Auto merge of #71049 - eddyb:const-err, r=oli-obk

Add `ConstKind::Error` and convert `ErrorHandled::Reported` to it.

By replicating the `ty::Error` approach to encoding "an error has occurred", all of the mechanisms that skip redundant/downstream errors are engaged and help out (see the reduction in test output).

This PR also adds `ErrorHandled::Linted` for the lint case because using `ErrorHandled::Reported` *without* having emitted an error that is *guaranteed* to stop compilation, is incorrect now.

r? @oli-obk cc @rust-lang/wg-const-eval @varkor @yodaldevoid

src/librustc_codegen_ssa/mir/operand.rs

@@ -6,6 +6,7 @@ use crate::glue;
 use crate::traits::*;
 use crate::MemFlags;
 
+use rustc_errors::ErrorReported;
 use rustc_middle::mir;
 use rustc_middle::mir::interpret::{ConstValue, ErrorHandled, Pointer, Scalar};
 use rustc_middle::ty::layout::TyAndLayout;
@@ -447,8 +448,10 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 self.eval_mir_constant_to_operand(bx, constant).unwrap_or_else(|err| {
                     match err {
                         // errored or at least linted
-                        ErrorHandled::Reported => {}
-                        ErrorHandled::TooGeneric => bug!("codgen encountered polymorphic constant"),
+                        ErrorHandled::Reported(ErrorReported) | ErrorHandled::Linted => {}
+                        ErrorHandled::TooGeneric => {
+                            bug!("codegen encountered polymorphic constant")
+                        }
                     }
                     // Allow RalfJ to sleep soundly knowing that even refactorings that remove
                     // the above error (or silence it under some conditions) will not cause UB.

src/librustc_infer/infer/error_reporting/mod.rs

@@ -304,8 +304,8 @@ pub fn unexpected_hidden_region_diagnostic(
         // down this path which gives a decent human readable
         // explanation.
         //
-        // (*) if not, the `tainted_by_errors` flag would be set to
-        // true in any case, so we wouldn't be here at all.
+        // (*) if not, the `tainted_by_errors` field would be set to
+        // `Some(ErrorReported)` in any case, so we wouldn't be here at all.
         note_and_explain_free_region(
             tcx,
             &mut err,

src/librustc_infer/infer/freshen.rs

@@ -251,7 +251,10 @@ impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
                 bug!("unexpected const {:?}", ct)
             }
 
-            ty::ConstKind::Param(_) | ty::ConstKind::Value(_) | ty::ConstKind::Unevaluated(..) => {}
+            ty::ConstKind::Param(_)
+            | ty::ConstKind::Value(_)
+            | ty::ConstKind::Unevaluated(..)
+            | ty::ConstKind::Error => {}
         }
 
         ct.super_fold_with(self)

src/librustc_infer/infer/resolve.rs

@@ -227,7 +227,7 @@ impl<'a, 'tcx> TypeFolder<'tcx> for FullTypeResolver<'a, 'tcx> {
             match c.val {
                 ty::ConstKind::Infer(InferConst::Var(vid)) => {
                     self.err = Some(FixupError::UnresolvedConst(vid));
-                    return self.tcx().consts.err;
+                    return self.tcx().mk_const(ty::Const { val: ty::ConstKind::Error, ty: c.ty });
                 }
                 ty::ConstKind::Infer(InferConst::Fresh(_)) => {
                     bug!("Unexpected const in full const resolver: {:?}", c);

src/librustc_middle/mir/interpret/error.rs

@@ -8,7 +8,7 @@ use crate::ty::{self, layout, Ty};
 
 use backtrace::Backtrace;
 use rustc_data_structures::sync::Lock;
-use rustc_errors::{struct_span_err, DiagnosticBuilder};
+use rustc_errors::{struct_span_err, DiagnosticBuilder, ErrorReported};
 use rustc_hir as hir;
 use rustc_hir::definitions::DefPathData;
 use rustc_macros::HashStable;
@@ -19,25 +19,16 @@ use std::{any::Any, fmt, mem};
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq, HashStable, RustcEncodable, RustcDecodable)]
 pub enum ErrorHandled {
-    /// Already reported a lint or an error for this evaluation.
-    Reported,
+    /// Already reported an error for this evaluation, and the compilation is
+    /// *guaranteed* to fail. Warnings/lints *must not* produce `Reported`.
+    Reported(ErrorReported),
+    /// Already emitted a lint for this evaluation.
+    Linted,
     /// Don't emit an error, the evaluation failed because the MIR was generic
     /// and the substs didn't fully monomorphize it.
     TooGeneric,
 }
 
