Make UserTypeProjection projections Opaque

Also shifts comments explaining why Stable MIR drops an optional variant
name field, for `Downcast` projection elements, to the `Place::stable`
function.

Author: klinvill

compiler/rustc_smir/src/rustc_smir/mod.rs

@@ -712,6 +712,11 @@ impl<'tcx> Stable<'tcx> for mir::PlaceElem<'tcx> {
                 to: *to,
                 from_end: *from_end,
             },
+            // MIR includes an `Option<Symbol>` argument for `Downcast` that is the name of the
+            // variant, used for printing MIR. However this information should also be accessible
+            // via a lookup using the `VariantIdx`. The `Option<Symbol>` argument is therefore
+            // dropped when converting to Stable MIR. A brief justification for this decision can be
+            // found at https://github.com/rust-lang/rust/pull/117517#issuecomment-1811683486
             Downcast(_, idx) => stable_mir::mir::ProjectionElem::Downcast(idx.stable(tables)),
             OpaqueCast(ty) => stable_mir::mir::ProjectionElem::OpaqueCast(ty.stable(tables)),
             Subtype(ty) => stable_mir::mir::ProjectionElem::Subtype(ty.stable(tables)),
@@ -723,7 +728,7 @@ impl<'tcx> Stable<'tcx> for mir::UserTypeProjection {
     type T = stable_mir::mir::UserTypeProjection;
 
     fn stable(&self, _tables: &mut Tables<'tcx>) -> Self::T {
-        UserTypeProjection { base: self.base.as_usize(), projection: format!("{:?}", self.projs) }
+        UserTypeProjection { base: self.base.as_usize(), projection: opaque(&self.projs) }
     }
 }
 

compiler/stable_mir/src/mir/body.rs

@@ -465,10 +465,6 @@ pub enum ProjectionElem {
     },
 
     /// "Downcast" to a variant of an enum or a coroutine.
-    //
-    // TODO(klinvill): MIR includes an Option<Symbol> argument that is the name of the variant, used
-    // for printing MIR. However I don't see it used anywhere. Is such a field needed or can we just
-    // include the VariantIdx which could be used to recover the field name if needed?
     Downcast(VariantIdx),
 
     /// Like an explicit cast from an opaque type to a concrete type, but without
@@ -488,7 +484,7 @@ pub enum ProjectionElem {
 pub struct UserTypeProjection {
     pub base: UserTypeAnnotationIndex,
 
-    pub projection: String,
+    pub projection: Opaque,
 }
 
 pub type Local = usize;