More tests and tweak comments

compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs

@@ -1640,9 +1640,9 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
             // }
             // ```
             // and a bound that looks like:
-            //    `for<'a> T::Trait<'a, x(): for<'b> Other<'b>>`
+            //    `for<'a> T::Trait<'a, x(..): for<'b> Other<'b>>`
             // this is going to expand to something like:
-            //    `for<'a> for<'r, T> <T as Trait<'a>>::x::<'r, T>::{opaque#0}: for<'b> Other<'b>`.
+            //    `for<'a> for<'r> <T as Trait<'a>>::x::<'r, T>::{opaque#0}: for<'b> Other<'b>`.
             if constraint.gen_args.parenthesized == hir::GenericArgsParentheses::ReturnTypeNotation
             {
                 let bound_vars = if let Some(type_def_id) = type_def_id
@@ -1853,13 +1853,13 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
     // For example, given
     // ```
     // trait Foo {
-    //     async fn x<'r, T>();
+    //     async fn x<'r>();
     // }
     // ```
     // and a bound that looks like:
     //    `for<'a, 'b> <T as Trait<'a>>::x(): Other<'b>`
     // this is going to expand to something like:
-    //    `for<'a, 'b, 'r, T> <T as Trait<'a>>::x::<'r, T>::{opaque#0}: Other<'b>`.
+    //    `for<'a, 'b, 'r> <T as Trait<'a>>::x::<'r, T>::{opaque#0}: Other<'b>`.
     //
     // We handle this similarly for associated-type-bound style return-type-notation
     // in `visit_segment_args`.
@@ -2000,6 +2000,17 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
         // the new bound vars. We do this because we need to know how many bound vars
         // are present on the binder explicitly (i.e. not return-type-notation vars)
         // to do bound var shifting correctly in HIR lowering.
+        //
+        // For example, in `where for<'a> <T as Trait<'a>>::method(..): Other`,
+        // the `late_bound_vars` of the where clause predicate (i.e. this HIR ty's
+        // parent) will include `'a` AND all the early- and late-bound vars of the
+        // method. But when lowering the RTN type, we just want the list of vars
+        // we used to resolve the trait ref. We explicitly stored those back onto
+        // the item segment, since there's no other good place to put them.
+        //
+        // See where these vars are used in `HirTyLowerer::lower_ty_maybe_return_type_notation`.
+        // And this is exercised in:
+        // `tests/ui/associated-type-bounds/return-type-notation/higher-ranked-bound-works.rs`.
         let existing_bound_vars = self.map.late_bound_vars.get_mut(&hir_id).unwrap();
         let existing_bound_vars_saved = existing_bound_vars.clone();
         existing_bound_vars.extend(bound_vars);

tests/ui/associated-type-bounds/return-type-notation/bad-inputs-and-output.rs

@@ -16,4 +16,22 @@ fn bar<T: Trait<method() -> (): Send>>() {}
 fn baz<T: Trait<method(): Send>>() {}
 //~^ ERROR return type notation arguments must be elided with `..`
 
+fn foo_path<T: Trait>() where T::method(i32): Send {}
+//~^ ERROR argument types not allowed with return type notation
+
+fn bar_path<T: Trait>() where T::method() -> (): Send {}
+//~^ ERROR return type not allowed with return type notation
+
+fn baz_path<T: Trait>() where T::method(): Send {}
+//~^ ERROR return type notation arguments must be elided with `..`
+
+fn foo_qualified<T: Trait>() where <T as Trait>::method(i32): Send {}
+//~^ ERROR expected associated type
+
+fn bar_qualified<T: Trait>() where <T as Trait>::method() -> (): Send {}
+//~^ ERROR expected associated type
+
+fn baz_qualified<T: Trait>() where <T as Trait>::method(): Send {}
+//~^ ERROR expected associated type
+
 fn main() {}

tests/ui/associated-type-bounds/return-type-notation/bad-inputs-and-output.stderr