-impl ErrorHandled {
-    pub fn assert_reported(self) {
-        match self {
-            ErrorHandled::Reported => {}
-            ErrorHandled::TooGeneric => bug!(
-                "MIR interpretation failed without reporting an error \
-                 even though it was fully monomorphized"
-            ),
-        }
-    }
-}
-
 CloneTypeFoldableImpls! {
     ErrorHandled,
 }
@@ -84,15 +75,12 @@ impl<'tcx> ConstEvalErr<'tcx> {
         tcx: TyCtxtAt<'tcx>,
         message: &str,
         emit: impl FnOnce(DiagnosticBuilder<'_>),
-    ) -> Result<(), ErrorHandled> {
+    ) -> ErrorHandled {
         self.struct_generic(tcx, message, emit, None)
     }
 
     pub fn report_as_error(&self, tcx: TyCtxtAt<'tcx>, message: &str) -> ErrorHandled {
-        match self.struct_error(tcx, message, |mut e| e.emit()) {
-            Ok(_) => ErrorHandled::Reported,
-            Err(x) => x,
-        }
+        self.struct_error(tcx, message, |mut e| e.emit())
     }
 
     pub fn report_as_lint(
@@ -102,7 +90,7 @@ impl<'tcx> ConstEvalErr<'tcx> {
         lint_root: hir::HirId,
         span: Option<Span>,
     ) -> ErrorHandled {
-        match self.struct_generic(
+        self.struct_generic(
             tcx,
             message,
             |mut lint: DiagnosticBuilder<'_>| {
@@ -122,10 +110,7 @@ impl<'tcx> ConstEvalErr<'tcx> {
                 lint.emit();
             },
             Some(lint_root),
-        ) {
-            Ok(_) => ErrorHandled::Reported,
-            Err(err) => err,
-        }
+        )
     }
 
     /// Create a diagnostic for this const eval error.
@@ -143,12 +128,14 @@ impl<'tcx> ConstEvalErr<'tcx> {
         message: &str,
         emit: impl FnOnce(DiagnosticBuilder<'_>),
         lint_root: Option<hir::HirId>,
-    ) -> Result<(), ErrorHandled> {
+    ) -> ErrorHandled {
         let must_error = match self.error {
             err_inval!(Layout(LayoutError::Unknown(_))) | err_inval!(TooGeneric) => {
-                return Err(ErrorHandled::TooGeneric);
+                return ErrorHandled::TooGeneric;
+            }
+            err_inval!(TypeckError(error_reported)) => {
+                return ErrorHandled::Reported(error_reported);
             }
-            err_inval!(TypeckError) => return Err(ErrorHandled::Reported),
             // We must *always* hard error on these, even if the caller wants just a lint.
             err_inval!(Layout(LayoutError::SizeOverflow(_))) => true,
             _ => false,
@@ -183,6 +170,7 @@ impl<'tcx> ConstEvalErr<'tcx> {
             // caller thinks anyway.
             // See <https://github.com/rust-lang/rust/pull/63152>.
             finish(struct_error(tcx, &err_msg), None);
+            ErrorHandled::Reported(ErrorReported)
         } else {
             // Regular case.
             if let Some(lint_root) = lint_root {
@@ -200,12 +188,13 @@ impl<'tcx> ConstEvalErr<'tcx> {
                     tcx.span,
                     |lint| finish(lint.build(message), Some(err_msg)),
                 );
+                ErrorHandled::Linted
             } else {
                 // Report as hard error.
                 finish(struct_error(tcx, message), Some(err_msg));
+                ErrorHandled::Reported(ErrorReported)
             }
         }
-        Ok(())
     }
 }
 
@@ -246,7 +235,9 @@ fn print_backtrace(backtrace: &mut Backtrace) {
 impl From<ErrorHandled> for InterpErrorInfo<'_> {
     fn from(err: ErrorHandled) -> Self {
         match err {
-            ErrorHandled::Reported => err_inval!(ReferencedConstant),
+            ErrorHandled::Reported(ErrorReported) | ErrorHandled::Linted => {
+                err_inval!(ReferencedConstant)
+            }
             ErrorHandled::TooGeneric => err_inval!(TooGeneric),
         }
         .into()
@@ -288,7 +279,7 @@ pub enum InvalidProgramInfo<'tcx> {
     /// which already produced an error.
     ReferencedConstant,
     /// Abort in case type errors are reached.
-    TypeckError,
+    TypeckError(ErrorReported),
     /// An error occurred during layout computation.
     Layout(layout::LayoutError<'tcx>),
     /// An invalid transmute happened.
@@ -301,7 +292,9 @@ impl fmt::Debug for InvalidProgramInfo<'_> {
         match self {
             TooGeneric => write!(f, "encountered overly generic constant"),
             ReferencedConstant => write!(f, "referenced constant has errors"),
-            TypeckError => write!(f, "encountered constants with type errors, stopping evaluation"),
+            TypeckError(ErrorReported) => {
+                write!(f, "encountered constants with type errors, stopping evaluation")
+            }
             Layout(ref err) => write!(f, "{}", err),
             TransmuteSizeDiff(from_ty, to_ty) => write!(
                 f,

