Rollup merge of #76695 - iximeow:trait-generic-bound-suggestion, r=estebank

fix syntax error in suggesting generic constraint in trait parameter

suggest `where T: Foo` for the first bound on a trait, then suggest
`, T: Foo` when the suggested bound would add to an existing set of
`where` clauses. `where T: Foo` may be the first bound if `T` has a
default, because we'd rather suggest
```
trait A<T=()> where T: Copy
```
than
```
trait A<T: Copy=()>
```
for legibility reasons.

the test case i added here is derived from [this reproduction](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=0bf3ace9f2a183d0bdbd748c6b8e3971):
```
struct B<T: Copy> {
    t: T
}

trait A<T = ()> {
    fn returns_constrained_type(&self, t: T) -> B<T> {
        B { t }
    }
}
```
where the suggested fix,
```
trait A<T = ()>, T: Copy { ... }
```
is in fact invalid syntax!

i also found an error in the existing suggestion for `trait Base<T = String>: Super<T>` where rustc would suggest `trait Base<T = String>: Super<T>, T: Copy`, but `T: Copy` is the first of the trait's `where` clauses and should be `where T: Copy` as well. the test for that suggestion expects invalid syntax, and has been revised to a compiler-pleasing `trait Base<T = String>: Super<T> where T: Copy`.

judging by https://github.com/rust-lang/rust/pull/70009 i'll.. cc @estebank ?

4 files changed, 59 insertions, 2 deletions

diff --git a/src/test/ui/trait-impl-bound-suggestions.fixed b/src/test/ui/trait-impl-bound-suggestions.fixed
new file mode 100644
index 00000000000..db3a95f5c4f
--- /dev/null
+++ b/src/test/ui/trait-impl-bound-suggestions.fixed
@@ -0,0 +1,20 @@
+// run-rustfix
+
+#[allow(unused)]
+use std::fmt::Debug;
+// Rustfix should add this, or use `std::fmt::Debug` instead.
+
+#[allow(dead_code)]
+struct ConstrainedStruct<X: Copy> {
+    x: X
+}
+
+#[allow(dead_code)]
+trait InsufficientlyConstrainedGeneric<X=()> where X: Copy {
+    fn return_the_constrained_type(&self, x: X) -> ConstrainedStruct<X> {
+        //~^ ERROR the trait bound `X: Copy` is not satisfied
+        ConstrainedStruct { x }
+    }
+}
+
+pub fn main() { }
diff --git a/src/test/ui/trait-impl-bound-suggestions.rs b/src/test/ui/trait-impl-bound-suggestions.rs
new file mode 100644
index 00000000000..bf75175179e
--- /dev/null
+++ b/src/test/ui/trait-impl-bound-suggestions.rs
@@ -0,0 +1,20 @@
+// run-rustfix
+
+#[allow(unused)]
+use std::fmt::Debug;
+// Rustfix should add this, or use `std::fmt::Debug` instead.
+
+#[allow(dead_code)]
+struct ConstrainedStruct<X: Copy> {
+    x: X
+}
+
+#[allow(dead_code)]
+trait InsufficientlyConstrainedGeneric<X=()> {
+    fn return_the_constrained_type(&self, x: X) -> ConstrainedStruct<X> {
+        //~^ ERROR the trait bound `X: Copy` is not satisfied
+        ConstrainedStruct { x }
+    }
+}
+
+pub fn main() { }
diff --git a/src/test/ui/trait-impl-bound-suggestions.stderr b/src/test/ui/trait-impl-bound-suggestions.stderr
new file mode 100644
index 00000000000..3a21e9c6b2a
--- /dev/null
+++ b/src/test/ui/trait-impl-bound-suggestions.stderr
@@ -0,0 +1,17 @@
+error[E0277]: the trait bound `X: Copy` is not satisfied
+  --> $DIR/trait-impl-bound-suggestions.rs:14:52
+   |
+LL | struct ConstrainedStruct<X: Copy> {
+   |                             ---- required by this bound in `ConstrainedStruct`
+...
+LL |     fn return_the_constrained_type(&self, x: X) -> ConstrainedStruct<X> {
+   |                                                    ^^^^^^^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `X`
+   |
+help: consider further restricting type parameter `X`
+   |
+LL | trait InsufficientlyConstrainedGeneric<X=()> where X: Copy {
+   |                                              ^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/type/type-check-defaults.stderr b/src/test/ui/type/type-check-defaults.stderr
index fa6f3422410..d8c7f595e62 100644
--- a/src/test/ui/type/type-check-defaults.stderr
+++ b/src/test/ui/type/type-check-defaults.stderr
@@ -56,8 +56,8 @@ LL | trait Base<T = String>: Super<T> { }
    |
 help: consider further restricting type parameter `T`
    |
-LL | trait Base<T = String>: Super<T>, T: Copy { }
-   |                                 ^^^^^^^^^
+LL | trait Base<T = String>: Super<T> where T: Copy { }
+   |                                  ^^^^^^^^^^^^^
 
 error[E0277]: cannot add `u8` to `i32`
   --> $DIR/type-check-defaults.rs:24:66