Elision: consider lifetimes from &T iff T has Self

Change the algorithm which determines whether a self lifetime can be
used for return type lifetime elision, such that we consider lifetimes
attached to any reference in the self type, so long as Self can be found
anywhere inside the type of that reference.

Author: adetaylor

compiler/rustc_resolve/src/late.rs

@@ -2205,16 +2205,55 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
 
     /// List all the lifetimes that appear in the provided type.
     fn find_lifetime_for_self(&self, ty: &'ast Ty) -> Set1<LifetimeRes> {
-        struct SelfVisitor<'r, 'a, 'tcx> {
+        /// Visits a type to find all the &references, and determines the
+        /// set of lifetimes for all of those references where the referent
+        /// contains Self.
+        struct FindReferenceVisitor<'r, 'a, 'tcx> {
             r: &'r Resolver<'a, 'tcx>,
             impl_self: Option<Res>,
             lifetime: Set1<LifetimeRes>,
+        }
+
+        impl<'a> Visitor<'a> for FindReferenceVisitor<'_, '_, '_> {
+            fn visit_ty(&mut self, ty: &'a Ty) {
+                trace!("FindReferenceVisitor considering ty={:?}", ty);
+                if let TyKind::Ref(lt, _) = ty.kind {
+                    // See if anything inside the &thing contains Self
+                    let mut visitor =
+                        SelfVisitor { r: self.r, impl_self: self.impl_self, self_found: false };
+                    visitor.visit_ty(ty);
+                    trace!("FindReferenceVisitor: SelfVisitor self_found={:?}", visitor.self_found);
+                    if visitor.self_found {
+                        let lt_id = if let Some(lt) = lt {
+                            lt.id
+                        } else {
+                            let res = self.r.lifetimes_res_map[&ty.id];
+                            let LifetimeRes::ElidedAnchor { start, .. } = res else { bug!() };
+                            start
+                        };
+                        let lt_res = self.r.lifetimes_res_map[&lt_id];
+                        trace!("FindReferenceVisitor inserting res={:?}", lt_res);
+                        self.lifetime.insert(lt_res);
+                    }
+                }
+                visit::walk_ty(self, ty)
+            }
+
+            // A type may have an expression as a const generic argument.
+            // We do not want to recurse into those.
+            fn visit_expr(&mut self, _: &'a Expr) {}
+        }
+
+        /// Visitor which checks the referent of a &Thing to see if the
+        /// Thing contains Self
+        struct SelfVisitor<'r, 'a, 'tcx> {
+            r: &'r Resolver<'a, 'tcx>,
+            impl_self: Option<Res>,
             self_found: bool,
         }
 
         impl SelfVisitor<'_, '_, '_> {
-            // Look for `self: &'a Self` - also desugared from `&'a self`,
-            // and if that matches, use it for elision and return early.
+            // Look for `self: &'a Self` - also desugared from `&'a self`
             fn is_self_ty(&self, ty: &Ty) -> bool {
                 match ty.kind {
                     TyKind::ImplicitSelf => true,
@@ -2237,18 +2276,6 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                     trace!("SelfVisitor found Self");
                     self.self_found = true;
                 }
-                if let TyKind::Ref(lt, _) = ty.kind {
-                    let lt_id = if let Some(lt) = lt {
-                        lt.id
-                    } else {
-                        let res = self.r.lifetimes_res_map[&ty.id];
-                        let LifetimeRes::ElidedAnchor { start, .. } = res else { bug!() };
-                        start
-                    };
-                    let lt_res = self.r.lifetimes_res_map[&lt_id];
-                    trace!("SelfVisitor inserting res={:?}", lt_res);
-                    self.lifetime.insert(lt_res);
-                }
                 visit::walk_ty(self, ty)
             }
 
@@ -2278,11 +2305,10 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                     Res::Def(DefKind::Struct | DefKind::Union | DefKind::Enum, _,) | Res::PrimTy(_)
                 )
             });
-        let mut visitor =
-            SelfVisitor { r: self.r, impl_self, lifetime: Set1::Empty, self_found: false };
+        let mut visitor = FindReferenceVisitor { r: self.r, impl_self, lifetime: Set1::Empty };
         visitor.visit_ty(ty);
-        trace!("SelfVisitor found={:?}, self_found={:?}", visitor.lifetime, visitor.self_found);
-        if visitor.self_found { visitor.lifetime } else { Set1::Empty }
+        trace!("FindReferenceVisitor found={:?}", visitor.lifetime);
+        visitor.lifetime
     }
 
     /// Searches the current set of local scopes for labels. Returns the `NodeId` of the resolved