Auto merge of #38605 - estebank:fix-38371, r=nikomatsakis

Suggest solutions for `fn foo(&foo: Foo)`

For a given file:

```rust
struct Foo {}

fn foo(&foo: Foo) {}
```

suggest:

```
error[E0308]: mismatched types
 --> file.rs:3:8
  |
3 | fn foo(&foo: Foo) {}
  |        ^^^^ expected struct `Foo`, found reference
  |
  = note: expected type `Foo`
  = note:    found type `&_`
  = help: did you mean `foo: &Foo`?
```

Fix #38371.

src/test/ui/mismatched_types/issue-38371.rs

@@ -0,0 +1,37 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+#![feature(slice_patterns)]
+
+
+struct Foo {
+}
+
+fn foo(&foo: Foo) {
+}
+
+fn bar(foo: Foo) {
+}
+
+fn qux(foo: &Foo) {
+}
+
+fn zar(&foo: &Foo) {
+}
+
+fn agh(&&bar: &u32) {
+}
+
+fn bgh(&&bar: u32) {
+}
+
+fn ugh(&[bar]: &u32) {
+}
+
+fn main() {}

src/test/ui/mismatched_types/issue-38371.stderr

@@ -0,0 +1,36 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-38371.rs:16:8
+   |
+16 | fn foo(&foo: Foo) {
+   |        ^^^^ expected struct `Foo`, found reference
+   |
+   = note: expected type `Foo`
+   = note:    found type `&_`
+   = help: did you mean `foo: &Foo`?
+
+error[E0308]: mismatched types
+  --> $DIR/issue-38371.rs:28:9
+   |
+28 | fn agh(&&bar: &u32) {
+   |         ^^^^ expected u32, found reference
+   |
+   = note: expected type `u32`
+   = note:    found type `&_`
+
+error[E0308]: mismatched types
+  --> $DIR/issue-38371.rs:31:8
+   |
+31 | fn bgh(&&bar: u32) {
+   |        ^^^^^ expected u32, found reference
+   |
+   = note: expected type `u32`
+   = note:    found type `&_`
+
+error[E0529]: expected an array or slice, found `u32`
+  --> $DIR/issue-38371.rs:34:9
+   |
+34 | fn ugh(&[bar]: &u32) {
+   |         ^^^^^ pattern cannot match with input type `u32`
+
+error: aborting due to 4 previous errors
+