Auto merge of #115025 - ouz-a:ouz_testing, r=lcnr

Make subtyping explicit in MIR

This adds new mir-opt that pushes new `ProjectionElem` called `ProjectionElem::Subtype(T)` to `Rvalue` of a subtyped assignment so we can unsoundness issues like https://github.com/rust-lang/rust/issues/107205

Addresses https://github.com/rust-lang/rust/issues/112651

r? `@lcnr`

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -2828,6 +2828,7 @@ fn classify_drop_access_kind(&self, place: PlaceRef<'tcx>) -> StorageDeadOrDrop<
                         }
                         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 @@ pub(super) fn describe_place_with_options(
                 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 @@ fn describe_field(
                     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 @@ pub(crate) fn report_mutability_error(
                     [
                         ..,
                         ProjectionElem::Index(_)
+                        | ProjectionElem::Subtype(_)
                         | ProjectionElem::ConstantIndex { .. }
                         | ProjectionElem::OpaqueCast { .. }
                         | ProjectionElem::Subslice { .. }

compiler/rustc_borrowck/src/lib.rs

@@ -1803,6 +1803,7 @@ fn check_if_assigned_path_is_moved(
         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 @@ fn is_mutable(
                     | 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 @@ fn next(&mut self) -> Option<Self::Item> {
                             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 @@ fn sanitize_projection(
                 }
                 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 @@ fn add_reborrow_constraint(
                 | 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 @@ fn transmute_scalar<'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 @@ pub fn codegen_place(
                 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 @@
     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 @@ pub fn project<P>(&self, base: &P, proj_elem: mir::PlaceElem<'tcx>) -> InterpRes
             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 @@ fn visit_projection_elem(
             | 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 @@ pub fn in_place<'tcx, Q, F>(cx: &ConstCx<'_, 'tcx>, in_local: &mut F, place: Pla
             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 @@ fn validate_place(&mut self, place: PlaceRef<'tcx>) -> Result<(), Unpromotable>
                         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_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 crate::util::is_within_packed;
 
+use crate::util::relate_types;
+
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 enum EdgeKind {
     Unwind,
@@ -602,7 +604,15 @@ fn mir_assign_valid_types(&self, src: Ty<'tcx>, dest: Ty<'tcx>) -> bool {
             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 @@ fn visit_projection_elem(
                     }
                 }
             }
+            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 @@ fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
                 // 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 @@
 
 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 @@ fn is_indirect(&self) -> bool {
             Self::Field(_, _)
             | Self::Index(_)
             | Self::OpaqueCast(_)
+            | Self::Subtype(_)
             | Self::ConstantIndex { .. }
             | Self::Subslice { .. }
             | Self::Downcast(_, _) => false,
@@ -70,6 +71,7 @@ pub fn is_stable_offset(&self) -> bool {
             Self::Deref | Self::Index(_) => false,
             Self::Field(_, _)
             | Self::OpaqueCast(_)
+            | Self::Subtype(_)
             | Self::ConstantIndex { .. }
             | Self::Subslice { .. }
             | Self::Downcast(_, _) => true,
@@ -95,6 +97,7 @@ pub fn can_use_in_debuginfo(&self) -> bool {
             | 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 @@ pub fn projection_ty_core<V, T>(
         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 @@ pub fn projection_ty_core<V, T>(
                 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

