Rollup merge of #88336 - jackh726:gats-where-constraints, r=estebank

 Detect stricter constraints on gats where clauses in impls vs trait

I might try to see if I can do a bit more to improve these diagnostics, but any initial feedback is appreciated. I can also do any additional work in a followup PR.

r? `@estebank`

5 files changed, 33 insertions, 17 deletions

diff --git a/src/test/ui/generic-associated-types/impl_bounds.rs b/src/test/ui/generic-associated-types/impl_bounds.rs
index 01edad00a89..27c135cb7cf 100644
--- a/src/test/ui/generic-associated-types/impl_bounds.rs
+++ b/src/test/ui/generic-associated-types/impl_bounds.rs
@@ -15,7 +15,7 @@ impl<T> Foo for Fooy<T> {
     type A<'a> where Self: 'static = (&'a ());
     //~^ ERROR the parameter type `T` may not live long enough
     type B<'a, 'b> where 'b: 'a = (&'a(), &'b ());
-    //~^ ERROR lifetime bound not satisfied
+    //~^ ERROR `impl` associated type
     //~| ERROR lifetime bound not satisfied
     type C where Self: Copy = String;
     //~^ ERROR the trait bound `T: Copy` is not satisfied
diff --git a/src/test/ui/generic-associated-types/impl_bounds.stderr b/src/test/ui/generic-associated-types/impl_bounds.stderr
index 8cf923ca3ac..73415e0faac 100644
--- a/src/test/ui/generic-associated-types/impl_bounds.stderr
+++ b/src/test/ui/generic-associated-types/impl_bounds.stderr
@@ -5,24 +5,16 @@ LL |     type A<'a> where Self: 'static = (&'a ());
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: consider adding an explicit lifetime bound `T: 'static`...
-   = note: ...so that the type `Fooy<T>` will meet its required lifetime bounds
+   = note: ...so that the definition in impl matches the definition from the trait
 
-error[E0478]: lifetime bound not satisfied
+error: `impl` associated type signature for `B` doesn't match `trait` associated type signature
   --> $DIR/impl_bounds.rs:17:5
    |
+LL |     type B<'a, 'b> where 'a: 'b;
+   |     ---------------------------- expected
+...
 LL |     type B<'a, 'b> where 'b: 'a = (&'a(), &'b ());
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-note: lifetime parameter instantiated with the lifetime `'b` as defined on the associated item at 17:16
-  --> $DIR/impl_bounds.rs:17:16
-   |
-LL |     type B<'a, 'b> where 'b: 'a = (&'a(), &'b ());
-   |                ^^
-note: but lifetime parameter must outlive the lifetime `'a` as defined on the associated item at 17:12
-  --> $DIR/impl_bounds.rs:17:12
-   |
-LL |     type B<'a, 'b> where 'b: 'a = (&'a(), &'b ());
-   |            ^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ found
 
 error[E0478]: lifetime bound not satisfied
   --> $DIR/impl_bounds.rs:17:5
diff --git a/src/test/ui/generic-associated-types/issue-86787.stderr b/src/test/ui/generic-associated-types/issue-86787.stderr
index 04cd84b0801..e1ff7729211 100644
--- a/src/test/ui/generic-associated-types/issue-86787.stderr
+++ b/src/test/ui/generic-associated-types/issue-86787.stderr
@@ -9,7 +9,7 @@ LL | |     <Left as HasChildrenOf>::T: 'a,
 LL | |     <Right as HasChildrenOf>::T: 'a
    | |                                    - help: consider adding a where clause: `, <Left as HasChildrenOf>::T: 'a`
 LL | |     = Either<&'a Left::T, &'a Right::T>;
-   | |________________________________________^ ...so that the type `<Left as HasChildrenOf>::T` will meet its required lifetime bounds
+   | |________________________________________^ ...so that the definition in impl matches the definition from the trait
 
 error[E0309]: the associated type `<Right as HasChildrenOf>::T` may not live long enough
   --> $DIR/issue-86787.rs:23:5
@@ -22,7 +22,7 @@ LL | |     <Left as HasChildrenOf>::T: 'a,
 LL | |     <Right as HasChildrenOf>::T: 'a
    | |                                    - help: consider adding a where clause: `, <Right as HasChildrenOf>::T: 'a`
 LL | |     = Either<&'a Left::T, &'a Right::T>;
-   | |________________________________________^ ...so that the type `<Right as HasChildrenOf>::T` will meet its required lifetime bounds
+   | |________________________________________^ ...so that the definition in impl matches the definition from the trait
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/generic-associated-types/missing-where-clause-on-trait.rs b/src/test/ui/generic-associated-types/missing-where-clause-on-trait.rs
new file mode 100644
index 00000000000..ad9f2e3e4ec
--- /dev/null
+++ b/src/test/ui/generic-associated-types/missing-where-clause-on-trait.rs
@@ -0,0 +1,13 @@
+// check-fail
+
+#![feature(generic_associated_types)]
+
+trait Foo {
+    type Assoc<'a, 'b>;
+}
+impl Foo for () {
+    type Assoc<'a, 'b> where 'a: 'b = ();
+    //~^ `impl` associated type
+}
+
+fn main() {}
diff --git a/src/test/ui/generic-associated-types/missing-where-clause-on-trait.stderr b/src/test/ui/generic-associated-types/missing-where-clause-on-trait.stderr
new file mode 100644
index 00000000000..0e183c8d69a
--- /dev/null
+++ b/src/test/ui/generic-associated-types/missing-where-clause-on-trait.stderr
@@ -0,0 +1,11 @@
+error: `impl` associated type signature for `Assoc` doesn't match `trait` associated type signature
+  --> $DIR/missing-where-clause-on-trait.rs:9:5
+   |
+LL |     type Assoc<'a, 'b>;
+   |     ------------------- expected
+...
+LL |     type Assoc<'a, 'b> where 'a: 'b = ();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ found
+
+error: aborting due to previous error
+