comment

compiler/rustc_hir_analysis/src/collect/lifetimes.rs

@@ -1834,6 +1834,27 @@ fn is_late_bound_map(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Option<&FxIndexSet<
     debug!(?late_bound);
     return Some(tcx.arena.alloc(late_bound));
 
+    /// Visits a `ty::Ty` collecting information about what generic parameters are constrained.
+    ///
+    /// The visitor does not operate on `hir::Ty` so that it can be called on the rhs of a `type Alias<...> = ...;`
+    /// which may live in a separate crate so there would not be any hir available. Instead we use the `type_of`
+    /// query to obtain a `ty::Ty` which will be present even in cross crate scenarios. It also naturally
+    /// handles cycle detection as we go through the query system.
+    ///
+    /// This is necessary in the first place for the following case:
+    /// ```
+    /// type Alias<'a, T> = <T as Trait<'a>>::Assoc;
+    /// fn foo<'a>(_: Alias<'a, ()>) -> Alias<'a, ()> { ... }
+    /// ```
+    ///
+    /// If we conservatively considered `'a` unconstrained then we could break users who had written code before
+    /// we started correctly handling aliases. If we considered `'a` constrained then it would become late bound
+    /// causing an error during astconv as the `'a` is not constrained by the input type `<() as Trait<'a>>::Assoc`
+    /// but appears in the output type `<() as Trait<'a>>::Assoc`.
+    ///
+    /// We must therefore "look into" the `Alias` to see whether we should consider `'a` constrained or not.
+    ///
+    /// See #100508 #85533 #47511 for additional context
     struct ConstrainedCollectorPostAstConv {
         arg_is_constrained: Box<[bool]>,
     }
@@ -1886,17 +1907,8 @@ fn is_late_bound_map(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Option<&FxIndexSet<
                     None,
                     hir::Path { res: Res::Def(DefKind::TyAlias, alias_def), segments, span },
                 )) => {
-                    // If this is a top level type alias attempt to "look through" it to see if the args
-                    // are constrained, instead of assuming they are and inserting all the lifetimes.
-                    // This is necessary for the following case:
-                    // ```
-                    // type Alias<'a, T> = <T as Trait<'a>>::Assoc;
-                    // fn foo<'a>(_: Alias<'a, ()>) -> Alias<'a, ()> { ... }
-                    // ```
-                    // If we considered `'a` constrained then it would become late bound causing an error
-                    // during astconv as the `'a` is not constrained by the input type `<() as Trait<'a>>::Assoc`
-                    // but appears in the output type `<() as Trait<'a>>::Assoc`.
-
+                    // See comments on `ConstrainedCollectorPostAstConv` for why this arm does not just consider
+                    // substs to be unconstrained.
                     let generics = self.tcx.generics_of(alias_def);
                     let mut walker = ConstrainedCollectorPostAstConv {
                         arg_is_constrained: vec![false; generics.params.len()].into_boxed_slice(),