Auto merge of rust-lang#117967 - adetaylor:fix-lifetime-elision-bug, r=<try>

Fix ambiguous cases of multiple & in elided self lifetimes

```rust
  struct Concrete(u32);

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

resulted in a confusing error.

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

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

Fixes rust-lang#117715

Author: bors

compiler/rustc_resolve/src/late.rs

@@ -2153,13 +2153,17 @@ 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>. In this case we won't consider
+                    // taking an alternative parameter lifetime; just avoid elision
+                    // entirely.
+                    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");
@@ -2179,15 +2183,55 @@ 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,
@@ -2206,19 +2250,9 @@ 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)
-                {
-                    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);
+                if self.is_self_ty(ty) {
+                    trace!("SelfVisitor found Self");
+                    self.self_found = true;
                 }
                 visit::walk_ty(self, ty)
             }
@@ -2249,9 +2283,9 @@ 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 = FindReferenceVisitor { r: self.r, impl_self, lifetime: Set1::Empty };
         visitor.visit_ty(ty);
-        trace!("SelfVisitor found={:?}", visitor.lifetime);
+        trace!("FindReferenceVisitor found={:?}", visitor.lifetime);
         visitor.lifetime
     }
 

tests/crashes/122903-1.rs

@@ -1,7 +1,7 @@
 //@ known-bug: #122903
 impl Struct {
-    async fn box_box_ref_Struct(
-        self: Box<Box<Self, impl FnMut(&mut Box<Box<Self, impl FnMut(&mut Self)>>)>>,
+    fn box_box_ref_Struct(
+        self: impl FnMut(Box<impl FnMut(&mut Self)>),
     ) -> &u32 {
         f
     }

tests/crashes/122903-2.rs

@@ -0,0 +1,22 @@
+//@ check-pass
+
+struct Foo<'a>(&'a str);
+
+impl<'b> Foo<'b> {
+    fn a<'a>(self: Self, a: &'a str) -> &str {
+        a
+    }
+    fn b<'a>(self: Foo<'b>, a: &'a str) -> &str {
+        a
+    }
+}
+
+struct Foo2<'a>(&'a u32);
+impl<'a> Foo2<'a> {
+    fn foo(self: &Self) -> &u32 { self.0 } // ok
+    fn bar(self: &Foo2<'a>) -> &u32 { self.0 } // ok (do not look into `Foo`)
+    fn baz2(self: Self, arg: &u32) -> &u32 { arg } // use lt from `arg`
+    fn baz3(self: Foo2<'a>, arg: &u32) -> &u32 { arg } // use lt from `arg`
+}
+
+fn main() {}

tests/ui/self/elision/ignore-non-reference-lifetimes.rs

@@ -1,4 +1,3 @@
-//@ check-pass
 //@ edition:2018
 
 #![feature(arbitrary_self_types)]
@@ -21,22 +20,27 @@ impl Struct {
     // Test using multiple `&Self`:
 
     async fn wrap_ref_Self_ref_Self(self: Wrap<&Self, &Self>, f: &u8) -> &u8 {
+        //~^ ERROR missing lifetime specifier
         f
     }
 
     async fn box_wrap_ref_Self_ref_Self(self: Box<Wrap<&Self, &Self>>, f: &u32) -> &u32 {
+        //~^ ERROR missing lifetime specifier
         f
     }
 
     async fn pin_wrap_ref_Self_ref_Self(self: Pin<Wrap<&Self, &Self>>, f: &u32) -> &u32 {
+        //~^ ERROR missing lifetime specifier
         f
     }
 
     async fn box_box_wrap_ref_Self_ref_Self(self: Box<Box<Wrap<&Self, &Self>>>, f: &u32) -> &u32 {
+        //~^ ERROR missing lifetime specifier
         f
     }
 
     async fn box_pin_wrap_ref_Self_ref_Self(self: Box<Pin<Wrap<&Self, &Self>>>, f: &u32) -> &u32 {
+        //~^ ERROR missing lifetime specifier
         f
     }
 }

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

@@ -0,0 +1,63 @@
+error[E0106]: missing lifetime specifier
+  --> $DIR/multiple-ref-self-async.rs:22:74
+   |
+LL |     async fn wrap_ref_Self_ref_Self(self: Wrap<&Self, &Self>, f: &u8) -> &u8 {
+   |                                           ------------------     ---     ^ 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 |     async fn wrap_ref_Self_ref_Self<'a>(self: Wrap<&'a Self, &'a Self>, f: &'a u8) -> &'a u8 {
+   |                                    ++++             ++        ++            ++         ++
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/multiple-ref-self-async.rs:27:84
+   |
+LL |     async fn box_wrap_ref_Self_ref_Self(self: Box<Wrap<&Self, &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 |     async fn box_wrap_ref_Self_ref_Self<'a>(self: Box<Wrap<&'a Self, &'a Self>>, f: &'a u32) -> &'a u32 {
+   |                                        ++++                 ++        ++             ++          ++
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/multiple-ref-self-async.rs:32:84
+   |
+LL |     async fn pin_wrap_ref_Self_ref_Self(self: Pin<Wrap<&Self, &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 |     async fn pin_wrap_ref_Self_ref_Self<'a>(self: Pin<Wrap<&'a Self, &'a Self>>, f: &'a u32) -> &'a u32 {
+   |                                        ++++                 ++        ++             ++          ++
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/multiple-ref-self-async.rs:37:93
+   |
+LL |     async fn box_box_wrap_ref_Self_ref_Self(self: Box<Box<Wrap<&Self, &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 |     async fn box_box_wrap_ref_Self_ref_Self<'a>(self: Box<Box<Wrap<&'a Self, &'a Self>>>, f: &'a u32) -> &'a u32 {
+   |                                            ++++                     ++        ++              ++          ++
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/multiple-ref-self-async.rs:42:93
+   |
+LL |     async fn box_pin_wrap_ref_Self_ref_Self(self: Box<Pin<Wrap<&Self, &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 |     async fn box_pin_wrap_ref_Self_ref_Self<'a>(self: Box<Pin<Wrap<&'a Self, &'a Self>>>, f: &'a u32) -> &'a u32 {
+   |                                            ++++                     ++        ++              ++          ++
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0106`.

