Make subtyping explicit in MIR

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -2828,6 +2828,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                         }
                         ProjectionElem::ConstantIndex { .. }
                         | ProjectionElem::Subslice { .. }
+                        | ProjectionElem::Subtype(_)
                         | ProjectionElem::Index(_) => kind,
                     },
                     place_ty.projection_ty(tcx, elem),

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -242,6 +242,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 ProjectionElem::Downcast(..) if opt.including_downcast => return None,
                 ProjectionElem::Downcast(..) => (),
                 ProjectionElem::OpaqueCast(..) => (),
+                ProjectionElem::Subtype(..) => (),
                 ProjectionElem::Field(field, _ty) => {
                     // FIXME(project-rfc_2229#36): print capture precisely here.
                     if let Some(field) = self.is_upvar_field_projection(PlaceRef {
@@ -322,7 +323,9 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                     PlaceRef { local, projection: proj_base }.ty(self.body, self.infcx.tcx)
                 }
                 ProjectionElem::Downcast(..) => place.ty(self.body, self.infcx.tcx),
-                ProjectionElem::OpaqueCast(ty) => PlaceTy::from_ty(*ty),
+                ProjectionElem::Subtype(ty) | ProjectionElem::OpaqueCast(ty) => {
+                    PlaceTy::from_ty(*ty)
+                }
                 ProjectionElem::Field(_, field_type) => PlaceTy::from_ty(*field_type),
             },
         };

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -159,6 +159,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                     [
                         ..,
                         ProjectionElem::Index(_)
+                        | ProjectionElem::Subtype(_)
                         | ProjectionElem::ConstantIndex { .. }
                         | ProjectionElem::OpaqueCast { .. }
                         | ProjectionElem::Subslice { .. }

compiler/rustc_borrowck/src/lib.rs

@@ -1803,6 +1803,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         for (place_base, elem) in place.iter_projections().rev() {
             match elem {
                 ProjectionElem::Index(_/*operand*/) |
+                ProjectionElem::Subtype(_) |
                 ProjectionElem::OpaqueCast(_) |
                 ProjectionElem::ConstantIndex { .. } |
                 // assigning to P[i] requires P to be valid.
@@ -2191,6 +2192,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                     | ProjectionElem::Index(..)
                     | ProjectionElem::ConstantIndex { .. }
                     | ProjectionElem::Subslice { .. }
+                    | ProjectionElem::Subtype(..)
                     | ProjectionElem::OpaqueCast { .. }
                     | ProjectionElem::Downcast(..) => {
                         let upvar_field_projection = self.is_upvar_field_projection(place);

compiler/rustc_borrowck/src/places_conflict.rs

@@ -249,6 +249,7 @@ fn place_components_conflict<'tcx>(
                 | (ProjectionElem::ConstantIndex { .. }, _, _)
                 | (ProjectionElem::Subslice { .. }, _, _)
                 | (ProjectionElem::OpaqueCast { .. }, _, _)
+                | (ProjectionElem::Subtype(_), _, _)
                 | (ProjectionElem::Downcast { .. }, _, _) => {
                     // Recursive case. This can still be disjoint on a
                     // further iteration if this a shallow access and
@@ -508,6 +509,7 @@ fn place_projection_conflict<'tcx>(
             | ProjectionElem::Field(..)
             | ProjectionElem::Index(..)
             | ProjectionElem::ConstantIndex { .. }
+            | ProjectionElem::Subtype(_)
             | ProjectionElem::OpaqueCast { .. }
             | ProjectionElem::Subslice { .. }
             | ProjectionElem::Downcast(..),

compiler/rustc_borrowck/src/prefixes.rs

@@ -89,6 +89,9 @@ impl<'cx, 'tcx> Iterator for Prefixes<'cx, 'tcx> {
                             cursor = cursor_base;
                             continue 'cursor;
                         }
+                        ProjectionElem::Subtype(..) => {
+                            panic!("Subtype projection is not allowed before borrow check")
+                        }
                         ProjectionElem::Deref => {
                             // (handled below)
                         }

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -716,6 +716,9 @@ impl<'a, 'b, 'tcx> TypeVerifier<'a, 'b, 'tcx> {
                 }
                 PlaceTy::from_ty(fty)
             }
+            ProjectionElem::Subtype(_) => {
+                bug!("ProjectionElem::Subtype shouldn't exist in borrowck")
+            }
             ProjectionElem::OpaqueCast(ty) => {
                 let ty = self.sanitize_type(place, ty);
                 let ty = self.cx.normalize(ty, location);
@@ -2563,6 +2566,9 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                 | ProjectionElem::Subslice { .. } => {
                     // other field access
                 }
+                ProjectionElem::Subtype(_) => {
+                    bug!("ProjectionElem::Subtype shouldn't exist in borrowck")
+                }
             }
         }
     }

