Auto merge of rust-lang#97039 - cjgillot:no-rpit-hrtb, r=jackh726

Forbid nested opaque types to reference HRTB from opaque types.

Avoids rust-lang#96194
Alternative to rust-lang#96970

r? `@oli-obk`

compiler/rustc_resolve/src/late/lifetimes.rs

@@ -1025,6 +1025,20 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
  }
  self.uninsert_lifetime_on_error(lifetime, def.unwrap());
  }
+ if let hir::Node::Item(hir::Item {
+ kind: hir::ItemKind::OpaqueTy { .. }, ..
+ }) = self.tcx.hir().get(parent_id)
+ {
+ if !self.trait_definition_only {
+ let mut err = self.tcx.sess.struct_span_err(
+ lifetime.span,
+ "higher kinded lifetime bounds on nested opaque types are not supported yet",
+ );
+ err.span_note(self.tcx.def_span(def_id), "lifetime declared here");
+ err.emit();
+ }
+ self.uninsert_lifetime_on_error(lifetime, def.unwrap());
+ }
  }
 
  // We want to start our early-bound indices at the end of the parent scope,

src/test/ui/impl-trait/issues/issue-54895.rs

@@ -1,5 +1,3 @@
-// check-pass
-
 trait Trait<'a> {
  type Out;
  fn call(&'a self) -> Self::Out;
@@ -15,6 +13,7 @@ impl<'a> Trait<'a> for X {
 }
 
 fn f() -> impl for<'a> Trait<'a, Out = impl Sized + 'a> {
+ //~^ ERROR higher kinded lifetime bounds on nested opaque types are not supported yet
  X(())
 }
 

src/test/ui/impl-trait/issues/issue-54895.stderr

@@ -0,0 +1,14 @@
+error: higher kinded lifetime bounds on nested opaque types are not supported yet
+ --> $DIR/issue-54895.rs:15:53
+ |
+LL | fn f() -> impl for<'a> Trait<'a, Out = impl Sized + 'a> {
+ | ^^
+ |
+note: lifetime declared here
+ --> $DIR/issue-54895.rs:15:20
+ |
+LL | fn f() -> impl for<'a> Trait<'a, Out = impl Sized + 'a> {
+ | ^^
+
+error: aborting due to previous error
+

src/test/ui/impl-trait/issues/issue-67830.rs

@@ -19,7 +19,7 @@ where
 
 struct A;
 fn test() -> impl for<'a> MyFn<&'a A, Output=impl Iterator + 'a> {
- //~^ ERROR implementation of `FnOnce` is not general enough
+ //~^ ERROR higher kinded lifetime bounds on nested opaque types are not supported yet
  Wrap(|a| Some(a).into_iter())
 }
 

src/test/ui/impl-trait/issues/issue-67830.stderr

@@ -1,11 +1,14 @@
-error: implementation of `FnOnce` is not general enough
- --> $DIR/issue-67830.rs:21:14
+error: higher kinded lifetime bounds on nested opaque types are not supported yet
+ --> $DIR/issue-67830.rs:21:62
  |
 LL | fn test() -> impl for<'a> MyFn<&'a A, Output=impl Iterator + 'a> {
- | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `FnOnce` is not general enough
+ |  ^^
  |
- = note: closure with signature `fn(&'2 A) -> std::option::IntoIter<&A>` must implement `FnOnce<(&'1 A,)>`, for any lifetime `'1`...
- = note: ...but it actually implements `FnOnce<(&'2 A,)>`, for some specific lifetime `'2`
+note: lifetime declared here
+ --> $DIR/issue-67830.rs:21:23
+ |
+LL | fn test() -> impl for<'a> MyFn<&'a A, Output=impl Iterator + 'a> {
+ | ^^
 