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

@@ -1,5 +1,3 @@
-//@ check-pass
-
 #![feature(arbitrary_self_types)]
 #![allow(non_snake_case)]
 
@@ -20,22 +18,27 @@ impl Struct {
     // Test using multiple `&Self`:
 
     fn wrap_ref_Self_ref_Self(self: Wrap<&Self, &Self>, f: &u8) -> &u8 {
+        //~^ ERROR missing lifetime specifier
         f
     }
 
     fn box_wrap_ref_Self_ref_Self(self: Box<Wrap<&Self, &Self>>, f: &u32) -> &u32 {
+        //~^ ERROR missing lifetime specifier
         f
     }
 
     fn pin_wrap_ref_Self_ref_Self(self: Pin<Wrap<&Self, &Self>>, f: &u32) -> &u32 {
+        //~^ ERROR missing lifetime specifier
         f
     }
 
     fn box_box_wrap_ref_Self_ref_Self(self: Box<Box<Wrap<&Self, &Self>>>, f: &u32) -> &u32 {
+        //~^ ERROR missing lifetime specifier
         f
     }
 
     fn box_pin_wrap_ref_Self_ref_Self(self: Box<Pin<Wrap<&Self, &Self>>>, f: &u32) -> &u32 {
+        //~^ ERROR missing lifetime specifier
         f
     }
 }

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

@@ -0,0 +1,63 @@
+error[E0106]: missing lifetime specifier
+  --> $DIR/multiple-ref-self.rs:20:68
+   |
+LL |     fn wrap_ref_Self_ref_Self(self: Wrap<&Self, &Self>, f: &u8) -> &u8 {
+   |                                     ------------------     ---     ^ 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 wrap_ref_Self_ref_Self<'a>(self: Wrap<&'a Self, &'a Self>, f: &'a u8) -> &'a u8 {
+   |                              ++++             ++        ++            ++         ++
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/multiple-ref-self.rs:25:78
+   |
+LL |     fn box_wrap_ref_Self_ref_Self(self: Box<Wrap<&Self, &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 box_wrap_ref_Self_ref_Self<'a>(self: Box<Wrap<&'a Self, &'a Self>>, f: &'a u32) -> &'a u32 {
+   |                                  ++++                 ++        ++             ++          ++
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/multiple-ref-self.rs:30:78
+   |
+LL |     fn pin_wrap_ref_Self_ref_Self(self: Pin<Wrap<&Self, &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 pin_wrap_ref_Self_ref_Self<'a>(self: Pin<Wrap<&'a Self, &'a Self>>, f: &'a u32) -> &'a u32 {
+   |                                  ++++                 ++        ++             ++          ++
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/multiple-ref-self.rs:35:87
+   |
+LL |     fn box_box_wrap_ref_Self_ref_Self(self: Box<Box<Wrap<&Self, &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 box_box_wrap_ref_Self_ref_Self<'a>(self: Box<Box<Wrap<&'a Self, &'a Self>>>, f: &'a u32) -> &'a u32 {
+   |                                      ++++                     ++        ++              ++          ++
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/multiple-ref-self.rs:40:87
+   |
+LL |     fn box_pin_wrap_ref_Self_ref_Self(self: Box<Pin<Wrap<&Self, &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 box_pin_wrap_ref_Self_ref_Self<'a>(self: Box<Pin<Wrap<&'a Self, &'a Self>>>, f: &'a u32) -> &'a u32 {
+   |                                      ++++                     ++        ++              ++          ++
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0106`.

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

@@ -0,0 +1,17 @@
+use std::pin::Pin;
+trait Trait {
+    fn method<'a>(self: Pin<&Self>, f: &'a u32) -> &'a u32 {
+        f
+    }
+}
+
+impl<P> Trait for Pin<P> {
+    // This should not hide `&Self`, which would cause this to compile.
+    fn method(self: Pin<&Self>, f: &u32) -> &u32 {
+        //~^ ERROR `impl` item signature doesn't match `trait`
+        f
+        //~^ ERROR lifetime may not live long enough
+    }
+}
+
+fn main() {}