Improved comments for UserAssertTy statement.

src/librustc/mir/mod.rs

@@ -1260,6 +1260,14 @@ pub enum StatementKind<'tcx> {
     ///
     /// Here we would insert a `UserAssertTy<(T, U)>(y)` instruction to check that the type of `y`
     /// is the right thing.
+    ///
+    /// `CanonicalTy` is used to capture "inference variables" from the user's types. For example:
+    ///
+    ///     let x: Vec<_> = ...;
+    ///     let y: &u32 = ...;
+    ///
+    /// would result in `Vec<?0>` and `&'?0 u32` respectively (where `?0` is a canonicalized
+    /// variable).
     UserAssertTy(CanonicalTy<'tcx>, Local),
 
     /// No-op. Useful for deleting instructions without affecting statement indices.

src/librustc/ty/context.rs

@@ -345,7 +345,8 @@ pub struct TypeckTables<'tcx> {
     /// method calls, including those of overloaded operators.
     type_dependent_defs: ItemLocalMap<Def>,
 
-    /// Stores the canonicalized types provided by the user.
+    /// Stores the canonicalized types provided by the user. See also `UserAssertTy` statement in
+    /// MIR.
     user_provided_tys: ItemLocalMap<CanonicalTy<'tcx>>,
 
     /// Stores the types for various nodes in the AST.  Note that this table

src/librustc_mir/build/block.rs

@@ -125,9 +125,9 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                                 })
                             }));
                     } else {
-                        // FIXME: We currently only insert `UserAssertTy` statements for patterns
-                        // that are bindings, this is as we do not want to deconstruct the type
-                        // being assertion to match the pattern.
+                        // FIXME(#47184): We currently only insert `UserAssertTy` statements for
+                        // patterns that are bindings, this is as we do not want to deconstruct
+                        // the type being assertion to match the pattern.
                         if let PatternKind::Binding { var, .. } = *pattern.kind {
                             if let Some(ty) = ty {
                                 this.user_assert_ty(block, ty, var, span);