nit: docs

Author: compiler-errors

compiler/rustc_middle/src/ty/sty.rs

@@ -1148,14 +1148,23 @@ impl<'tcx, T: IntoIterator> Binder<'tcx, T> {
 #[derive(HashStable, TypeFoldable, TypeVisitable, Lift)]
 pub struct AliasTy<'tcx> {
     /// The parameters of the associated or opaque item.
+    ///
+    /// For a projection, these are the substitutions for the trait and the
+    /// GAT substitutions, if there are any.
+    ///
+    /// For RPIT the substitutions are for the generics of the function,
+    /// while for TAIT it is used for the generic parameters of the alias.
     pub substs: SubstsRef<'tcx>,
 
     /// The `DefId` of the `TraitItem` for the associated type `N` if this is a projection,
     /// or the `OpaqueType` item if this is an opaque.
     ///
-    /// Note that this is not the `DefId` of the `TraitRef` containing this
-    /// associated type, which is in `tcx.associated_item(item_def_id).container`,
-    /// aka. `tcx.parent(item_def_id).unwrap()`.
+    /// During codegen, `tcx.type_of(def_id)` can be used to get the type of the
+    /// underlying type if the type is an opaque.
+    ///
+    /// Note that if this is an associated type, this is not the `DefId` of the
+    /// `TraitRef` containing this associated type, which is in `tcx.associated_item(def_id).container`,
+    /// aka. `tcx.parent(def_id)`.
     pub def_id: DefId,
 }