compiler/rustc_middle/src/mir/visit.rs

@@ -1109,6 +1109,11 @@ fn process_projection_elem(
                     self.visit_ty(&mut new_ty, TyContext::Location(location));
                     if ty != new_ty { Some(PlaceElem::OpaqueCast(new_ty)) } else { None }
                 }
+                PlaceElem::Subtype(ty) => {
+                    let mut new_ty = ty;
+                    self.visit_ty(&mut new_ty, TyContext::Location(location));
+                    if ty != new_ty { Some(PlaceElem::Subtype(new_ty)) } else { None }
+                }
                 PlaceElem::Deref
                 | PlaceElem::ConstantIndex { .. }
                 | PlaceElem::Subslice { .. }
@@ -1175,7 +1180,9 @@ fn super_projection_elem(
             location: Location,
         ) {
             match elem {
-                ProjectionElem::OpaqueCast(ty) | ProjectionElem::Field(_, ty) => {
+                ProjectionElem::OpaqueCast(ty)
+                | ProjectionElem::Subtype(ty)
+                | ProjectionElem::Field(_, ty) => {
                     self.visit_ty(ty, TyContext::Location(location));
                 }
                 ProjectionElem::Index(local) => {

compiler/rustc_mir_build/src/build/expr/as_place.rs

@@ -102,7 +102,7 @@ fn convert_to_hir_projections_and_truncate_for_capture(
                 continue;
             }
             // These do not affect anything, they just make sure we know the right type.
-            ProjectionElem::OpaqueCast(_) => continue,
+            ProjectionElem::OpaqueCast(_) | ProjectionElem::Subtype(..) => continue,
             ProjectionElem::Index(..)
             | ProjectionElem::ConstantIndex { .. }
             | ProjectionElem::Subslice { .. } => {
@@ -709,6 +709,7 @@ fn add_fake_borrows_of_base(
                     ProjectionElem::Field(..)
                     | ProjectionElem::Downcast(..)
                     | ProjectionElem::OpaqueCast(..)
+                    | ProjectionElem::Subtype(..)
                     | ProjectionElem::ConstantIndex { .. }
                     | ProjectionElem::Subslice { .. } => (),
                 }

compiler/rustc_mir_dataflow/src/move_paths/abs_domain.rs

@@ -57,6 +57,7 @@ fn lift(&self) -> Self::Abstract {
                 ProjectionElem::ConstantIndex { offset, min_length, from_end }
             }
             ProjectionElem::Downcast(a, u) => ProjectionElem::Downcast(a, u),
+            ProjectionElem::Subtype(ty) => ProjectionElem::Subtype(ty.lift()),
         }
     }
 }

compiler/rustc_mir_dataflow/src/move_paths/builder.rs

@@ -227,10 +227,13 @@ fn move_path_for(&mut self, place: Place<'tcx>) -> Result<MovePathIndex, MoveErr
                     }
                     _ => bug!("Unexpected type {place_ty:#?}"),
                 },
-                // `OpaqueCast` only transmutes the type, so no moves there and
-                // `Downcast` only changes information about a `Place` without moving
+                // `OpaqueCast`:Only transmutes the type, so no moves there.
+                // `Downcast`  :Only changes information about a `Place` without moving.
+                // `Subtype`   :Only transmutes the type, so moves.
                 // So it's safe to skip these.
-                ProjectionElem::OpaqueCast(_) | ProjectionElem::Downcast(_, _) => (),
+                ProjectionElem::OpaqueCast(_)
+                | ProjectionElem::Subtype(_)
+                | ProjectionElem::Downcast(_, _) => (),
             }
             if union_path.is_none() {
                 // inlined from add_move_path because of a borrowck conflict with the iterator

