Auto merge of #52242 - ashtneoi:suggest-ref-mut, r=pnkfelix

NLL: Suggest `ref mut` and `&mut self`

Fixes #51244. Supersedes #51249, I think.

Under the old lexical lifetimes, the compiler provided helpful suggestions about adding `mut` when you tried to mutate a variable bound as `&self` or (explicit) `ref`. NLL doesn't have those suggestions yet. This pull request adds them.

I didn't bother making the help text exactly the same as without NLL, but I can if that's important.

(Originally this was supposed to be part of #51612, but I got bogged down trying to fit everything in one PR.)

src/test/ui/did_you_mean/issue-38147-1.nll.stderr

@@ -2,7 +2,7 @@ error[E0596]: cannot borrow immutable item `*self.s` as mutable
   --> $DIR/issue-38147-1.rs:27:9
    |
 LL |     fn f(&self) {
-   |          ----- help: consider changing this to be a mutable reference: `&mut Foo<'_>`
+   |          ----- help: consider changing this to be a mutable reference: `&mut self`
 LL |         self.s.push('x'); //~ ERROR cannot borrow data mutably
    |         ^^^^^^ `self` is a `&` reference, so the data it refers to cannot be borrowed as mutable
 

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

@@ -10,7 +10,7 @@ error[E0596]: cannot borrow immutable item `self.x` as mutable
   --> $DIR/issue-39544.rs:26:17
    |
 LL |     fn foo<'z>(&'z self) {
-   |                -------- help: consider changing this to be a mutable reference: `&mut Z`
+   |                -------- help: consider changing this to be a mutable reference: `&mut self`
 LL |         let _ = &mut self.x; //~ ERROR cannot borrow
    |                 ^^^^^^^^^^^ `self` is a `&` reference, so the data it refers to cannot be borrowed as mutable
 
@@ -18,7 +18,7 @@ error[E0596]: cannot borrow immutable item `self.x` as mutable
   --> $DIR/issue-39544.rs:30:17
    |
 LL |     fn foo1(&self, other: &Z) {
-   |             ----- help: consider changing this to be a mutable reference: `&mut Z`
+   |             ----- help: consider changing this to be a mutable reference: `&mut self`
 LL |         let _ = &mut self.x; //~ ERROR cannot borrow
    |                 ^^^^^^^^^^^ `self` is a `&` reference, so the data it refers to cannot be borrowed as mutable
 
@@ -35,7 +35,7 @@ error[E0596]: cannot borrow immutable item `self.x` as mutable
   --> $DIR/issue-39544.rs:35:17
    |
 LL |     fn foo2<'a>(&'a self, other: &Z) {
-   |                 -------- help: consider changing this to be a mutable reference: `&mut Z`
+   |                 -------- help: consider changing this to be a mutable reference: `&mut self`
 LL |         let _ = &mut self.x; //~ ERROR cannot borrow
    |                 ^^^^^^^^^^^ `self` is a `&` reference, so the data it refers to cannot be borrowed as mutable
 

src/test/ui/nll/issue-51244.rs

@@ -0,0 +1,17 @@
+// Copyright 2018 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)]
+
+fn main() {
+    let ref my_ref @ _ = 0;
+    *my_ref = 0;
+    //~^ ERROR cannot assign to `*my_ref` which is behind a `&` reference [E0594]
+}

src/test/ui/nll/issue-51244.stderr

@@ -0,0 +1,11 @@
+error[E0594]: cannot assign to `*my_ref` which is behind a `&` reference
+  --> $DIR/issue-51244.rs:15:5
+   |
+LL |     let ref my_ref @ _ = 0;
+   |         -------------- help: consider changing this to be a mutable reference: `ref mut my_ref @ _`
+LL |     *my_ref = 0;
+   |     ^^^^^^^^^^^ `my_ref` is a `&` reference, so the data it refers to cannot be written
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0594`.

src/test/ui/rfc-2005-default-binding-mode/enum.nll.stderr

@@ -2,19 +2,19 @@ error[E0594]: cannot assign to `*x` which is behind a `&` reference
   --> $DIR/enum.rs:19:5
    |
 LL |     *x += 1; //~ ERROR cannot assign to immutable
-   |     ^^^^^^^ cannot assign
+   |     ^^^^^^^ `x` is a `&` reference, so the data it refers to cannot be written
 
 error[E0594]: cannot assign to `*x` which is behind a `&` reference
   --> $DIR/enum.rs:23:9
    |
 LL |         *x += 1; //~ ERROR cannot assign to immutable
-   |         ^^^^^^^ cannot assign
+   |         ^^^^^^^ `x` is a `&` reference, so the data it refers to cannot be written
 
