borrowck: consolidate mut suggestions

This converts all of borrowck's `mut` suggestions to a new
`mc::ImmutabilityBlame` API instead of the current mix of various hacks.

Fixes #35937.
Fixes #40823.

src/test/ui/codemap_tests/huge_multispan_highlight.stderr

@@ -2,7 +2,7 @@ error: cannot borrow immutable local variable `x` as mutable
    --> $DIR/huge_multispan_highlight.rs:100:18
     |
 12  |     let x = "foo";
-    |         - use `mut x` here to make mutable
+    |         - consider changing this to `mut x`
 ...
 100 |     let y = &mut x;
     |                  ^ cannot borrow mutably

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

@@ -10,6 +10,8 @@ error: cannot borrow immutable argument `self` as mutable
 error: cannot borrow immutable argument `self` as mutable
   --> $DIR/issue-31424.rs:23:15
    |
+22 |     fn bar(self: &mut Self) {
+   |            ---- consider changing this to `mut self`
 23 |         (&mut self).bar();
    |               ^^^^ cannot borrow mutably
 

src/test/ui/did_you_mean/issue-35937.rs

@@ -0,0 +1,31 @@
+// 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.
+
+struct Foo {
+    pub v: Vec<String>
+}
+
+fn main() {
+    let f = Foo { v: Vec::new() };
+    f.v.push("cat".to_string());
+}
+
+
+struct S {
+    x: i32,
+}
+fn foo() {
+    let s = S { x: 42 };
+    s.x += 1;
+}
+
+fn bar(s: S) {
+    s.x += 1;
+}

src/test/ui/did_you_mean/issue-35937.stderr

@@ -0,0 +1,26 @@
+error: cannot borrow immutable field `f.v` as mutable
+  --> $DIR/issue-35937.rs:17:5
+   |
+16 |     let f = Foo { v: Vec::new() };
+   |         - consider changing this to `mut f`
+17 |     f.v.push("cat".to_string());
+   |     ^^^ cannot mutably borrow immutable field
+
+error: cannot assign to immutable field `s.x`
+  --> $DIR/issue-35937.rs:26:5
+   |
+25 |     let s = S { x: 42 };
+   |         - consider changing this to `mut s`
+26 |     s.x += 1;
+   |     ^^^^^^^^ cannot mutably borrow immutable field
+
+error: cannot assign to immutable field `s.x`
+  --> $DIR/issue-35937.rs:30:5
+   |
+29 | fn bar(s: S) {
+   |        - consider changing this to `mut s`
+30 |     s.x += 1;
+   |     ^^^^^^^^ cannot mutably borrow immutable field
+
+error: aborting due to 3 previous errors
+

src/test/ui/did_you_mean/issue-38147-2.stderr

@@ -2,7 +2,7 @@ error: cannot borrow immutable borrowed content `*self.s` as mutable
   --> $DIR/issue-38147-2.rs:17:9
    |
 12 |     s: &'a String
-   |     ------------- use `&'a mut String` here to make mutable
+   |        ---------- use `&'a mut String` here to make mutable
 ...
 17 |         self.s.push('x');
    |         ^^^^^^ cannot borrow as mutable

src/test/ui/did_you_mean/issue-38147-3.stderr

@@ -2,10 +2,8 @@ error: cannot borrow immutable borrowed content `*self.s` as mutable
   --> $DIR/issue-38147-3.rs:17:9
    |
 12 |     s: &'a String
-   |     ------------- use `&'a mut String` here to make mutable
+   |        ---------- use `&'a mut String` here to make mutable
 ...
-16 |     fn f(&self) {
-   |          ----- use `&mut self` here to make mutable
 17 |         self.s.push('x');
    |         ^^^^^^ cannot borrow as mutable
 

src/test/ui/did_you_mean/issue-39544.rs

@@ -8,15 +8,20 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-enum X {
+pub enum X {
     Y
 }
 
-struct Z {
+pub struct Z {
     x: X
 }
 
-fn main() {
+pub fn main() {
     let z = Z { x: X::Y };
     let _ = &mut z.x;
 }
+
+pub fn with_arg(z: Z, w: &Z) {
+    let _ = &mut z.x;
+    let _ = &mut w.x;
+}

src/test/ui/did_you_mean/issue-39544.stderr

@@ -6,5 +6,22 @@ error: cannot borrow immutable field `z.x` as mutable
 21 |     let _ = &mut z.x;
    |                  ^^^ cannot mutably borrow immutable field
 