compiler/rustc_codegen_cranelift/src/base.rs

@@ -876,6 +876,7 @@ pub(crate) fn codegen_place<'tcx>(
                 cplace = cplace.place_deref(fx);
             }
             PlaceElem::OpaqueCast(ty) => bug!("encountered OpaqueCast({ty}) in codegen"),
+            PlaceElem::Subtype(ty) => cplace = cplace.place_transmute_type(fx, fx.monomorphize(ty)),
             PlaceElem::Field(field, _ty) => {
                 cplace = cplace.place_field(fx, field);
             }

compiler/rustc_codegen_cranelift/src/value_and_place.rs

@@ -674,6 +674,16 @@ impl<'tcx> CPlace<'tcx> {
         }
     }
 
+    /// Used for `ProjectionElem::Subtype`, `ty` has to be monomorphized before
+    /// passed on.
+    pub(crate) fn place_transmute_type(
+        self,
+        fx: &mut FunctionCx<'_, '_, 'tcx>,
+        ty: Ty<'tcx>,
+    ) -> CPlace<'tcx> {
+        CPlace { inner: self.inner, layout: fx.layout_of(ty) }
+    }
+
     pub(crate) fn place_field(
         self,
         fx: &mut FunctionCx<'_, '_, 'tcx>,

compiler/rustc_codegen_ssa/src/mir/place.rs

@@ -466,6 +466,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 mir::ProjectionElem::OpaqueCast(ty) => {
                     bug!("encountered OpaqueCast({ty}) in codegen")
                 }
+                mir::ProjectionElem::Subtype(ty) => cg_base.project_type(bx, self.monomorphize(ty)),
                 mir::ProjectionElem::Index(index) => {
                     let index = &mir::Operand::Copy(mir::Place::from(index));
                     let index = self.codegen_operand(bx, index);

compiler/rustc_const_eval/src/interpret/eval_context.rs

@@ -13,7 +13,7 @@ use rustc_middle::ty::layout::{
     self, FnAbiError, FnAbiOfHelpers, FnAbiRequest, LayoutError, LayoutOf, LayoutOfHelpers,
     TyAndLayout,
 };
-use rustc_middle::ty::{self, GenericArgsRef, ParamEnv, Ty, TyCtxt, TypeFoldable};
+use rustc_middle::ty::{self, GenericArgsRef, ParamEnv, Ty, TyCtxt, TypeFoldable, Variance};
 use rustc_mir_dataflow::storage::always_storage_live_locals;
 use rustc_session::Limit;
 use rustc_span::Span;
@@ -384,7 +384,7 @@ pub(super) fn mir_assign_valid_types<'tcx>(
     // all normal lifetimes are erased, higher-ranked types with their
     // late-bound lifetimes are still around and can lead to type
     // differences.
-    if util::is_subtype(tcx, param_env, src.ty, dest.ty) {
+    if util::relate_types(tcx, param_env, Variance::Covariant, 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

compiler/rustc_const_eval/src/interpret/projection.rs

@@ -319,6 +319,8 @@ where
             OpaqueCast(ty) => {
                 span_bug!(self.cur_span(), "OpaqueCast({ty}) encountered after borrowck")
             }
+            // We don't want anything happening here, this is here as a dummy.
+            Subtype(_) => base.transmute(base.layout(), self)?,
             Field(field, _) => self.project_field(base, field.index())?,
             Downcast(_, variant) => self.project_downcast(base, variant)?,
             Deref => self.deref_pointer(&base.to_op(self)?)?.into(),

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -664,6 +664,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
             | ProjectionElem::Downcast(..)
             | ProjectionElem::OpaqueCast(..)
             | ProjectionElem::Subslice { .. }
+            | ProjectionElem::Subtype(..)
             | ProjectionElem::Field(..)
             | ProjectionElem::Index(_) => {}
         }

compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs

@@ -306,6 +306,7 @@ where
             ProjectionElem::Index(index) if in_local(index) => return true,
 
             ProjectionElem::Deref
+            | ProjectionElem::Subtype(_)
             | ProjectionElem::Field(_, _)
             | ProjectionElem::OpaqueCast(_)
             | ProjectionElem::ConstantIndex { .. }

compiler/rustc_const_eval/src/transform/promote_consts.rs

@@ -357,7 +357,9 @@ impl<'tcx> Validator<'_, 'tcx> {
                         return Err(Unpromotable);
                     }
 
-                    ProjectionElem::ConstantIndex { .. } | ProjectionElem::Subslice { .. } => {}
+                    ProjectionElem::ConstantIndex { .. }
+                    | ProjectionElem::Subtype(_)
+                    | ProjectionElem::Subslice { .. } => {}
 