compiler/rustc_mir_transform/src/add_subtyping_projections.rs

@@ -0,0 +1,66 @@
+use crate::MirPass;
+use rustc_index::IndexVec;
+use rustc_middle::mir::patch::MirPatch;
+use rustc_middle::mir::visit::MutVisitor;
+use rustc_middle::mir::*;
+use rustc_middle::ty::TyCtxt;
+
+pub struct Subtyper;
+
+pub struct SubTypeChecker<'a, 'tcx> {
+    tcx: TyCtxt<'tcx>,
+    patcher: MirPatch<'tcx>,
+    local_decls: &'a IndexVec<Local, LocalDecl<'tcx>>,
+}
+
+impl<'a, 'tcx> MutVisitor<'tcx> for SubTypeChecker<'a, 'tcx> {
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn visit_assign(
+        &mut self,
+        place: &mut Place<'tcx>,
+        rvalue: &mut Rvalue<'tcx>,
+        location: Location,
+    ) {
+        let mut place_ty = place.ty(self.local_decls, self.tcx).ty;
+        let mut rval_ty = rvalue.ty(self.local_decls, self.tcx);
+        // Not erasing this causes `Free Regions` errors in validator,
+        // when rval is `ReStatic`.
+        rval_ty = self.tcx.erase_regions_ty(rval_ty);
+        place_ty = self.tcx.erase_regions(place_ty);
+        if place_ty != rval_ty {
+            let temp = self
+                .patcher
+                .new_temp(rval_ty, self.local_decls[place.as_ref().local].source_info.span);
+            let new_place = Place::from(temp);
+            self.patcher.add_assign(location, new_place, rvalue.clone());
+            let subtyped = new_place.project_deeper(&[ProjectionElem::Subtype(place_ty)], self.tcx);
+            *rvalue = Rvalue::Use(Operand::Move(subtyped));
+        }
+    }
+}
+
+// Aim here is to do this kind of transformation:
+//
+// let place: place_ty = rval;
+// // gets transformed to
+// let temp: rval_ty = rval;
+// let place: place_ty = temp as place_ty;
+//
+pub fn subtype_finder<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+    let patch = MirPatch::new(body);
+    let mut checker = SubTypeChecker { tcx, patcher: patch, local_decls: &body.local_decls };
+
+    for (bb, data) in body.basic_blocks.as_mut_preserves_cfg().iter_enumerated_mut() {
+        checker.visit_basic_block_data(bb, data);
+    }
+    checker.patcher.apply(body);
+}
+
+impl<'tcx> MirPass<'tcx> for Subtyper {
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        subtype_finder(tcx, body);
+    }
+}

compiler/rustc_mir_transform/src/gvn.rs

@@ -306,6 +306,7 @@ fn simplify_place_value(
                 }
                 ProjectionElem::Downcast(name, index) => ProjectionElem::Downcast(name, index),
                 ProjectionElem::OpaqueCast(ty) => ProjectionElem::OpaqueCast(ty),
+                ProjectionElem::Subtype(ty) => ProjectionElem::Subtype(ty),
             };
             value = self.insert(Value::Projection(value, proj));
         }

compiler/rustc_mir_transform/src/inline.rs

@@ -218,7 +218,13 @@ fn try_inlining(
         // Normally, this shouldn't be required, but trait normalization failure can create a
         // validation ICE.
         let output_type = callee_body.return_ty();
-        if !util::is_subtype(self.tcx, self.param_env, output_type, destination_ty) {
+        if !util::relate_types(
+            self.tcx,
+            self.param_env,
+            ty::Variance::Covariant,
+            output_type,
+            destination_ty,
+        ) {
             trace!(?output_type, ?destination_ty);
             return Err("failed to normalize return type");
         }
@@ -248,7 +254,13 @@ fn try_inlining(
                 self_arg_ty.into_iter().chain(arg_tuple_tys).zip(callee_body.args_iter())
             {
                 let input_type = callee_body.local_decls[input].ty;
-                if !util::is_subtype(self.tcx, self.param_env, input_type, arg_ty) {
+                if !util::relate_types(
+                    self.tcx,
+                    self.param_env,
+                    ty::Variance::Covariant,
+                    input_type,
+                    arg_ty,
+                ) {
                     trace!(?arg_ty, ?input_type);
                     return Err("failed to normalize tuple argument type");
                 }
@@ -257,7 +269,13 @@ fn try_inlining(
             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 !util::is_subtype(self.tcx, self.param_env, input_type, arg_ty) {
+                if !util::relate_types(
+                    self.tcx,
+                    self.param_env,
+                    ty::Variance::Covariant,
+                    input_type,
+                    arg_ty,
+                ) {
                     trace!(?arg_ty, ?input_type);
                     return Err("failed to normalize argument type");
                 }

