Rollup merge of #106712 - Ezrashaw:impl-ref-trait, r=estebank

make error emitted on `impl &Trait` nicer

Fixes #106694

Turned out to be simpler than I thought, also added UI test.

Before: ([playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=9bda53271ef3a8886793cf427b8cea91))
```text
error: expected one of `:`, ``@`,` or `|`, found `)`
 --> src/main.rs:2:22
  |
2 | fn foo(_: impl &Trait) {}
  |                      ^ expected one of `:`, ``@`,` or `|`
  |
  = note: anonymous parameters are removed in the 2018 edition (see RFC 1685)
help: if this is a parameter name, give it a type
  |
2 | fn foo(_: impl Trait: &TypeName) {}
  |                ~~~~~~~~~~~~~~~~
help: if this is a type, explicitly ignore the parameter name
  |
2 | fn foo(_: impl _: &Trait) {}
  |                ++

error: expected one of `!`, `(`, `)`, `,`, `?`, `for`, `~`, lifetime, or path, found `&`
 --> src/main.rs:2:16
  |
2 | fn foo(_: impl &Trait) {}
  |               -^ expected one of 9 possible tokens
  |               |
  |               help: missing `,`

error: expected one of `!`, `(`, `,`, `=`, `>`, `?`, `for`, `~`, lifetime, or path, found `&`
 --> src/main.rs:3:11
  |
3 | fn bar<T: &Trait>(_: T) {}
  |           ^ expected one of 10 possible tokens
```

After:
```text
error: expected a trait, found type
 --> <anon>:2:16
  |
2 | fn foo(_: impl &Trait) {}
  |                -^^^^^
  |                |
  |                help: consider removing the indirection

error: expected a trait, found type
 --> <anon>:3:11
  |
3 | fn bar<T: &Trait>(_: T) {}
  |           -^^^^^
  |           |
  |           help: consider removing the indirection
```

2 files changed, 117 insertions, 0 deletions

diff --git a/tests/ui/generics/issue-106694.rs b/tests/ui/generics/issue-106694.rs
new file mode 100644
index 00000000000..c4b02ee81ec
--- /dev/null
+++ b/tests/ui/generics/issue-106694.rs
@@ -0,0 +1,24 @@
+trait Trait {}
+
+fn foo(_: impl &Trait) {}
+//~^ ERROR expected a trait, found type
+
+fn bar<T: &Trait>(_: T) {}
+//~^ ERROR expected a trait, found type
+
+fn partially_correct_impl(_: impl &*const &Trait + Copy) {}
+//~^ ERROR expected a trait, found type
+
+fn foo_bad(_: impl &BadTrait) {}
+//~^ ERROR expected a trait, found type
+//~^^ ERROR cannot find trait `BadTrait` in this scope
+
+fn bar_bad<T: &BadTrait>(_: T) {}
+//~^ ERROR expected a trait, found type
+//~^^ ERROR cannot find trait `BadTrait` in this scope
+
+fn partially_correct_impl_bad(_: impl &*const &BadTrait + Copy) {}
+//~^ ERROR expected a trait, found type
+//~^^ ERROR cannot find trait `BadTrait` in this scope
+
+fn main() {}
diff --git a/tests/ui/generics/issue-106694.stderr b/tests/ui/generics/issue-106694.stderr
new file mode 100644
index 00000000000..235b8982a99
--- /dev/null
+++ b/tests/ui/generics/issue-106694.stderr
@@ -0,0 +1,93 @@
+error: expected a trait, found type
+  --> $DIR/issue-106694.rs:3:16
+   |
+LL | fn foo(_: impl &Trait) {}
+   |                ^^^^^^
+   |
+help: consider removing the indirection
+   |
+LL - fn foo(_: impl &Trait) {}
+LL + fn foo(_: impl Trait) {}
+   |
+
+error: expected a trait, found type
+  --> $DIR/issue-106694.rs:6:11
+   |
+LL | fn bar<T: &Trait>(_: T) {}
+   |           ^^^^^^
+   |
+help: consider removing the indirection
+   |
+LL - fn bar<T: &Trait>(_: T) {}
+LL + fn bar<T: Trait>(_: T) {}
+   |
+
+error: expected a trait, found type
+  --> $DIR/issue-106694.rs:9:35
+   |
+LL | fn partially_correct_impl(_: impl &*const &Trait + Copy) {}
+   |                                   ^^^^^^^^^^^^^^
+   |
+help: consider removing the indirection
+   |
+LL - fn partially_correct_impl(_: impl &*const &Trait + Copy) {}
+LL + fn partially_correct_impl(_: impl Trait + Copy) {}
+   |
+
+error: expected a trait, found type
+  --> $DIR/issue-106694.rs:12:20
+   |
+LL | fn foo_bad(_: impl &BadTrait) {}
+   |                    ^^^^^^^^^
+   |
+help: consider removing the indirection
+   |
+LL - fn foo_bad(_: impl &BadTrait) {}
+LL + fn foo_bad(_: impl BadTrait) {}
+   |
+
+error: expected a trait, found type
+  --> $DIR/issue-106694.rs:16:15
+   |
+LL | fn bar_bad<T: &BadTrait>(_: T) {}
+   |               ^^^^^^^^^
+   |
+help: consider removing the indirection
+   |
+LL - fn bar_bad<T: &BadTrait>(_: T) {}
+LL + fn bar_bad<T: BadTrait>(_: T) {}
+   |
+
+error: expected a trait, found type
+  --> $DIR/issue-106694.rs:20:39
+   |
+LL | fn partially_correct_impl_bad(_: impl &*const &BadTrait + Copy) {}
+   |                                       ^^^^^^^^^^^^^^^^^
+   |
+help: consider removing the indirection
+   |
+LL - fn partially_correct_impl_bad(_: impl &*const &BadTrait + Copy) {}
+LL + fn partially_correct_impl_bad(_: impl BadTrait + Copy) {}
+   |
+
+error[E0405]: cannot find trait `BadTrait` in this scope
+  --> $DIR/issue-106694.rs:12:21
+   |
+LL | fn foo_bad(_: impl &BadTrait) {}
+   |                     ^^^^^^^^ not found in this scope
+
+error[E0405]: cannot find trait `BadTrait` in this scope
+  --> $DIR/issue-106694.rs:16:16
+   |
+LL | fn bar_bad<T: &BadTrait>(_: T) {}
+   |                ^^^^^^^^ not found in this scope
+
+error[E0405]: cannot find trait `BadTrait` in this scope
+  --> $DIR/issue-106694.rs:20:48
+   |
+LL | fn partially_correct_impl_bad(_: impl &*const &BadTrait + Copy) {}
+   |                                                ^^^^^^^^ not found in this scope
+
+error: aborting due to 9 previous errors
+
+For more information about this error, try `rustc --explain E0405`.