                     ProjectionElem::Index(local) => {
                         let mut promotable = false;

compiler/rustc_const_eval/src/transform/validate.rs

@@ -7,7 +7,7 @@ use rustc_infer::traits::Reveal;
 use rustc_middle::mir::interpret::Scalar;
 use rustc_middle::mir::visit::{NonUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::*;
-use rustc_middle::ty::{self, InstanceDef, ParamEnv, Ty, TyCtxt, TypeVisitableExt};
+use rustc_middle::ty::{self, InstanceDef, ParamEnv, Ty, TyCtxt, TypeVisitableExt, Variance};
 use rustc_mir_dataflow::impls::MaybeStorageLive;
 use rustc_mir_dataflow::storage::always_storage_live_locals;
 use rustc_mir_dataflow::{Analysis, ResultsCursor};
@@ -16,6 +16,8 @@ use rustc_target::spec::abi::Abi;
 
 use crate::util::is_within_packed;
 
+use crate::util::relate_types;
+
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 enum EdgeKind {
     Unwind,
@@ -602,7 +604,15 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             return true;
         }
 
-        crate::util::is_subtype(self.tcx, self.param_env, src, dest)
+        // After borrowck subtyping should be fully explicit via
+        // `Subtype` projections.
+        let variance = if self.mir_phase >= MirPhase::Runtime(RuntimePhase::Initial) {
+            Variance::Invariant
+        } else {
+            Variance::Covariant
+        };
+
+        crate::util::relate_types(self.tcx, self.param_env, variance, src, dest)
     }
 }
 
@@ -753,6 +763,23 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                     }
                 }
             }
+            ProjectionElem::Subtype(ty) => {
+                if !relate_types(
+                    self.tcx,
+                    self.param_env,
+                    Variance::Covariant,
+                    ty,
+                    place_ref.ty(&self.body.local_decls, self.tcx).ty,
+                ) {
+                    self.fail(
+                        location,
+                        format!(
+                            "Failed subtyping {ty:#?} and {:#?}",
+                            place_ref.ty(&self.body.local_decls, self.tcx).ty
+                        ),
+                    )
+                }
+            }
             _ => {}
         }
         self.super_projection_elem(place_ref, elem, context, location);
@@ -1088,6 +1115,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                 // LHS and RHS of the assignment must have the same type.
                 let left_ty = dest.ty(&self.body.local_decls, self.tcx).ty;
                 let right_ty = rvalue.ty(&self.body.local_decls, self.tcx);
+
                 if !self.mir_assign_valid_types(right_ty, left_ty) {
                     self.fail(
                         location,

compiler/rustc_const_eval/src/util/compare_types.rs

@@ -5,7 +5,7 @@
 
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::traits::{DefiningAnchor, ObligationCause};
-use rustc_middle::ty::{ParamEnv, Ty, TyCtxt};
+use rustc_middle::ty::{ParamEnv, Ty, TyCtxt, Variance};
 use rustc_trait_selection::traits::ObligationCtxt;
 
 /// Returns whether the two types are equal up to subtyping.
@@ -24,16 +24,22 @@ pub fn is_equal_up_to_subtyping<'tcx>(
     }
 
     // Check for subtyping in either direction.
-    is_subtype(tcx, param_env, src, dest) || is_subtype(tcx, param_env, dest, src)
+    relate_types(tcx, param_env, Variance::Covariant, src, dest)
+        || relate_types(tcx, param_env, Variance::Covariant, dest, src)
 }
 
 /// Returns whether `src` is a subtype of `dest`, i.e. `src <: dest`.
 ///
+/// When validating assignments, the variance should be `Covariant`. When checking
+/// during `MirPhase` >= `MirPhase::Runtime(RuntimePhase::Initial)` variance should be `Invariant`
+/// because we want to check for type equality.
+///
 /// 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>(
+pub fn relate_types<'tcx>(
     tcx: TyCtxt<'tcx>,
     param_env: ParamEnv<'tcx>,
+    variance: Variance,
     src: Ty<'tcx>,
     dest: Ty<'tcx>,
 ) -> bool {
@@ -48,7 +54,7 @@ pub fn is_subtype<'tcx>(
     let cause = ObligationCause::dummy();
     let src = ocx.normalize(&cause, param_env, src);
     let dest = ocx.normalize(&cause, param_env, dest);
-    match ocx.sub(&cause, param_env, src, dest) {
+    match ocx.relate(&cause, param_env, variance, src, dest) {
         Ok(()) => {}
         Err(_) => return false,
     };

compiler/rustc_const_eval/src/util/mod.rs

@@ -7,7 +7,7 @@ mod type_name;
 
 pub use self::alignment::{is_disaligned, is_within_packed};
 pub use self::check_validity_requirement::check_validity_requirement;
-pub use self::compare_types::{is_equal_up_to_subtyping, is_subtype};
+pub use self::compare_types::{is_equal_up_to_subtyping, relate_types};
 pub use self::type_name::type_name;
 