@@ -1,3 +1,21 @@
+error[E0575]: expected associated type, found associated function `Trait::method`
+  --> $DIR/bad-inputs-and-output.rs:28:36
+   |
+LL | fn foo_qualified<T: Trait>() where <T as Trait>::method(i32): Send {}
+   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^ not a associated type
+
+error[E0575]: expected associated type, found associated function `Trait::method`
+  --> $DIR/bad-inputs-and-output.rs:31:36
+   |
+LL | fn bar_qualified<T: Trait>() where <T as Trait>::method() -> (): Send {}
+   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not a associated type
+
+error[E0575]: expected associated type, found associated function `Trait::method`
+  --> $DIR/bad-inputs-and-output.rs:34:36
+   |
+LL | fn baz_qualified<T: Trait>() where <T as Trait>::method(): Send {}
+   |                                    ^^^^^^^^^^^^^^^^^^^^^^ not a associated type
+
 warning: the feature `return_type_notation` is incomplete and may not be safe to use and/or cause compiler crashes
   --> $DIR/bad-inputs-and-output.rs:3:12
    |
@@ -25,5 +43,24 @@ error: return type notation arguments must be elided with `..`
 LL | fn baz<T: Trait<method(): Send>>() {}
    |                       ^^ help: add `..`: `(..)`
 
-error: aborting due to 3 previous errors; 1 warning emitted
+error: argument types not allowed with return type notation
+  --> $DIR/bad-inputs-and-output.rs:19:40
+   |
+LL | fn foo_path<T: Trait>() where T::method(i32): Send {}
+   |                                        ^^^^^ help: remove the input types: `()`
+
+error: return type not allowed with return type notation
+  --> $DIR/bad-inputs-and-output.rs:22:42
+   |
+LL | fn bar_path<T: Trait>() where T::method() -> (): Send {}
+   |                                          ^^^^^^ help: remove the return type
+
+error: return type notation arguments must be elided with `..`
+  --> $DIR/bad-inputs-and-output.rs:25:40
+   |
+LL | fn baz_path<T: Trait>() where T::method(): Send {}
+   |                                        ^^ help: add `..`: `(..)`
+
+error: aborting due to 9 previous errors; 1 warning emitted
 
+For more information about this error, try `rustc --explain E0575`.

tests/ui/associated-type-bounds/return-type-notation/higher-ranked-bound-works.rs

@@ -0,0 +1,52 @@
+//@ check-pass
+
+#![feature(return_type_notation)]
+//~^ WARN the feature `return_type_notation` is incomplete
+
+trait Trait<'a> {
+    fn late<'b>(&'b self, _: &'a ()) -> impl Sized;
+    fn early<'b: 'b>(&'b self, _: &'a ()) -> impl Sized;
+}
+
+#[allow(refining_impl_trait_internal)]
+impl<'a> Trait<'a> for () {
+    fn late<'b>(&'b self, _: &'a ()) -> i32 { 1 }
+    fn early<'b: 'b>(&'b self, _: &'a ()) -> i32 { 1 }
+}
+
+trait Other<'c> {}
+impl Other<'_> for i32 {}
+
+fn test<T>(t: &T)
+where
+    T: for<'a, 'c> Trait<'a, late(..): Other<'c>>,
+    // which is basically:
+    // for<'a, 'c> Trait<'a, for<'b> method<'b>: Other<'c>>,
+    T: for<'a, 'c> Trait<'a, early(..): Other<'c>>,
+    // which is basically:
+    // for<'a, 'c> Trait<'a, for<'b> method<'b>: Other<'c>>,
+{
+    is_other_impl(t.late(&()));
+    is_other_impl(t.early(&()));
+}
+
+fn test_path<T>(t: &T)
+where
+T: for<'a> Trait<'a>,
+    for<'a, 'c> <T as Trait<'a>>::late(..): Other<'c>,
+    // which is basically:
+    // for<'a, 'b, 'c> <T as Trait<'a>>::method::<'b>: Other<'c>
+    for<'a, 'c> <T as Trait<'a>>::early(..): Other<'c>,
+    // which is basically:
+    // for<'a, 'b, 'c> <T as Trait<'a>>::method::<'b>: Other<'c>
+{
+    is_other_impl(t.late(&()));
+    is_other_impl(t.early(&()));
+}
+
+fn is_other_impl(_: impl for<'c> Other<'c>) {}
+
+fn main() {
+    test(&());
+    test(&());
+}

tests/ui/associated-type-bounds/return-type-notation/higher-ranked-bound-works.stderr

