Auto merge of #147055 - beepster4096:subtype_is_not_a_projection, r=lcnr

Turn ProjectionElem::Subtype into CastKind::Subtype

I noticed that drop elaboration can't, in general, handle `ProjectionElem::SubType`. It creates a disjoint move path that overlaps with other move paths. (`Subslice` does too, and I'm working on a different PR to make that special case less fragile.) If its skipped and treated as the same move path as its parent then `MovePath.place` has multiple possible projections. (It would probably make sense to remove all `Subtype` projections for the canonical place but it doesn't make sense to have this special case for a problem that doesn't actually occur in real MIR.)

The only reason this doesn't break is that `Subtype` is always the sole projection of the local its applied to. For the same reason, it works fine as a `CastKind` so I figured that makes more sense than documenting and validating this hidden invariant.

cc rust-lang/rust#112651, rust-lang/rust#133258

r? Icnr (bc you've been the main person dealing with `Subtype` it looks like)

Author: bors

rustc_public/src/mir/body.rs

@@ -836,14 +836,6 @@ pub enum ProjectionElem {
     /// Like an explicit cast from an opaque type to a concrete type, but without
     /// requiring an intermediate variable.
     OpaqueCast(Ty),
-
-    /// 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.
-    Subtype(Ty),
 }
 
 #[derive(Clone, Debug, Eq, PartialEq, Serialize)]
@@ -1028,6 +1020,7 @@ pub enum CastKind {
     PtrToPtr,
     FnPtrToPtr,
     Transmute,
+    Subtype,
 }
 
 #[derive(Clone, Debug, Eq, PartialEq, Hash, Serialize)]
@@ -1089,7 +1082,7 @@ impl ProjectionElem {
                 Self::subslice_ty(ty, *from, *to, *from_end)
             }
             ProjectionElem::Downcast(_) => Ok(ty),
-            ProjectionElem::OpaqueCast(ty) | ProjectionElem::Subtype(ty) => Ok(*ty),
+            ProjectionElem::OpaqueCast(ty) => Ok(*ty),
         }
     }
 

rustc_public/src/mir/visit.rs

@@ -471,7 +471,6 @@ macro_rules! visit_place_fns {
                 ProjectionElem::Subslice { from: _, to: _, from_end: _ } => {}
                 ProjectionElem::Downcast(_idx) => {}
                 ProjectionElem::OpaqueCast(ty) => self.visit_ty(ty, location),
-                ProjectionElem::Subtype(ty) => self.visit_ty(ty, location),
             }
         }
     };
@@ -512,7 +511,6 @@ macro_rules! visit_place_fns {
                 ProjectionElem::Subslice { from: _, to: _, from_end: _ } => {}
                 ProjectionElem::Downcast(_idx) => {}
                 ProjectionElem::OpaqueCast(ty) => self.visit_ty(ty, location),
-                ProjectionElem::Subtype(ty) => self.visit_ty(ty, location),
             }
         }
     };

rustc_public/src/unstable/convert/internal.rs

@@ -703,9 +703,6 @@ impl RustcInternal for ProjectionElem {
             ProjectionElem::OpaqueCast(ty) => {
                 rustc_middle::mir::PlaceElem::OpaqueCast(ty.internal(tables, tcx))
             }
-            ProjectionElem::Subtype(ty) => {
-                rustc_middle::mir::PlaceElem::Subtype(ty.internal(tables, tcx))
-            }
         }
     }
 }

rustc_public/src/unstable/convert/stable/mir.rs

@@ -356,6 +356,7 @@ impl<'tcx> Stable<'tcx> for mir::CastKind {
             PtrToPtr => crate::mir::CastKind::PtrToPtr,
             FnPtrToPtr => crate::mir::CastKind::FnPtrToPtr,
             Transmute => crate::mir::CastKind::Transmute,
+            Subtype => crate::mir::CastKind::Subtype,
         }
     }
 }
@@ -453,7 +454,6 @@ impl<'tcx> Stable<'tcx> for mir::PlaceElem<'tcx> {
             // found at https://github.com/rust-lang/rust/pull/117517#issuecomment-1811683486
             Downcast(_, idx) => crate::mir::ProjectionElem::Downcast(idx.stable(tables, cx)),
             OpaqueCast(ty) => crate::mir::ProjectionElem::OpaqueCast(ty.stable(tables, cx)),
-            Subtype(ty) => crate::mir::ProjectionElem::Subtype(ty.stable(tables, cx)),
             UnwrapUnsafeBinder(..) => todo!("FIXME(unsafe_binders):"),
         }
     }