compiler/rustc_mir_transform/src/lib.rs

@@ -54,6 +54,7 @@
 pub mod check_unsafety;
 mod remove_place_mention;
 // This pass is public to allow external drivers to perform MIR cleanup
+mod add_subtyping_projections;
 pub mod cleanup_post_borrowck;
 mod const_debuginfo;
 mod const_goto;
@@ -466,6 +467,7 @@ pub fn run_analysis_to_runtime_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'
 /// After this series of passes, no lifetime analysis based on borrowing can be done.
 fn run_analysis_cleanup_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
     let passes: &[&dyn MirPass<'tcx>] = &[
+        &add_subtyping_projections::Subtyper,
         &cleanup_post_borrowck::CleanupPostBorrowck,
         &remove_noop_landing_pads::RemoveNoopLandingPads,
         &simplify::SimplifyCfg::EarlyOpt,

compiler/rustc_trait_selection/src/traits/engine.rs

@@ -23,6 +23,7 @@
 use rustc_middle::ty::error::TypeError;
 use rustc_middle::ty::ToPredicate;
 use rustc_middle::ty::TypeFoldable;
+use rustc_middle::ty::Variance;
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_session::config::TraitSolver;
 
@@ -156,6 +157,20 @@ pub fn sub<T: ToTrace<'tcx>>(
             .map(|infer_ok| self.register_infer_ok_obligations(infer_ok))
     }
 
+    pub fn relate<T: ToTrace<'tcx>>(
+        &self,
+        cause: &ObligationCause<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        variance: Variance,
+        expected: T,
+        actual: T,
+    ) -> Result<(), TypeError<'tcx>> {
+        self.infcx
+            .at(cause, param_env)
+            .relate(DefineOpaqueTypes::Yes, expected, variance, 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,

src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs

@@ -272,6 +272,7 @@ fn check_place<'tcx>(tcx: TyCtxt<'tcx>, place: Place<'tcx>, span: Span, body: &B
             | ProjectionElem::Downcast(..)
             | ProjectionElem::Subslice { .. }
             | ProjectionElem::Deref
+            | ProjectionElem::Subtype(_)
             | ProjectionElem::Index(_) => {},
         }
     }

tests/mir-opt/inline/inline_generator.main.Inline.panic-abort.diff

@@ -7,11 +7,12 @@
       let mut _2: std::pin::Pin<&mut {generator@$DIR/inline_generator.rs:16:5: 16:8}>;
       let mut _3: &mut {generator@$DIR/inline_generator.rs:16:5: 16:8};
       let mut _4: {generator@$DIR/inline_generator.rs:16:5: 16:8};
-+     let mut _5: bool;
++     let mut _6: bool;
       scope 1 {
           debug _r => _1;
       }
 +     scope 2 (inlined g) {
++         let mut _5: {generator@$DIR/inline_generator.rs:16:5: 16:8};
 +     }
 +     scope 3 (inlined Pin::<&mut {generator@$DIR/inline_generator.rs:16:5: 16:8}>::new) {
 +         debug pointer => _3;
@@ -22,10 +23,10 @@
 +         }
 +     }
 +     scope 6 (inlined g::{closure#0}) {
-+         debug a => _5;
-+         let mut _6: &mut {generator@$DIR/inline_generator.rs:16:5: 16:8};
-+         let mut _7: u32;
-+         let mut _8: i32;
++         debug a => _6;
++         let mut _7: &mut {generator@$DIR/inline_generator.rs:16:5: 16:8};
++         let mut _8: u32;
++         let mut _9: i32;
 +     }
   
