Auto merge of #54720 - davidtwco:issue-51191, r=nikomatsakis

NLL fails to suggest "try removing `&mut` here"

Fixes #51191.

This PR adds ``try removing `&mut` here`` suggestions to functions where a mutable borrow is being taken of a `&mut self` or a `self: &mut Self`. This PR also enables the suggestion for adding a `mut` pattern to by-value implicit self arguments without `mut` patterns already.

r? @nikomatsakis

src/test/ui/did_you_mean/issue-31424.nll.stderr

@@ -2,15 +2,19 @@ error[E0596]: cannot borrow `self` as mutable, as it is not declared as mutable
   --> $DIR/issue-31424.rs:17:9
    |
 LL |         (&mut self).bar(); //~ ERROR cannot borrow
-   |         ^^^^^^^^^^^ cannot borrow as mutable
+   |         ^^^^^^^^^^^
+   |         |
+   |         cannot borrow as mutable
+   |         try removing `&mut` here
 
 error[E0596]: cannot borrow `self` as mutable, as it is not declared as mutable
   --> $DIR/issue-31424.rs:23:9
    |
-LL |     fn bar(self: &mut Self) {
-   |            ---- help: consider changing this to be mutable: `mut self`
 LL |         (&mut self).bar(); //~ ERROR cannot borrow
-   |         ^^^^^^^^^^^ cannot borrow as mutable
+   |         ^^^^^^^^^^^
+   |         |
+   |         cannot borrow as mutable
+   |         try removing `&mut` here
 
 error: aborting due to 2 previous errors
 

src/test/ui/nll/issue-51191.rs

@@ -0,0 +1,42 @@
+// Copyright 2016 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(nll)]
+
+struct Struct;
+
+impl Struct {
+    fn bar(self: &mut Self) {
+        (&mut self).bar();
+        //~^ ERROR cannot borrow `self` as mutable, as it is not declared as mutable [E0596]
+    }
+
+    fn imm(self) {
+        (&mut self).bar();
+        //~^ ERROR cannot borrow `self` as mutable, as it is not declared as mutable [E0596]
+    }
+
+    fn mtbl(mut self) {
+        (&mut self).bar();
+    }
+
+    fn immref(&self) {
+        (&mut self).bar();
+        //~^ ERROR cannot borrow `self` as mutable, as it is not declared as mutable [E0596]
+        //~^^ ERROR cannot borrow data in a `&` reference as mutable [E0596]
+    }
+
+    fn mtblref(&mut self) {
+        (&mut self).bar();
+        //~^ ERROR cannot borrow `self` as mutable, as it is not declared as mutable [E0596]
+    }
+}
+
+fn main () {}

src/test/ui/nll/issue-51191.stderr

@@ -0,0 +1,41 @@
+error[E0596]: cannot borrow `self` as mutable, as it is not declared as mutable
+  --> $DIR/issue-51191.rs:17:9
+   |
+LL |         (&mut self).bar();
+   |         ^^^^^^^^^^^
+   |         |
+   |         cannot borrow as mutable
+   |         try removing `&mut` here
+
+error[E0596]: cannot borrow `self` as mutable, as it is not declared as mutable
+  --> $DIR/issue-51191.rs:22:9
+   |
+LL |     fn imm(self) {
+   |            ---- help: consider changing this to be mutable: `mut self`
+LL |         (&mut self).bar();
+   |         ^^^^^^^^^^^ cannot borrow as mutable
+
+error[E0596]: cannot borrow `self` as mutable, as it is not declared as mutable
+  --> $DIR/issue-51191.rs:31:9
+   |
+LL |         (&mut self).bar();
+   |         ^^^^^^^^^^^ cannot borrow as mutable
+
+error[E0596]: cannot borrow data in a `&` reference as mutable
+  --> $DIR/issue-51191.rs:31:9
+   |
+LL |         (&mut self).bar();
+   |         ^^^^^^^^^^^ cannot borrow as mutable
+
+error[E0596]: cannot borrow `self` as mutable, as it is not declared as mutable
+  --> $DIR/issue-51191.rs:37:9
+   |
+LL |         (&mut self).bar();
+   |         ^^^^^^^^^^^
+   |         |
+   |         cannot borrow as mutable
+   |         try removing `&mut` here
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0596`.