 error: aborting due to previous error
 

src/test/ui/impl-trait/issues/issue-88236-2.rs

@@ -13,11 +13,16 @@ impl<'a> Hrtb<'a> for &'a () {
 }
 
 fn make_impl() -> impl for<'a> Hrtb<'a, Assoc = impl Send + 'a> {}
+//~^ ERROR higher kinded lifetime bounds on nested opaque types are not supported yet
+
 fn make_weird_impl<'b>(x: &'b ()) -> impl for<'a> Hrtb<'a, Assoc = impl Send + 'a> {
- &() //~^ ERROR implementation of `Hrtb` is not general enough
+ //~^ ERROR higher kinded lifetime bounds on nested opaque types are not supported yet
+ &()
 }
+
 fn make_bad_impl<'b>(x: &'b ()) -> impl for<'a> Hrtb<'a, Assoc = impl Send + 'a> {
- x //~^ ERROR implementation of `Hrtb` is not general enough
+ //~^ ERROR higher kinded lifetime bounds on nested opaque types are not supported yet
+ x
 }
 
 fn main() {}

src/test/ui/impl-trait/issues/issue-88236-2.stderr

@@ -1,20 +1,38 @@
-error: implementation of `Hrtb` is not general enough
- --> $DIR/issue-88236-2.rs:16:38
+error: higher kinded lifetime bounds on nested opaque types are not supported yet
+ --> $DIR/issue-88236-2.rs:15:61
+ |
+LL | fn make_impl() -> impl for<'a> Hrtb<'a, Assoc = impl Send + 'a> {}
+ | ^^
+ |
+note: lifetime declared here
+ --> $DIR/issue-88236-2.rs:15:28
+ |
+LL | fn make_impl() -> impl for<'a> Hrtb<'a, Assoc = impl Send + 'a> {}
+ | ^^
+
+error: higher kinded lifetime bounds on nested opaque types are not supported yet
+ --> $DIR/issue-88236-2.rs:18:80
  |
 LL | fn make_weird_impl<'b>(x: &'b ()) -> impl for<'a> Hrtb<'a, Assoc = impl Send + 'a> {
- | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Hrtb` is not general enough
+ |  ^^
  |
- = note: `Hrtb<'0>` would have to be implemented for the type `&()`, for any lifetime `'0`...
- = note: ...but `Hrtb<'1>` is actually implemented for the type `&'1 ()`, for some specific lifetime `'1`
+note: lifetime declared here
+ --> $DIR/issue-88236-2.rs:18:47
+ |
+LL | fn make_weird_impl<'b>(x: &'b ()) -> impl for<'a> Hrtb<'a, Assoc = impl Send + 'a> {
+ | ^^
 
-error: implementation of `Hrtb` is not general enough
- --> $DIR/issue-88236-2.rs:19:36
+error: higher kinded lifetime bounds on nested opaque types are not supported yet
+ --> $DIR/issue-88236-2.rs:23:78
  |
 LL | fn make_bad_impl<'b>(x: &'b ()) -> impl for<'a> Hrtb<'a, Assoc = impl Send + 'a> {
- | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Hrtb` is not general enough
+ |  ^^
  |
- = note: `Hrtb<'1>` would have to be implemented for the type `&()`, for any lifetime `'1`...
- = note: ...but `Hrtb<'_>` is actually implemented for the type `&()`
+note: lifetime declared here
+ --> $DIR/issue-88236-2.rs:23:45
+ |
+LL | fn make_bad_impl<'b>(x: &'b ()) -> impl for<'a> Hrtb<'a, Assoc = impl Send + 'a> {
+ | ^^
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 

src/test/ui/impl-trait/issues/issue-88236.rs

@@ -1,5 +1,3 @@
-// check-pass
-
 // this used to cause stack overflows
 
 trait Hrtb<'a> {
@@ -15,5 +13,6 @@ impl<'a> Hrtb<'a> for &'a () {
 }
 
 fn make_impl() -> impl for<'a> Hrtb<'a, Assoc = impl Send + 'a> {}
+//~^ ERROR higher kinded lifetime bounds on nested opaque types are not supported yet
 
