5 files changed, 34 insertions, 3 deletions
diff --git a/compiler/rustc_middle/src/mir/pretty.rs b/compiler/rustc_middle/src/mir/pretty.rs
index 27105eae665..f032fd29d05 100644
--- a/compiler/rustc_middle/src/mir/pretty.rs
+++ b/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})")?;
             }
diff --git a/compiler/rustc_middle/src/mir/statement.rs b/compiler/rustc_middle/src/mir/statement.rs
index b308fb51993..3471d620ee6 100644
--- a/compiler/rustc_middle/src/mir/statement.rs
+++ b/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,
         }
diff --git a/compiler/rustc_middle/src/mir/syntax.rs b/compiler/rustc_middle/src/mir/syntax.rs
index e30172727f2..0b95fdfa1fb 100644
--- a/compiler/rustc_middle/src/mir/syntax.rs
+++ b/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
diff --git a/compiler/rustc_middle/src/mir/tcx.rs b/compiler/rustc_middle/src/mir/tcx.rs
index bcbc5aa4744..7df25fc5c03 100644
--- a/compiler/rustc_middle/src/mir/tcx.rs
+++ b/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
diff --git a/compiler/rustc_middle/src/mir/visit.rs b/compiler/rustc_middle/src/mir/visit.rs
index ea447d1538c..f2745b32cf9 100644
--- a/compiler/rustc_middle/src/mir/visit.rs
+++ b/compiler/rustc_middle/src/mir/visit.rs
@@ -1109,6 +1109,11 @@ macro_rules! visit_place_fns {
                     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 @@ macro_rules! visit_place_fns {
             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) => {