Suggest borrowing if a trait implementation is found for &/&mut <type>

src/test/ui/suggestions/imm-ref-trait-object-literal.stderr

@@ -21,10 +21,10 @@ LL | fn foo<X: Trait>(_: X) {}
    |           ----- required by this bound in `foo`
 ...
 LL |   foo(s);
-   |       ^ the trait `Trait` is not implemented for `S`
-   |
-   = help: the following implementations were found:
-             <&'a mut S as Trait>
+   |       ^
+   |       |
+   |       expected an implementor of trait `Trait`
+   |       help: consider borrowing mutably here: `&mut s`
 
 error: aborting due to 2 previous errors
 

src/test/ui/suggestions/issue-84973-2.rs

@@ -0,0 +1,13 @@
+// A slight variation of issue-84973.rs. Here, a mutable borrow is
+// required (and the obligation kind is different).
+
+trait Tr {}
+impl Tr for &mut i32 {}
+
+fn foo<T: Tr>(i: T) {}
+
+fn main() {
+    let a: i32 = 32;
+    foo(a);
+    //~^ ERROR: the trait bound `i32: Tr` is not satisfied [E0277]
+}

src/test/ui/suggestions/issue-84973-2.stderr

@@ -0,0 +1,15 @@
+error[E0277]: the trait bound `i32: Tr` is not satisfied
+  --> $DIR/issue-84973-2.rs:11:9
+   |
+LL | fn foo<T: Tr>(i: T) {}
+   |           -- required by this bound in `foo`
+...
+LL |     foo(a);
+   |         ^
+   |         |
+   |         expected an implementor of trait `Tr`
+   |         help: consider borrowing mutably here: `&mut a`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/suggestions/issue-84973.rs

@@ -0,0 +1,33 @@
+// Checks whether borrowing is suggested when a trait bound is not satisfied
+// for found type `T`, but is for `&/&mut T`.
+
+fn main() {
+    let f = Fancy{};
+    let o = Other::new(f);
+    //~^ ERROR: the trait bound `Fancy: SomeTrait` is not satisfied [E0277]
+}
+
+struct Fancy {}
+
+impl <'a> SomeTrait for &'a Fancy {
+}
+
+trait SomeTrait {}
+
+struct Other<'a, G> {
+    a: &'a str,
+    g: G,
+}
+
+// Broadly copied from https://docs.rs/petgraph/0.5.1/src/petgraph/dot.rs.html#70
+impl<'a, G> Other<'a, G>
+where
+    G: SomeTrait,
+{
+    pub fn new(g: G) -> Self {
+        Other {
+            a: "hi",
+            g: g,
+        }
+    }
+}

src/test/ui/suggestions/issue-84973.stderr

@@ -0,0 +1,15 @@
+error[E0277]: the trait bound `Fancy: SomeTrait` is not satisfied
+  --> $DIR/issue-84973.rs:6:24
+   |
+LL |     let o = Other::new(f);
+   |                        ^
+   |                        |
+   |                        expected an implementor of trait `SomeTrait`
+   |                        help: consider borrowing here: `&f`
+...
+LL |     pub fn new(g: G) -> Self {
+   |     ------------------------ required by `Other::<'a, G>::new`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.