       bb0: {
@@ -34,21 +35,24 @@
           StorageLive(_3);
           StorageLive(_4);
 -         _4 = g() -> [return: bb1, unwind unreachable];
-+         _4 = {generator@$DIR/inline_generator.rs:16:5: 16:8 (#0)};
++         StorageLive(_5);
++         _5 = {generator@$DIR/inline_generator.rs:16:5: 16:8 (#0)};
++         _4 = move (_5 as subtype {generator@$DIR/inline_generator.rs:16:5: 16:8});
++         StorageDead(_5);
 +         _3 = &mut _4;
 +         _2 = Pin::<&mut {generator@$DIR/inline_generator.rs:16:5: 16:8}> { pointer: move _3 };
 +         StorageDead(_3);
-+         StorageLive(_5);
-+         _5 = const false;
-+         _6 = (_2.0: &mut {generator@$DIR/inline_generator.rs:16:5: 16:8});
-+         _7 = discriminant((*_6));
-+         switchInt(move _7) -> [0: bb3, 1: bb7, 3: bb8, otherwise: bb9];
++         StorageLive(_6);
++         _6 = const false;
++         _7 = (_2.0: &mut {generator@$DIR/inline_generator.rs:16:5: 16:8});
++         _8 = discriminant((*_7));
++         switchInt(move _8) -> [0: bb3, 1: bb7, 3: bb8, otherwise: bb9];
       }
   
       bb1: {
 -         _3 = &mut _4;
 -         _2 = Pin::<&mut {generator@$DIR/inline_generator.rs:16:5: 16:8}>::new(move _3) -> [return: bb2, unwind unreachable];
-+         StorageDead(_5);
++         StorageDead(_6);
 +         StorageDead(_2);
 +         drop(_4) -> [return: bb2, unwind unreachable];
       }
@@ -65,8 +69,8 @@
       bb3: {
 -         StorageDead(_2);
 -         drop(_4) -> [return: bb4, unwind unreachable];
-+         StorageLive(_8);
-+         switchInt(_5) -> [0: bb4, otherwise: bb5];
++         StorageLive(_9);
++         switchInt(_6) -> [0: bb4, otherwise: bb5];
       }
   
       bb4: {
@@ -74,18 +78,18 @@
 -         _0 = const ();
 -         StorageDead(_1);
 -         return;
-+         _8 = const 13_i32;
++         _9 = const 13_i32;
 +         goto -> bb6;
 +     }
 + 
 +     bb5: {
-+         _8 = const 7_i32;
++         _9 = const 7_i32;
 +         goto -> bb6;
 +     }
 + 
 +     bb6: {
-+         _1 = GeneratorState::<i32, bool>::Yielded(move _8);
-+         discriminant((*_6)) = 3;
++         _1 = GeneratorState::<i32, bool>::Yielded(move _9);
++         discriminant((*_7)) = 3;
 +         goto -> bb1;
 +     }
 + 
@@ -94,10 +98,10 @@
 +     }
 + 
 +     bb8: {
-+         StorageLive(_8);
-+         StorageDead(_8);
-+         _1 = GeneratorState::<i32, bool>::Complete(_5);
-+         discriminant((*_6)) = 1;
++         StorageLive(_9);
++         StorageDead(_9);
++         _1 = GeneratorState::<i32, bool>::Complete(_6);
++         discriminant((*_7)) = 1;
 +         goto -> bb1;
 +     }
 + 

tests/mir-opt/inline/inline_generator.main.Inline.panic-unwind.diff

@@ -7,11 +7,12 @@
       let mut _2: std::pin::Pin<&mut {generator@$DIR/inline_generator.rs:16:5: 16:8}>;
       let mut _3: &mut {generator@$DIR/inline_generator.rs:16:5: 16:8};
       let mut _4: {generator@$DIR/inline_generator.rs:16:5: 16:8};
-+     let mut _5: bool;
++     let mut _6: bool;
       scope 1 {
           debug _r => _1;
       }
 +     scope 2 (inlined g) {
++         let mut _5: {generator@$DIR/inline_generator.rs:16:5: 16:8};
 +     }
 +     scope 3 (inlined Pin::<&mut {generator@$DIR/inline_generator.rs:16:5: 16:8}>::new) {
 +         debug pointer => _3;
@@ -22,10 +23,10 @@
 +         }
 +     }
 +     scope 6 (inlined g::{closure#0}) {
-+         debug a => _5;
-+         let mut _6: &mut {generator@$DIR/inline_generator.rs:16:5: 16:8};
-+         let mut _7: u32;
-+         let mut _8: i32;
++         debug a => _6;
++         let mut _7: &mut {generator@$DIR/inline_generator.rs:16:5: 16:8};
++         let mut _8: u32;
++         let mut _9: i32;
 +     }
   