 fn main() {}

src/test/ui/impl-trait/issues/issue-88236.stderr

@@ -0,0 +1,14 @@
+error: higher kinded lifetime bounds on nested opaque types are not supported yet
+ --> $DIR/issue-88236.rs:15:61
+ |
+LL | fn make_impl() -> impl for<'a> Hrtb<'a, Assoc = impl Send + 'a> {}
+ | ^^
+ |
+note: lifetime declared here
+ --> $DIR/issue-88236.rs:15:28
+ |
+LL | fn make_impl() -> impl for<'a> Hrtb<'a, Assoc = impl Send + 'a> {}
+ | ^^
+
+error: aborting due to previous error
+

src/test/ui/impl-trait/nested-rpit-hrtb.rs

@@ -0,0 +1,64 @@
+// Test the interaction between rested RPIT and HRTB.
+
+trait Foo<'a> {
+ type Assoc;
+}
+
+impl Foo<'_> for () {
+ type Assoc = ();
+}
+
+// Alternative version of `Foo` whose impl uses `'a`.
+trait Bar<'a> {
+ type Assoc;
+}
+
+impl<'a> Bar<'a> for () {
+ type Assoc = &'a ();
+}
+
+trait Qux<'a> {}
+
+impl Qux<'_> for () {}
+
+// This is not supported.
+fn one_hrtb_outlives() -> impl for<'a> Foo<'a, Assoc = impl Sized + 'a> {}
+//~^ ERROR higher kinded lifetime bounds on nested opaque types are not supported yet
+
+// This is not supported.
+fn one_hrtb_trait_param() -> impl for<'a> Foo<'a, Assoc = impl Qux<'a>> {}
+//~^ ERROR higher kinded lifetime bounds on nested opaque types are not supported yet
+
+fn one_hrtb_outlives_uses() -> impl for<'a> Bar<'a, Assoc = impl Sized + 'a> {}
+//~^ ERROR higher kinded lifetime bounds on nested opaque types are not supported yet
+
+fn one_hrtb_trait_param_uses() -> impl for<'a> Bar<'a, Assoc = impl Qux<'a>> {}
+//~^ ERROR higher kinded lifetime bounds on nested opaque types are not supported yet
+
+// This should pass.
+fn one_hrtb_mention_fn_trait_param<'b>() -> impl for<'a> Foo<'a, Assoc = impl Qux<'b>> {}
+
+// This should pass.
+fn one_hrtb_mention_fn_outlives<'b>() -> impl for<'a> Foo<'a, Assoc = impl Sized + 'b> {}
+
+// This should pass.
+fn one_hrtb_mention_fn_trait_param_uses<'b>() -> impl for<'a> Bar<'a, Assoc = impl Qux<'b>> {}
+
+// This should pass.
+fn one_hrtb_mention_fn_outlives_uses<'b>() -> impl for<'a> Bar<'a, Assoc = impl Sized + 'b> {}
+
+// This should pass.
+fn two_htrb_trait_param() -> impl for<'a> Foo<'a, Assoc = impl for<'b> Qux<'b>> {}
+
+// `'b` is not in scope for the outlives bound.
+fn two_htrb_outlives() -> impl for<'a> Foo<'a, Assoc = impl for<'b> Sized + 'b> {}
+//~^ ERROR use of undeclared lifetime name `'b` [E0261]
+
+// This should pass.
+fn two_htrb_trait_param_uses() -> impl for<'a> Bar<'a, Assoc = impl for<'b> Qux<'b>> {}
+
+// `'b` is not in scope for the outlives bound.
+fn two_htrb_outlives_uses() -> impl for<'a> Bar<'a, Assoc = impl for<'b> Sized + 'b> {}
+//~^ ERROR use of undeclared lifetime name `'b` [E0261]
+
+fn main() {}