Improve implicit self mutability suggestions.

This commit adds an `ImplicitSelfKind` to the HIR and the MIR that keeps
track of whether a implicit self argument is immutable by-value, mutable
by-value, immutable reference or mutable reference so that the addition
of the `mut` keyword can be suggested for the immutable by-value case.

src/test/incremental/hashes/trait_defs.rs

@@ -270,7 +270,7 @@ trait TraitChangeModeSelfOwnToMut: Sized {
 #[rustc_clean(label="Hir", cfg="cfail2")]
 #[rustc_clean(label="Hir", cfg="cfail3")]
 trait TraitChangeModeSelfOwnToMut: Sized {
-    #[rustc_clean(label="Hir", cfg="cfail2")]
+    #[rustc_dirty(label="Hir", cfg="cfail2")]
     #[rustc_clean(label="Hir", cfg="cfail3")]
     #[rustc_dirty(label="HirBody", cfg="cfail2")]
     #[rustc_clean(label="HirBody", cfg="cfail3")]

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

@@ -0,0 +1,37 @@
+// 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
+    }
+
+    fn imm(self) {
+        (&mut self).bar(); //~ ERROR cannot borrow
+    }
+
+    fn mtbl(mut self) {
+        (&mut self).bar();
+    }
+
+    fn immref(&self) {
+        (&mut self).bar(); //~ ERROR cannot borrow
+    }
+
+    fn mtblref(&mut self) {
+        (&mut self).bar();
+    }
+}
+
+fn main () {}

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

@@ -0,0 +1,37 @@
+error[E0596]: cannot borrow `self` as mutable, as it is not declared as mutable
+  --> $DIR/issue-51191.rs:17:9
+   |
+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
+
+error[E0596]: cannot borrow `self` as mutable, as it is not declared as mutable
+  --> $DIR/issue-51191.rs:21:9
+   |
+LL |     fn imm(self) {
+   |            ---- help: consider changing this to be mutable: `mut self`
+LL |         (&mut self).bar(); //~ ERROR cannot borrow
+   |         ^^^^^^^^^^^ cannot borrow as mutable
+
+error[E0596]: cannot borrow `self` as mutable, as it is not declared as mutable
+  --> $DIR/issue-51191.rs:29:9
+   |
+LL |         (&mut self).bar(); //~ ERROR cannot borrow
+   |         ^^^^^^^^^^^ cannot borrow as mutable
+
+error[E0596]: cannot borrow data in a `&` reference as mutable
+  --> $DIR/issue-51191.rs:29:9
+   |
+LL |         (&mut self).bar(); //~ ERROR cannot borrow
+   |         ^^^^^^^^^^^ cannot borrow as mutable
+
+error[E0596]: cannot borrow `self` as mutable, as it is not declared as mutable
+  --> $DIR/issue-51191.rs:33:9
+   |
+LL |         (&mut self).bar();
+   |         ^^^^^^^^^^^ cannot borrow as mutable
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0596`.