src/librustc_middle/ty/context.rs

@@ -182,7 +182,7 @@ pub struct CommonLifetimes<'tcx> {
 }
 
 pub struct CommonConsts<'tcx> {
-    pub err: &'tcx Const<'tcx>,
+    pub unit: &'tcx Const<'tcx>,
 }
 
 pub struct LocalTableInContext<'a, V> {
@@ -410,8 +410,8 @@ pub struct TypeckTables<'tcx> {
     pub used_trait_imports: Lrc<DefIdSet>,
 
     /// If any errors occurred while type-checking this body,
-    /// this field will be set to `true`.
-    pub tainted_by_errors: bool,
+    /// this field will be set to `Some(ErrorReported)`.
+    pub tainted_by_errors: Option<ErrorReported>,
 
     /// All the opaque types that are restricted to concrete types
     /// by this function.
@@ -447,7 +447,7 @@ impl<'tcx> TypeckTables<'tcx> {
             fru_field_types: Default::default(),
             coercion_casts: Default::default(),
             used_trait_imports: Lrc::new(Default::default()),
-            tainted_by_errors: false,
+            tainted_by_errors: None,
             concrete_opaque_types: Default::default(),
             upvar_list: Default::default(),
             generator_interior_types: Default::default(),
@@ -858,9 +858,9 @@ impl<'tcx> CommonConsts<'tcx> {
         let mk_const = |c| interners.const_.intern(c, |c| Interned(interners.arena.alloc(c))).0;
 
         CommonConsts {
-            err: mk_const(ty::Const {
+            unit: mk_const(ty::Const {
                 val: ty::ConstKind::Value(ConstValue::Scalar(Scalar::zst())),
-                ty: types.err,
+                ty: types.unit,
             }),
         }
     }

src/librustc_middle/ty/flags.rs

@@ -70,14 +70,7 @@ impl FlagComputation {
             | &ty::Str
             | &ty::Foreign(..) => {}
 
-            // You might think that we could just return Error for
-            // any type containing Error as a component, and get
-            // rid of the TypeFlags::HAS_TY_ERR flag -- likewise for ty_bot (with
-            // the exception of function types that return bot).
-            // But doing so caused sporadic memory corruption, and
-            // neither I (tjc) nor nmatsakis could figure out why,
-            // so we're doing it this way.
-            &ty::Error => self.add_flags(TypeFlags::HAS_TY_ERR),
+            &ty::Error => self.add_flags(TypeFlags::HAS_ERROR),
 
             &ty::Param(_) => {
                 self.add_flags(TypeFlags::HAS_TY_PARAM);
@@ -239,6 +232,7 @@ impl FlagComputation {
                 self.add_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE);
             }
             ty::ConstKind::Value(_) => {}
+            ty::ConstKind::Error => self.add_flags(TypeFlags::HAS_ERROR),
         }
     }
 

src/librustc_middle/ty/fold.rs

@@ -82,7 +82,7 @@ pub trait TypeFoldable<'tcx>: fmt::Debug + Clone {
         self.has_type_flags(TypeFlags::HAS_TY_OPAQUE)
     }
     fn references_error(&self) -> bool {
-        self.has_type_flags(TypeFlags::HAS_TY_ERR)
+        self.has_type_flags(TypeFlags::HAS_ERROR)
     }
     fn has_param_types_or_consts(&self) -> bool {
         self.has_type_flags(TypeFlags::HAS_TY_PARAM | TypeFlags::HAS_CT_PARAM)

src/librustc_middle/ty/mod.rs

@@ -27,6 +27,7 @@ use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::sorted_map::SortedIndexMultiMap;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::sync::{self, par_iter, ParallelIterator};
+use rustc_errors::ErrorReported;
 use rustc_hir as hir;
 use rustc_hir::def::{CtorKind, CtorOf, DefKind, Namespace, Res};
 use rustc_hir::def_id::{CrateNum, DefId, DefIdMap, LocalDefId, CRATE_DEF_INDEX};
@@ -567,8 +568,8 @@ bitflags! {
                                           | TypeFlags::HAS_TY_OPAQUE.bits
                                           | TypeFlags::HAS_CT_PROJECTION.bits;
 
