Rollup merge of #97656 - EdwinRy:error_ast_low_type_contraint_parentheses, r=cjgillot

Add a suggestion to replace parentheses with angle brackets on associated trait constraint

This implements a requested suggestion FIXME in [`compiler/rustc_ast_lowering/src/lib.rs` ](https://github.com/rust-lang/rust/blob/9598b4b594c97dff66feb93522e22db500deea07/compiler/rustc_ast_lowering/src/lib.rs#L921)

The suggestion asks for the parentheses to either be replaced with angle brackets or removed completely depending on whether there are arguments provided within.

![image](https://user-images.githubusercontent.com/20026256/171770414-ab60bfe2-ae27-44b0-964d-9ffcc32a7475.png)

![image](https://user-images.githubusercontent.com/20026256/171770383-ca8a2766-b3aa-43e3-8ba4-ae18874886ce.png)

r? `@oli-obk`

2 files changed, 35 insertions, 1 deletions

diff --git a/src/test/ui/generic-associated-types/gat-trait-path-parenthesised-args.rs b/src/test/ui/generic-associated-types/gat-trait-path-parenthesised-args.rs
index f01da8c61ed..c55b0530c9d 100644
--- a/src/test/ui/generic-associated-types/gat-trait-path-parenthesised-args.rs
+++ b/src/test/ui/generic-associated-types/gat-trait-path-parenthesised-args.rs
@@ -10,4 +10,9 @@ fn foo<'a>(arg: Box<dyn X<Y('a) = &'a ()>>) {}
   //~| ERROR this associated type takes 0 generic arguments but 1 generic argument
   //~| ERROR this associated type takes 1 lifetime argument but 0 lifetime arguments
 
+
+fn bar<'a>(arg: Box<dyn X<Y() = ()>>) {}
+  //~^ ERROR: parenthesized generic arguments cannot be used
+  //~| ERROR this associated type takes 1 lifetime argument but 0 lifetime arguments
+
 fn main() {}
diff --git a/src/test/ui/generic-associated-types/gat-trait-path-parenthesised-args.stderr b/src/test/ui/generic-associated-types/gat-trait-path-parenthesised-args.stderr
index 6014a02c4d9..162214063e7 100644
--- a/src/test/ui/generic-associated-types/gat-trait-path-parenthesised-args.stderr
+++ b/src/test/ui/generic-associated-types/gat-trait-path-parenthesised-args.stderr
@@ -9,6 +9,19 @@ error: parenthesized generic arguments cannot be used in associated type constra
    |
 LL | fn foo<'a>(arg: Box<dyn X<Y('a) = &'a ()>>) {}
    |                           ^^^^^
+   |
+help: use angle brackets instead
+   |
+LL | fn foo<'a>(arg: Box<dyn X<Y<'a> = &'a ()>>) {}
+   |                            ~  ~
+
+error: parenthesized generic arguments cannot be used in associated type constraints
+  --> $DIR/gat-trait-path-parenthesised-args.rs:14:27
+   |
+LL | fn bar<'a>(arg: Box<dyn X<Y() = ()>>) {}
+   |                           ^--
+   |                            |
+   |                            help: remove these parentheses
 
 error[E0107]: this associated type takes 1 lifetime argument but 0 lifetime arguments were supplied
   --> $DIR/gat-trait-path-parenthesised-args.rs:7:27
@@ -40,6 +53,22 @@ note: associated type defined here, with 0 generic parameters
 LL |   type Y<'a>;
    |        ^
 
-error: aborting due to 4 previous errors
+error[E0107]: this associated type takes 1 lifetime argument but 0 lifetime arguments were supplied
+  --> $DIR/gat-trait-path-parenthesised-args.rs:14:27
+   |
+LL | fn bar<'a>(arg: Box<dyn X<Y() = ()>>) {}
+   |                           ^ expected 1 lifetime argument
+   |
+note: associated type defined here, with 1 lifetime parameter: `'a`
+  --> $DIR/gat-trait-path-parenthesised-args.rs:4:8
+   |
+LL |   type Y<'a>;
+   |        ^ --
+help: add missing lifetime argument
+   |
+LL | fn bar<'a>(arg: Box<dyn X<Y('a) = ()>>) {}
+   |                             ++
+
+error: aborting due to 6 previous errors
 
 For more information about this error, try `rustc --explain E0107`.