Rollup merge of rust-lang#41489 - estebank:trait-bounds-diagnosstic, r=arielb1

Make unsatisfied trait bounds note multiline

Make diagnostic note for existing method with unsatisfied trait bounds
multiline for cleaner output.

```
  = note: the method `count` exists but the following trait bounds were not satisfied:
          `[closure@../../src/test/compile-fail/issue-36053-2.rs:17:39: 17:53] : std::ops::FnMut<(&_,)>`
          `std::iter::Filter<std::iter::Fuse<std::iter::Once<&str>> [closure@../../src/test/compile-fail/issue-36053-2.rs:17:39: 17:53]> : std::iter::Iterator`
```

Before:

```
  = note: the method `count` exists but the following trait bounds were not satisfied: `[closure@../../src/test/compile-fail/issue-36053-2.rs:17:39: 17:53] : std::ops::FnMut<(&_,)>`, `std::iter::Filter<std::iter::Fuse<std::iter::Once<&str>>, [closure@../../src/test/compile-fail/issue-36053-2.rs:17:39: 17:53]> : std::iter::Iterator`
```

Author: Mark-Simulacrum

src/librustc_typeck/check/method/suggest.rs

@@ -251,9 +251,9 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                     let bound_list = unsatisfied_predicates.iter()
                         .map(|p| format!("`{} : {}`", p.self_ty(), p))
                         .collect::<Vec<_>>()
-                        .join(", ");
+                        .join("\n");
                     err.note(&format!("the method `{}` exists but the following trait bounds \
-                                       were not satisfied: {}",
+                                       were not satisfied:\n{}",
                                       item_name,
                                       bound_list));
                 }

src/test/ui/mismatched_types/issue-36053-2.stderr

@@ -4,7 +4,9 @@ error: no method named `count` found for type `std::iter::Filter<std::iter::Fuse
 17 |     once::<&str>("str").fuse().filter(|a: &str| true).count();
    |                                                       ^^^^^
    |
-   = note: the method `count` exists but the following trait bounds were not satisfied: `[closure@$DIR/issue-36053-2.rs:17:39: 17:53] : std::ops::FnMut<(&_,)>`, `std::iter::Filter<std::iter::Fuse<std::iter::Once<&str>>, [closure@$DIR/issue-36053-2.rs:17:39: 17:53]> : std::iter::Iterator`
+   = note: the method `count` exists but the following trait bounds were not satisfied:
+           `[closure@$DIR/issue-36053-2.rs:17:39: 17:53] : std::ops::FnMut<(&_,)>`
+           `std::iter::Filter<std::iter::Fuse<std::iter::Once<&str>>, [closure@$DIR/issue-36053-2.rs:17:39: 17:53]> : std::iter::Iterator`
 
 error[E0281]: type mismatch: `[closure@$DIR/issue-36053-2.rs:17:39: 17:53]` implements the trait `for<'r> std::ops::FnMut<(&'r str,)>`, but the trait `for<'r> std::ops::FnMut<(&'r &str,)>` is required
   --> $DIR/issue-36053-2.rs:17:32

src/test/compile-fail/method-help-unsatisfied-bound.rs renamed to src/test/ui/mismatched_types/method-help-unsatisfied-bound.rs

@@ -14,5 +14,5 @@ fn main() {
     let a: Result<(), Foo> = Ok(());
     a.unwrap();
     //~^ ERROR no method named `unwrap` found for type `std::result::Result<(), Foo>`
-    //~| NOTE the following trait bounds were not satisfied: `Foo : std::fmt::Debug`
+    //~| NOTE the method `unwrap` exists but the following trait bounds were not satisfied
 }

src/test/ui/mismatched_types/method-help-unsatisfied-bound.stderr

@@ -0,0 +1,11 @@
+error: no method named `unwrap` found for type `std::result::Result<(), Foo>` in the current scope
+  --> $DIR/method-help-unsatisfied-bound.rs:15:7
+   |
+15 |     a.unwrap();
+   |       ^^^^^^
+   |
+   = note: the method `unwrap` exists but the following trait bounds were not satisfied:
+           `Foo : std::fmt::Debug`
+
+error: aborting due to previous error
+