3 files changed, 12 insertions, 5 deletions
diff --git a/compiler/rustc_middle/src/mir/pretty.rs b/compiler/rustc_middle/src/mir/pretty.rs
index 6fec4517b8c..9361d32a0d5 100644
--- a/compiler/rustc_middle/src/mir/pretty.rs
+++ b/compiler/rustc_middle/src/mir/pretty.rs
@@ -1127,7 +1127,7 @@ fn post_fmt_projection(projection: &[PlaceElem<'_>], fmt: &mut Formatter<'_>) ->
                 write!(fmt, " as {ty})")?;
             }
             ProjectionElem::Subtype(ty) => {
-                write!(fmt, "as {ty})")?;
+                write!(fmt, " as subtype {ty})")?;
             }
             ProjectionElem::Downcast(Some(name), _index) => {
                 write!(fmt, " as {name})")?;
diff --git a/compiler/rustc_middle/src/mir/syntax.rs b/compiler/rustc_middle/src/mir/syntax.rs
index e7db6a67240..49f4def6dfb 100644
--- a/compiler/rustc_middle/src/mir/syntax.rs
+++ b/compiler/rustc_middle/src/mir/syntax.rs
@@ -1077,9 +1077,12 @@ pub enum ProjectionElem<V, T> {
     OpaqueCast(T),
 
     /// A `Subtype(T)` projection is applied to any `StatementKind::Assign` where
-    /// type of lvalue doesn't match type of rvalue, primary goal being making subtyping
+    /// 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`.
diff --git a/compiler/rustc_middle/src/mir/tcx.rs b/compiler/rustc_middle/src/mir/tcx.rs
index f534f0f5f3c..44ae75e2de7 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,
@@ -110,8 +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::Subtype(_) => PlaceTy::from_ty(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