Forbid nested opaque types to reference HRTB from opaque types.

compiler/rustc_resolve/src/late/lifetimes.rs

@@ -1036,6 +1036,20 @@ 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 +1,66 @@
+// 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]
+//~| ERROR lifetime name `'b` shadows a lifetime name that is already in scope [E0496]
+
+// 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]
+//~| ERROR lifetime name `'b` shadows a lifetime name that is already in scope [E0496]
+
+fn main() {}

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

@@ -0,0 +1,80 @@
+error: higher kinded lifetime bounds on nested opaque types are not supported yet
+  --> $DIR/nested-rpit-hrtb.rs:25:69
+   |
+LL | fn one_hrtb_outlives() -> impl for<'a> Foo<'a, Assoc = impl Sized + 'a> {}
+   |                                                                     ^^
+   |
+note: lifetime declared here
+  --> $DIR/nested-rpit-hrtb.rs:25:36
+   |
+LL | 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
+  --> $DIR/nested-rpit-hrtb.rs:29:68
+   |
+LL | fn one_hrtb_trait_param() -> impl for<'a> Foo<'a, Assoc = impl Qux<'a>> {}
+   |                                                                    ^^
+   |
+note: lifetime declared here
+  --> $DIR/nested-rpit-hrtb.rs:29:39
+   |
+LL | 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
+  --> $DIR/nested-rpit-hrtb.rs:32:74
+   |
+LL | fn one_hrtb_outlives_uses() -> impl for<'a> Bar<'a, Assoc = impl Sized + 'a> {}
+   |                                                                          ^^
+   |
+note: lifetime declared here
+  --> $DIR/nested-rpit-hrtb.rs:32:41
+   |
+LL | 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
+  --> $DIR/nested-rpit-hrtb.rs:35:73
+   |
+LL | fn one_hrtb_trait_param_uses() -> impl for<'a> Bar<'a, Assoc = impl Qux<'a>> {}
+   |                                                                         ^^
+   |
+note: lifetime declared here
+  --> $DIR/nested-rpit-hrtb.rs:35:44
+   |
+LL | fn one_hrtb_trait_param_uses() -> impl for<'a> Bar<'a, Assoc = impl Qux<'a>> {}
+   |                                            ^^
+
+error[E0261]: use of undeclared lifetime name `'b`
+  --> $DIR/nested-rpit-hrtb.rs:54:77
+   |
+LL | fn two_htrb_outlives() -> impl for<'a> Foo<'a, Assoc = impl for<'b> Sized + 'b> {}
+   |                     - help: consider introducing lifetime `'b` here: `<'b>` ^^ undeclared lifetime
+
+error[E0496]: lifetime name `'b` shadows a lifetime name that is already in scope
+  --> $DIR/nested-rpit-hrtb.rs:54:65
+   |
+LL | fn two_htrb_outlives() -> impl for<'a> Foo<'a, Assoc = impl for<'b> Sized + 'b> {}
+   |                                                                 ^^          -- first declared here
+   |                                                                 |
+   |                                                                 lifetime `'b` already in scope
+
+error[E0261]: use of undeclared lifetime name `'b`
+  --> $DIR/nested-rpit-hrtb.rs:62:82
+   |
+LL | fn two_htrb_outlives_uses() -> impl for<'a> Bar<'a, Assoc = impl for<'b> Sized + 'b> {}
+   |                          - help: consider introducing lifetime `'b` here: `<'b>` ^^ undeclared lifetime
+
+error[E0496]: lifetime name `'b` shadows a lifetime name that is already in scope
+  --> $DIR/nested-rpit-hrtb.rs:62:70
+   |
+LL | fn two_htrb_outlives_uses() -> impl for<'a> Bar<'a, Assoc = impl for<'b> Sized + 'b> {}
+   |                                                                      ^^          -- first declared here
+   |                                                                      |
+   |                                                                      lifetime `'b` already in scope
+
+error: aborting due to 8 previous errors
+
+Some errors have detailed explanations: E0261, E0496.
+For more information about an error, try `rustc --explain E0261`.