 /// Classify whether an operator is "left-homogeneous", i.e., the LHS has the

compiler/rustc_middle/src/mir/pretty.rs

@@ -1103,6 +1103,7 @@ fn pre_fmt_projection(projection: &[PlaceElem<'_>], fmt: &mut Formatter<'_>) ->
     for &elem in projection.iter().rev() {
         match elem {
             ProjectionElem::OpaqueCast(_)
+            | ProjectionElem::Subtype(_)
             | ProjectionElem::Downcast(_, _)
             | ProjectionElem::Field(_, _) => {
                 write!(fmt, "(").unwrap();
@@ -1125,6 +1126,9 @@ fn post_fmt_projection(projection: &[PlaceElem<'_>], fmt: &mut Formatter<'_>) ->
             ProjectionElem::OpaqueCast(ty) => {
                 write!(fmt, " as {ty})")?;
             }
+            ProjectionElem::Subtype(ty) => {
+                write!(fmt, " as subtype {ty})")?;
+            }
             ProjectionElem::Downcast(Some(name), _index) => {
                 write!(fmt, " as {name})")?;
             }

compiler/rustc_middle/src/mir/statement.rs

@@ -57,6 +57,7 @@ impl<V, T> ProjectionElem<V, T> {
             Self::Field(_, _)
             | Self::Index(_)
             | Self::OpaqueCast(_)
+            | Self::Subtype(_)
             | Self::ConstantIndex { .. }
             | Self::Subslice { .. }
             | Self::Downcast(_, _) => false,
@@ -70,6 +71,7 @@ impl<V, T> ProjectionElem<V, T> {
             Self::Deref | Self::Index(_) => false,
             Self::Field(_, _)
             | Self::OpaqueCast(_)
+            | Self::Subtype(_)
             | Self::ConstantIndex { .. }
             | Self::Subslice { .. }
             | Self::Downcast(_, _) => true,
@@ -95,6 +97,7 @@ impl<V, T> ProjectionElem<V, T> {
             | Self::Field(_, _) => true,
             Self::ConstantIndex { from_end: true, .. }
             | Self::Index(_)
+            | Self::Subtype(_)
             | Self::OpaqueCast(_)
             | Self::Subslice { .. } => false,
         }

compiler/rustc_middle/src/mir/syntax.rs

@@ -1075,6 +1075,18 @@ pub enum ProjectionElem<V, T> {
     /// Like an explicit cast from an opaque type to a concrete type, but without
     /// requiring an intermediate variable.
     OpaqueCast(T),
+
+    /// A `Subtype(T)` projection is applied to any `StatementKind::Assign` where
+    /// type of lvalue doesn't match the type of rvalue, the primary goal is making subtyping
+    /// explicit during optimizations and codegen.
+    ///
+    /// This projection doesn't impact the runtime behavior of the program except for potentially changing
+    /// some type metadata of the interpreter or codegen backend.
+    ///
+    /// This goal is achieved with mir_transform pass `Subtyper`, which runs right after
+    /// borrowchecker, as we only care about subtyping that can affect trait selection and
+    /// `TypeId`.
+    Subtype(T),
 }
 
 /// Alias for projections as they appear in places, where the base is a place

compiler/rustc_middle/src/mir/tcx.rs

@@ -69,7 +69,7 @@ impl<'tcx> PlaceTy<'tcx> {
         param_env: ty::ParamEnv<'tcx>,
         elem: &ProjectionElem<V, T>,
         mut handle_field: impl FnMut(&Self, FieldIdx, T) -> Ty<'tcx>,
-        mut handle_opaque_cast: impl FnMut(&Self, T) -> Ty<'tcx>,
+        mut handle_opaque_cast_and_subtype: impl FnMut(&Self, T) -> Ty<'tcx>,
     ) -> PlaceTy<'tcx>
     where
         V: ::std::fmt::Debug,
@@ -110,7 +110,12 @@ impl<'tcx> PlaceTy<'tcx> {
                 PlaceTy { ty: self.ty, variant_index: Some(index) }
             }
             ProjectionElem::Field(f, fty) => PlaceTy::from_ty(handle_field(&self, f, fty)),
-            ProjectionElem::OpaqueCast(ty) => PlaceTy::from_ty(handle_opaque_cast(&self, ty)),
+            ProjectionElem::OpaqueCast(ty) => {
+                PlaceTy::from_ty(handle_opaque_cast_and_subtype(&self, ty))
+            }
+            ProjectionElem::Subtype(ty) => {
+                PlaceTy::from_ty(handle_opaque_cast_and_subtype(&self, ty))
+            }
         };
         debug!("projection_ty self: {:?} elem: {:?} yields: {:?}", self, elem, answer);
         answer