note: required for `T` to implement `Trait<_>`
-  --> $DIR/issue-45814.rs:8:20
+  --> $DIR/issue-45814.rs:9:20
    |
 LL | default impl<T, U> Trait<T> for U {}
    |                    ^^^^^^^^     ^
diff --git a/src/test/ui/coherence/issue-100191-2.stderr b/src/test/ui/specialization/issue-45814.negative.stderr
similarity index 81%
rename from src/test/ui/coherence/issue-100191-2.stderr
rename to src/test/ui/specialization/issue-45814.negative.stderr
index d50c220bcfd48..5013559b80ea9 100644
--- a/src/test/ui/coherence/issue-100191-2.stderr
+++ b/src/test/ui/specialization/issue-45814.negative.stderr
@@ -1,8 +1,8 @@
 error[E0275]: overflow evaluating the requirement `T: Trait<_>`
    |
-   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`issue_100191_2`)
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`issue_45814`)
 note: required for `T` to implement `Trait<_>`
-  --> $DIR/issue-100191-2.rs:8:20
+  --> $DIR/issue-45814.rs:9:20
    |
 LL | default impl<T, U> Trait<T> for U {}
    |                    ^^^^^^^^     ^
diff --git a/src/test/ui/specialization/issue-45814.rs b/src/test/ui/specialization/issue-45814.rs
index 8ee5d3e2e58da..fce236390c2b0 100644
--- a/src/test/ui/specialization/issue-45814.rs
+++ b/src/test/ui/specialization/issue-45814.rs
@@ -1,6 +1,7 @@
 //~ ERROR overflow evaluating the requirement `T: Trait<_>`
-
+// revisions: current negative
 #![feature(specialization)]
+#![cfg_attr(negative, feature(with_negative_coherence))]
 #![allow(incomplete_features)]
 
 pub trait Trait<T> {}
diff --git a/src/test/ui/suggestions/issue-102354.stderr b/src/test/ui/suggestions/issue-102354.stderr
index b17c4dc5dfb50..08d4b99559031 100644
--- a/src/test/ui/suggestions/issue-102354.stderr
+++ b/src/test/ui/suggestions/issue-102354.stderr
@@ -2,7 +2,10 @@ error[E0599]: no method named `func` found for type `i32` in the current scope
   --> $DIR/issue-102354.rs:9:7
    |
 LL |     x.func();
-   |       ^^^^ this is an associated function, not a method
+   |     --^^^^--
+   |     | |
+   |     | this is an associated function, not a method
+   |     help: use associated function syntax instead: `i32::func()`
    |
    = note: found the following associated functions; to be used as methods, functions must have a `self` parameter
 note: the candidate is defined in the trait `Trait`
@@ -10,14 +13,6 @@ note: the candidate is defined in the trait `Trait`
    |
 LL |     fn func() {}
    |     ^^^^^^^^^
-help: use associated function syntax instead
-   |
-LL |     i32::func();
-   |     ~~~~~~~~~~~
-help: disambiguate the associated function for the candidate
-   |
-LL |     <i32 as Trait>::func(x);
-   |     ~~~~~~~~~~~~~~~~~~~~~~~
 
 error: aborting due to previous error
 
diff --git a/src/tools/miropt-test-tools/src/lib.rs b/src/tools/miropt-test-tools/src/lib.rs
index 96819d3547b29..cfba7d583b138 100644
--- a/src/tools/miropt-test-tools/src/lib.rs
+++ b/src/tools/miropt-test-tools/src/lib.rs
@@ -11,7 +11,7 @@ pub fn files_for_miropt_test(testfile: &std::path::Path, bit_width: u32) -> Vec<
     let test_file_contents = fs::read_to_string(&testfile).unwrap();
 
     let test_dir = testfile.parent().unwrap();
-    let test_crate = testfile.file_stem().unwrap().to_str().unwrap().replace("-", "_");
+    let test_crate = testfile.file_stem().unwrap().to_str().unwrap().replace('-', "_");
 
     let bit_width = if test_file_contents.lines().any(|l| l == "// EMIT_MIR_FOR_EACH_BIT_WIDTH") {
         format!(".{}bit", bit_width)