-error: aborting due to previous error
+error: cannot borrow immutable field `z.x` as mutable
+  --> $DIR/issue-39544.rs:25:18
+   |
+24 | pub fn with_arg(z: Z, w: &Z) {
+   |                 - consider changing this to `mut z`
+25 |     let _ = &mut z.x;
+   |                  ^^^ cannot mutably borrow immutable field
+
+error: cannot borrow immutable field `w.x` as mutable
+  --> $DIR/issue-39544.rs:26:18
+   |
+24 | pub fn with_arg(z: Z, w: &Z) {
+   |                          -- use `&mut Z` here to make mutable
+25 |     let _ = &mut z.x;
+26 |     let _ = &mut w.x;
+   |                  ^^^ cannot mutably borrow immutable field
+
+error: aborting due to 3 previous errors
 

src/test/ui/did_you_mean/issue-40823.rs

@@ -0,0 +1,14 @@
+// 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.
+
+fn main() {
+    let mut buf = &[1, 2, 3, 4];
+    buf.iter_mut();
+}

src/test/ui/did_you_mean/issue-40823.stderr

@@ -0,0 +1,8 @@
+error: cannot borrow immutable borrowed content `*buf` as mutable
+  --> $DIR/issue-40823.rs:13:5
+   |
+13 |     buf.iter_mut();
+   |     ^^^ cannot borrow as mutable
+
+error: aborting due to previous error
+

src/test/ui/span/borrowck-borrow-overloaded-auto-deref-mut.stderr

@@ -2,7 +2,7 @@ error: cannot borrow immutable argument `x` as mutable
   --> $DIR/borrowck-borrow-overloaded-auto-deref-mut.rs:63:24
    |
 62 | fn deref_mut_field1(x: Own<Point>) {
-   |                     - use `mut x` here to make mutable
+   |                     - consider changing this to `mut x`
 63 |     let __isize = &mut x.y; //~ ERROR cannot borrow
    |                        ^ cannot borrow mutably
 
@@ -28,7 +28,7 @@ error: cannot borrow immutable argument `x` as mutable
   --> $DIR/borrowck-borrow-overloaded-auto-deref-mut.rs:98:5
    |
 97 | fn assign_field1<'a>(x: Own<Point>) {
-   |                      - use `mut x` here to make mutable
+   |                      - consider changing this to `mut x`
 98 |     x.y = 3; //~ ERROR cannot borrow
    |     ^ cannot borrow mutably
 
@@ -54,7 +54,7 @@ error: cannot borrow immutable argument `x` as mutable
    --> $DIR/borrowck-borrow-overloaded-auto-deref-mut.rs:119:5
     |
 118 | fn deref_mut_method1(x: Own<Point>) {
-    |                      - use `mut x` here to make mutable
+    |                      - consider changing this to `mut x`
 119 |     x.set(0, 0); //~ ERROR cannot borrow
     |     ^ cannot borrow mutably
 
@@ -70,7 +70,7 @@ error: cannot borrow immutable argument `x` as mutable
    --> $DIR/borrowck-borrow-overloaded-auto-deref-mut.rs:139:6
     |
 138 | fn assign_method1<'a>(x: Own<Point>) {
-    |                       - use `mut x` here to make mutable
+    |                       - consider changing this to `mut x`
 139 |     *x.y_mut() = 3; //~ ERROR cannot borrow
     |      ^ cannot borrow mutably
 

src/test/ui/span/borrowck-borrow-overloaded-deref-mut.stderr

@@ -2,7 +2,7 @@ error: cannot borrow immutable argument `x` as mutable
   --> $DIR/borrowck-borrow-overloaded-deref-mut.rs:39:25
    |
 38 | fn deref_mut1(x: Own<isize>) {
-   |               - use `mut x` here to make mutable
+   |               - consider changing this to `mut x`
 39 |     let __isize = &mut *x; //~ ERROR cannot borrow
    |                         ^ cannot borrow mutably
 
@@ -18,7 +18,7 @@ error: cannot borrow immutable argument `x` as mutable
   --> $DIR/borrowck-borrow-overloaded-deref-mut.rs:59:6
    |
 58 | fn assign1<'a>(x: Own<isize>) {
-   |                - use `mut x` here to make mutable
+   |                - consider changing this to `mut x`
 59 |     *x = 3; //~ ERROR cannot borrow
    |      ^ cannot borrow mutably
 

src/test/ui/span/borrowck-object-mutability.stderr

@@ -10,6 +10,9 @@ error: cannot borrow immutable borrowed content `*x` as mutable
 error: cannot borrow immutable `Box` content `*x` as mutable
   --> $DIR/borrowck-object-mutability.rs:29:5
    |
+27 | fn owned_receiver(x: Box<Foo>) {
+   |                   - consider changing this to `mut x`
+28 |     x.borrowed();
 29 |     x.borrowed_mut(); //~ ERROR cannot borrow
    |     ^ cannot borrow as mutable