-        /// Is an error type reachable?
-        const HAS_TY_ERR                  = 1 << 13;
+        /// Is an error type/const reachable?
+        const HAS_ERROR                   = 1 << 13;
 
         /// Does this have any region that "appears free" in the type?
         /// Basically anything but [ReLateBound] and [ReErased].
@@ -2388,7 +2389,7 @@ impl<'tcx> AdtDef {
                     None
                 }
             }
-            Err(ErrorHandled::Reported) => {
+            Err(ErrorHandled::Reported(ErrorReported) | ErrorHandled::Linted) => {
                 if !expr_did.is_local() {
                     span_bug!(
                         tcx.def_span(expr_did),

src/librustc_middle/ty/print/pretty.rs

@@ -938,6 +938,7 @@ pub trait PrettyPrinter<'tcx>:
                 self.pretty_print_bound_var(debruijn, bound_var)?
             }
             ty::ConstKind::Placeholder(placeholder) => p!(write("Placeholder({:?})", placeholder)),
+            ty::ConstKind::Error => p!(write("[const error]")),
         };
         Ok(self)
     }

src/librustc_middle/ty/relate.rs

@@ -510,12 +510,21 @@ pub fn super_relate_consts<R: TypeRelation<'tcx>>(
     let tcx = relation.tcx();
 
     let eagerly_eval = |x: &'tcx ty::Const<'tcx>| {
+        // FIXME(eddyb) this doesn't account for lifetime inference variables
+        // being erased by `eval`, *nor* for the polymorphic aspect of `eval`.
+        // That is, we could always use `eval` and it will just return the
+        // old value back if it doesn't succeed.
         if !x.val.needs_infer() {
             return x.eval(tcx, relation.param_env()).val;
         }
         x.val
     };
 
+    // FIXME(eddyb) doesn't look like everything below checks that `a.ty == b.ty`.
+    // We could probably always assert it early, as `const` generic parameters
+    // are not allowed to depend on other generic parameters, i.e. are concrete.
+    // (although there could be normalization differences)
+
     // Currently, the values that can be unified are primitive types,
     // and those that derive both `PartialEq` and `Eq`, corresponding
     // to structural-match types.
@@ -524,6 +533,9 @@ pub fn super_relate_consts<R: TypeRelation<'tcx>>(
             // The caller should handle these cases!
             bug!("var types encountered in super_relate_consts: {:?} {:?}", a, b)
         }
+
+        (ty::ConstKind::Error, _) | (_, ty::ConstKind::Error) => Ok(ty::ConstKind::Error),
+
         (ty::ConstKind::Param(a_p), ty::ConstKind::Param(b_p)) if a_p.index == b_p.index => {
             return Ok(a);
         }

src/librustc_middle/ty/structural_impls.rs

@@ -1022,9 +1022,10 @@ impl<'tcx> TypeFoldable<'tcx> for ty::ConstKind<'tcx> {
             ty::ConstKind::Unevaluated(did, substs, promoted) => {
                 ty::ConstKind::Unevaluated(did, substs.fold_with(folder), promoted)
             }
-            ty::ConstKind::Value(_) | ty::ConstKind::Bound(..) | ty::ConstKind::Placeholder(..) => {
-                *self
-            }
+            ty::ConstKind::Value(_)
+            | ty::ConstKind::Bound(..)
+            | ty::ConstKind::Placeholder(..)
+            | ty::ConstKind::Error => *self,
         }
     }
 
@@ -1033,9 +1034,10 @@ impl<'tcx> TypeFoldable<'tcx> for ty::ConstKind<'tcx> {
             ty::ConstKind::Infer(ic) => ic.visit_with(visitor),
             ty::ConstKind::Param(p) => p.visit_with(visitor),
             ty::ConstKind::Unevaluated(_, substs, _) => substs.visit_with(visitor),
-            ty::ConstKind::Value(_) | ty::ConstKind::Bound(..) | ty::ConstKind::Placeholder(_) => {
-                false
-            }
+            ty::ConstKind::Value(_)
+            | ty::ConstKind::Bound(..)
+            | ty::ConstKind::Placeholder(_)
+            | ty::ConstKind::Error => false,
         }
     }
 }