Comment nits

compiler/rustc_hir_typeck/src/coercion.rs

@@ -510,9 +510,11 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         success(adjustments, ty, obligations)
     }
 
-    // &[T; n] or &mut [T; n] -> &[T]
-    // or &mut [T; n] -> &mut [T]
-    // or &Concrete -> &Trait, etc.
+    /// Performs [unsized coercion] by emulating a fulfillment loop on a
+    /// `CoerceUnsized` goal until all `CoerceUnsized` and `Unsize` goals
+    /// are successfully selected.
+    ///
+    /// [unsized coercion](https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions)
     #[instrument(skip(self), level = "debug")]
     fn coerce_unsized(&self, mut source: Ty<'tcx>, mut target: Ty<'tcx>) -> CoerceResult<'tcx> {
         source = self.shallow_resolve(source);

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -618,8 +618,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         match *self_ty.kind() {
             ty::Infer(ty::TyVar(found_vid)) => {
-                // FIXME: consider using `sub_root_var` here so we
-                // can see through subtyping.
                 let found_vid = self.root_var(found_vid);
                 debug!("self_type_matches_expected_vid - found_vid={:?}", found_vid);
                 expected_vid == found_vid
@@ -634,8 +632,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         self_ty: ty::TyVid,
     ) -> impl DoubleEndedIterator<Item = traits::PredicateObligation<'tcx>> + Captures<'tcx> + 'b
     {
-        // FIXME: consider using `sub_root_var` here so we
-        // can see through subtyping.
         let ty_var_root = self.root_var(self_ty);
         trace!("pending_obligations = {:#?}", self.fulfillment_cx.borrow().pending_obligations());
 

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -2394,7 +2394,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
             Ok(args) => args,
             Err(()) => {
                 // FIXME: A rematch may fail when a candidate cache hit occurs
-                // on thefreshened form of the trait predicate, but the match
+                // on the freshened form of the trait predicate, but the match
                 // fails for some reason that is not captured in the freshened
                 // cache key. For example, equating an impl trait ref against
                 // the placeholder trait ref may fail due the Generalizer relation

compiler/rustc_type_ir/src/sty.rs

@@ -200,7 +200,9 @@ pub enum TyKind<I: Interner> {
     /// A tuple type. For example, `(i32, bool)`.
     Tuple(I::ListTy),
 
-    /// A projection or opaque type. Both of these types
+    /// A projection, opaque type, weak type alias, or inherent associated type.
+    /// All of these types are represented as pairs of def-id and args, and can
+    /// be normalized, so they are grouped conceptually.
     Alias(AliasKind, I::AliasTy),
 
     /// A type parameter; for example, `T` in `fn f<T>(x: T) {}`.