@@ -0,0 +1,11 @@
+warning: the feature `return_type_notation` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/higher-ranked-bound-works.rs:3:12
+   |
+LL | #![feature(return_type_notation)]
+   |            ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #109417 </~https://github.com/rust-lang/rust/issues/109417> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+warning: 1 warning emitted
+

tests/ui/associated-type-bounds/return-type-notation/namespace-conflict.rs

@@ -0,0 +1,42 @@
+//@ check-pass
+
+#![allow(non_camel_case_types)]
+#![feature(return_type_notation)]
+//~^ WARN the feature `return_type_notation` is incomplete
+
+trait Foo {
+    type test;
+
+    fn test() -> impl Bar;
+}
+
+fn call_path<T: Foo>()
+where
+    T::test(..): Bar,
+{
+}
+
+fn call_bound<T: Foo<test(..): Bar>>() {}
+
+trait Bar {}
+struct NotBar;
+struct YesBar;
+impl Bar for YesBar {}
+
+impl Foo for () {
+    type test = NotBar;
+
+    // Use refinement here so we can observe `YesBar: Bar`.
+    #[allow(refining_impl_trait_internal)]
+    fn test() -> YesBar {
+        YesBar
+    }
+}
+
+fn main() {
+    // If `T::test(..)` resolved to the GAT (erroneously), then this would be
+    // an error since `<() as Foo>::bar` -- the associated type -- does not
+    // implement `Bar`, but the return type of the method does.
+    call_path::<()>();
+    call_bound::<()>();
+}

tests/ui/associated-type-bounds/return-type-notation/namespace-conflict.stderr

@@ -0,0 +1,11 @@
+warning: the feature `return_type_notation` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/namespace-conflict.rs:4:12
+   |
+LL | #![feature(return_type_notation)]
+   |            ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #109417 </~https://github.com/rust-lang/rust/issues/109417> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+warning: 1 warning emitted
+

tests/ui/associated-type-bounds/return-type-notation/non-rpitit.rs

@@ -5,7 +5,10 @@ trait Trait {
     fn method() {}
 }
 
-fn test<T: Trait<method(..): Send>>() {}
-//~^ ERROR  return type notation used on function that is not `async` and does not return `impl Trait`
+fn bound<T: Trait<method(..): Send>>() {}
+//~^ ERROR return type notation used on function that is not `async` and does not return `impl Trait`
+
+fn path<T>() where T: Trait, T::method(..): Send {}
+//~^ ERROR return type notation used on function that is not `async` and does not return `impl Trait`
 
 fn main() {}

tests/ui/associated-type-bounds/return-type-notation/non-rpitit.stderr

@@ -8,15 +8,26 @@ LL | #![feature(return_type_notation)]
    = note: `#[warn(incomplete_features)]` on by default
 
 error: return type notation used on function that is not `async` and does not return `impl Trait`
-  --> $DIR/non-rpitit.rs:8:18
+  --> $DIR/non-rpitit.rs:8:19
    |
 LL |     fn method() {}
    |     ----------- this function must be `async` or return `impl Trait`
 ...
-LL | fn test<T: Trait<method(..): Send>>() {}
-   |                  ^^^^^^^^^^^^^^^^
+LL | fn bound<T: Trait<method(..): Send>>() {}
+   |                   ^^^^^^^^^^^^^^^^
    |
    = note: function returns `()`, which is not compatible with associated type return bounds
 
-error: aborting due to 1 previous error; 1 warning emitted
+error: return type notation used on function that is not `async` and does not return `impl Trait`
+  --> $DIR/non-rpitit.rs:11:30
+   |
+LL |     fn method() {}
+   |     ----------- this function must be `async` or return `impl Trait`
+...
+LL | fn path<T>() where T: Trait, T::method(..): Send {}
+   |                              ^^^^^^^^^^^^^
+   |
+   = note: function returns `()`, which is not compatible with associated type return bounds
+
+error: aborting due to 2 previous errors; 1 warning emitted