On incorrect equality constraint likely to be assoc type, suggest appropriate syntax

When encountering `where <A as Foo>::Bar = B`, it is possible that `Bar`
is an associated type. If so, suggest `where A: Foo<Bar = B>`.

CC #20041.

3 files changed, 51 insertions, 1 deletions

diff --git a/src/test/ui/generic-associated-types/missing-bounds.fixed b/src/test/ui/generic-associated-types/missing-bounds.fixed
index c2e619d1a7e..364d2388741 100644
--- a/src/test/ui/generic-associated-types/missing-bounds.fixed
+++ b/src/test/ui/generic-associated-types/missing-bounds.fixed
@@ -32,4 +32,15 @@ impl<B: std::ops::Add<Output = B>> Add for D<B> {
     }
 }
 
+struct E<B>(B);
+
+impl<B: Add> Add for E<B> where B: Add<Output = B>, B: std::ops::Add<Output = B> {
+    //~^ ERROR equality constraints are not yet supported in `where` clauses
+    type Output = Self;
+
+    fn add(self, rhs: Self) -> Self {
+        Self(self.0 + rhs.0) //~ ERROR mismatched types
+    }
+}
+
 fn main() {}
diff --git a/src/test/ui/generic-associated-types/missing-bounds.rs b/src/test/ui/generic-associated-types/missing-bounds.rs
index 1852ef62fe6..ffafff5e9f5 100644
--- a/src/test/ui/generic-associated-types/missing-bounds.rs
+++ b/src/test/ui/generic-associated-types/missing-bounds.rs
@@ -32,4 +32,15 @@ impl<B> Add for D<B> {
     }
 }
 
+struct E<B>(B);
+
+impl<B: Add> Add for E<B> where <B as Add>::Output = B {
+    //~^ ERROR equality constraints are not yet supported in `where` clauses
+    type Output = Self;
+
+    fn add(self, rhs: Self) -> Self {
+        Self(self.0 + rhs.0) //~ ERROR mismatched types
+    }
+}
+
 fn main() {}
diff --git a/src/test/ui/generic-associated-types/missing-bounds.stderr b/src/test/ui/generic-associated-types/missing-bounds.stderr
index 630ceac093e..50536fdaca9 100644
--- a/src/test/ui/generic-associated-types/missing-bounds.stderr
+++ b/src/test/ui/generic-associated-types/missing-bounds.stderr
@@ -1,3 +1,15 @@
+error: equality constraints are not yet supported in `where` clauses
+  --> $DIR/missing-bounds.rs:37:33
+   |
+LL | impl<B: Add> Add for E<B> where <B as Add>::Output = B {
+   |                                 ^^^^^^^^^^^^^^^^^^^^^^ not supported
+   |
+   = note: see issue #20041 <https://github.com/rust-lang/rust/issues/20041> for more information
+help: if `Output` is an associated type you're trying to set, use the associated type binding syntax
+   |
+LL | impl<B: Add> Add for E<B> where B: Add<Output = B> {
+   |                                 ^^^^^^^^^^^^^^^^^^
+
 error[E0308]: mismatched types
   --> $DIR/missing-bounds.rs:11:11
    |
@@ -43,7 +55,23 @@ help: consider restricting type parameter `B`
 LL | impl<B: std::ops::Add<Output = B>> Add for D<B> {
    |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 3 previous errors
+error[E0308]: mismatched types
+  --> $DIR/missing-bounds.rs:42:14
+   |
+LL | impl<B: Add> Add for E<B> where <B as Add>::Output = B {
+   |      - this type parameter
+...
+LL |         Self(self.0 + rhs.0)
+   |              ^^^^^^^^^^^^^^ expected type parameter `B`, found associated type
+   |
+   = note: expected type parameter `B`
+             found associated type `<B as std::ops::Add>::Output`
+help: consider further restricting type parameter `B`
+   |
+LL | impl<B: Add> Add for E<B> where <B as Add>::Output = B, B: std::ops::Add<Output = B> {
+   |                                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 5 previous errors
 
 Some errors have detailed explanations: E0308, E0369.
 For more information about an error, try `rustc --explain E0308`.