       bb0: {
@@ -37,7 +38,10 @@
 -     }
 - 
 -     bb1: {
-+         _4 = {generator@$DIR/inline_generator.rs:16:5: 16:8 (#0)};
++         StorageLive(_5);
++         _5 = {generator@$DIR/inline_generator.rs:16:5: 16:8 (#0)};
++         _4 = move (_5 as subtype {generator@$DIR/inline_generator.rs:16:5: 16:8});
++         StorageDead(_5);
           _3 = &mut _4;
 -         _2 = Pin::<&mut {generator@$DIR/inline_generator.rs:16:5: 16:8}>::new(move _3) -> [return: bb2, unwind: bb5];
 -     }
@@ -46,16 +50,16 @@
 +         _2 = Pin::<&mut {generator@$DIR/inline_generator.rs:16:5: 16:8}> { pointer: move _3 };
           StorageDead(_3);
 -         _1 = <{generator@$DIR/inline_generator.rs:16:5: 16:8} as Generator<bool>>::resume(move _2, const false) -> [return: bb3, unwind: bb5];
-+         StorageLive(_5);
-+         _5 = const false;
-+         _6 = (_2.0: &mut {generator@$DIR/inline_generator.rs:16:5: 16:8});
-+         _7 = discriminant((*_6));
-+         switchInt(move _7) -> [0: bb5, 1: bb9, 3: bb10, otherwise: bb11];
++         StorageLive(_6);
++         _6 = const false;
++         _7 = (_2.0: &mut {generator@$DIR/inline_generator.rs:16:5: 16:8});
++         _8 = discriminant((*_7));
++         switchInt(move _8) -> [0: bb5, 1: bb9, 3: bb10, otherwise: bb11];
       }
   
 -     bb3: {
 +     bb1: {
-+         StorageDead(_5);
++         StorageDead(_6);
           StorageDead(_2);
 -         drop(_4) -> [return: bb4, unwind: bb6];
 +         drop(_4) -> [return: bb2, unwind: bb4];
@@ -81,23 +85,23 @@
 +     }
 + 
 +     bb5: {
-+         StorageLive(_8);
-+         switchInt(_5) -> [0: bb6, otherwise: bb7];
++         StorageLive(_9);
++         switchInt(_6) -> [0: bb6, otherwise: bb7];
 +     }
 + 
 +     bb6: {
-+         _8 = const 13_i32;
++         _9 = const 13_i32;
 +         goto -> bb8;
 +     }
 + 
 +     bb7: {
-+         _8 = const 7_i32;
++         _9 = const 7_i32;
 +         goto -> bb8;
 +     }
 + 
 +     bb8: {
-+         _1 = GeneratorState::<i32, bool>::Yielded(move _8);
-+         discriminant((*_6)) = 3;
++         _1 = GeneratorState::<i32, bool>::Yielded(move _9);
++         discriminant((*_7)) = 3;
 +         goto -> bb1;
 +     }
 + 
@@ -106,10 +110,10 @@
 +     }
 + 
 +     bb10: {
-+         StorageLive(_8);
-+         StorageDead(_8);
-+         _1 = GeneratorState::<i32, bool>::Complete(_5);
-+         discriminant((*_6)) = 1;
++         StorageLive(_9);
++         StorageDead(_9);
++         _1 = GeneratorState::<i32, bool>::Complete(_6);
++         discriminant((*_7)) = 1;
 +         goto -> bb1;
 +     }
 +