Fix lifetime elision

  struct Concrete(u32);

  impl Concrete {
      fn m(self: &Box<Self>) -> &u32 {
          &self.0
      }
  }

resulted in a confusing error.

  impl Concrete {
      fn n(self: &Box<&Self>) -> &u32 {
          &self.0
      }
  }

resulted in no error or warning, despite apparent ambiguity over the elided lifetime.

Fixes #117715

Author: adetaylor

compiler/rustc_resolve/src/late.rs

@@ -2052,13 +2052,15 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
             // Handle `self` specially.
             if index == 0 && has_self {
                 let self_lifetime = self.find_lifetime_for_self(ty);
-                if let Set1::One(lifetime) = self_lifetime {
+                elision_lifetime = match self_lifetime {
                     // We found `self` elision.
-                    elision_lifetime = Elision::Self_(lifetime);
-                } else {
+                    Set1::One(lifetime) => Elision::Self_(lifetime),
+                    // `self` itself had ambiguous lifetimes, e.g.
+                    // &Box<&Self>
+                    Set1::Many => Elision::Err,
                     // We do not have `self` elision: disregard the `Elision::Param` that we may
                     // have found.
-                    elision_lifetime = Elision::None;
+                    Set1::Empty => Elision::None,
                 }
             }
             debug!("(resolving function / closure) recorded parameter");
@@ -2082,6 +2084,7 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
             r: &'r Resolver<'a, 'tcx>,
             impl_self: Option<Res>,
             lifetime: Set1<LifetimeRes>,
+            self_found: bool,
         }
 
         impl SelfVisitor<'_, '_, '_> {
@@ -2105,7 +2108,11 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
         impl<'a> Visitor<'a> for SelfVisitor<'_, '_, '_> {
             fn visit_ty(&mut self, ty: &'a Ty) {
                 trace!("SelfVisitor considering ty={:?}", ty);
-                if let TyKind::Ref(lt, ref mt) = ty.kind && self.is_self_ty(&mt.ty) {
+                if self.is_self_ty(ty) {
+                    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 {
@@ -2146,10 +2153,11 @@ 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 };
+        let mut visitor =
+            SelfVisitor { r: self.r, impl_self, lifetime: Set1::Empty, self_found: false };
         visitor.visit_ty(ty);
-        trace!("SelfVisitor found={:?}", visitor.lifetime);
-        visitor.lifetime
+        trace!("SelfVisitor found={:?}, self_found={:?}", visitor.lifetime, visitor.self_found);
+        if visitor.self_found { visitor.lifetime } else { Set1::Empty }
     }
 
     /// Searches the current set of local scopes for labels. Returns the `NodeId` of the resolved

tests/ui/self/elision/ref-self-multi.rs

@@ -0,0 +1,24 @@
+#![feature(arbitrary_self_types)]
+#![allow(non_snake_case)]
+
+use std::marker::PhantomData;
+use std::ops::Deref;
+use std::pin::Pin;
+
+struct Struct { }
+
+struct Wrap<T, P>(T, PhantomData<P>);
+
+impl<T, P> Deref for Wrap<T, P> {
+    type Target = T;
+    fn deref(&self) -> &T { &self.0 }
+}
+
+impl Struct {
+    fn ref_box_ref_Self(self: &Box<&Self>, f: &u32) -> &u32 {
+        f
+        //~^ ERROR missing lifetime specifier
+    }
+}
+
+fn main() { }

tests/ui/self/elision/ref-self-multi.stderr

@@ -0,0 +1,15 @@
+error[E0106]: missing lifetime specifier
+  --> $DIR/ref-self-multi.rs:18:56
+   |
+LL |     fn ref_box_ref_Self(self: &Box<&Self>, f: &u32) -> &u32 {
+   |                               -----------     ----     ^ expected named lifetime parameter
+   |
+   = help: this function's return type contains a borrowed value, but the signature does not say whether it is borrowed from one of `self`'s 2 lifetimes or `f`
+help: consider introducing a named lifetime parameter
+   |
+LL |     fn ref_box_ref_Self<'a>(self: &'a Box<&'a Self>, f: &'a u32) -> &'a u32 {
+   |                        ++++        ++      ++            ++          ++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0106`.

tests/ui/self/elision/ref-self.rs

@@ -53,6 +53,11 @@ impl Struct {
         f
         //~^ ERROR lifetime may not live long enough
     }
+
+    fn ref_box_Self(self: &Box<Self>, f: &u32) -> &u32 {
+        f
+        //~^ ERROR lifetime may not live long enough
+    }
 }
 
 fn main() { }

tests/ui/self/elision/ref-self.stderr

@@ -103,5 +103,20 @@ help: consider introducing a named lifetime parameter and update trait if needed
 LL |     fn wrap_ref_Self_Self<'a>(self: Wrap<&'a Self, Self>, f: &'a u8) -> &u8 {
    |                          ++++             ++                  ++
 
-error: aborting due to 7 previous errors
+error: lifetime may not live long enough
+  --> $DIR/ref-self.rs:58:9
+   |
+LL |     fn ref_box_Self(self: &Box<Self>, f: &u32) -> &u32 {
+   |                           -              - let's call the lifetime of this reference `'1`
+   |                           |
+   |                           let's call the lifetime of this reference `'2`
+LL |         f
+   |         ^ method was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
+   |
+help: consider introducing a named lifetime parameter and update trait if needed
+   |
+LL |     fn ref_box_Self<'a>(self: &'a Box<Self>, f: &'a u32) -> &u32 {
+   |                    ++++        ++                ++
+
+error: aborting due to 8 previous errors