 error[E0594]: cannot assign to `*x` which is behind a `&` reference
   --> $DIR/enum.rs:29:9
    |
 LL |         *x += 1; //~ ERROR cannot assign to immutable
-   |         ^^^^^^^ cannot assign
+   |         ^^^^^^^ `x` is a `&` reference, so the data it refers to cannot be written
 
 error: aborting due to 3 previous errors
 

src/test/ui/rfc-2005-default-binding-mode/enum.stderr

@@ -1,24 +1,18 @@
 error[E0594]: cannot assign to immutable borrowed content `*x`
   --> $DIR/enum.rs:19:5
    |
-LL |     let Wrap(x) = &Wrap(3);
-   |              - consider changing this to `x`
 LL |     *x += 1; //~ ERROR cannot assign to immutable
    |     ^^^^^^^ cannot borrow as mutable
 
 error[E0594]: cannot assign to immutable borrowed content `*x`
   --> $DIR/enum.rs:23:9
    |
-LL |     if let Some(x) = &Some(3) {
-   |                 - consider changing this to `x`
 LL |         *x += 1; //~ ERROR cannot assign to immutable
    |         ^^^^^^^ cannot borrow as mutable
 
 error[E0594]: cannot assign to immutable borrowed content `*x`
   --> $DIR/enum.rs:29:9
    |
-LL |     while let Some(x) = &Some(3) {
-   |                    - consider changing this to `x`
 LL |         *x += 1; //~ ERROR cannot assign to immutable
    |         ^^^^^^^ cannot borrow as mutable
 

src/test/ui/rfc-2005-default-binding-mode/explicit-mut.nll.stderr

@@ -2,19 +2,19 @@ error[E0594]: cannot assign to `*n` which is behind a `&` reference
   --> $DIR/explicit-mut.rs:17:13
    |
 LL |             *n += 1; //~ ERROR cannot assign to immutable
-   |             ^^^^^^^ cannot assign
+   |             ^^^^^^^ `n` is a `&` reference, so the data it refers to cannot be written
 
 error[E0594]: cannot assign to `*n` which is behind a `&` reference
   --> $DIR/explicit-mut.rs:25:13
    |
 LL |             *n += 1; //~ ERROR cannot assign to immutable
-   |             ^^^^^^^ cannot assign
+   |             ^^^^^^^ `n` is a `&` reference, so the data it refers to cannot be written
 
 error[E0594]: cannot assign to `*n` which is behind a `&` reference
   --> $DIR/explicit-mut.rs:33:13
    |
 LL |             *n += 1; //~ ERROR cannot assign to immutable
-   |             ^^^^^^^ cannot assign
+   |             ^^^^^^^ `n` is a `&` reference, so the data it refers to cannot be written
 
 error: aborting due to 3 previous errors
 

src/test/ui/rfc-2005-default-binding-mode/explicit-mut.stderr

@@ -1,24 +1,18 @@
 error[E0594]: cannot assign to immutable borrowed content `*n`
   --> $DIR/explicit-mut.rs:17:13
    |
-LL |         Some(n) => {
-   |              - consider changing this to `n`
 LL |             *n += 1; //~ ERROR cannot assign to immutable
    |             ^^^^^^^ cannot borrow as mutable
 
 error[E0594]: cannot assign to immutable borrowed content `*n`
   --> $DIR/explicit-mut.rs:25:13
    |
-LL |         Some(n) => {
-   |              - consider changing this to `n`
 LL |             *n += 1; //~ ERROR cannot assign to immutable
    |             ^^^^^^^ cannot borrow as mutable
 
 error[E0594]: cannot assign to immutable borrowed content `*n`
   --> $DIR/explicit-mut.rs:33:13
    |
-LL |         Some(n) => {
-   |              - consider changing this to `n`
 LL |             *n += 1; //~ ERROR cannot assign to immutable
    |             ^^^^^^^ cannot borrow as mutable
 

src/test/ui/suggestions/issue-51244.nll.stderr

@@ -0,0 +1,11 @@
+error[E0594]: cannot assign to `*my_ref` which is behind a `&` reference
+  --> $DIR/issue-51244.rs:13:5
+   |
+LL |     let ref my_ref @ _ = 0;
+   |         -------------- help: consider changing this to be a mutable reference: `ref mut my_ref @ _`
+LL |     *my_ref = 0; //~ ERROR cannot assign to immutable borrowed content `*my_ref` [E0594]
+   |     ^^^^^^^^^^^ `my_ref` is a `&` reference, so the data it refers to cannot be written
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0594`.

src/test/ui/suggestions/issue-51244.rs

@@ -0,0 +1,14 @@
+// Copyright 2018 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 ref my_ref @ _ = 0;
+    *my_ref = 0; //~ ERROR cannot assign to immutable borrowed content `*my_ref` [E0594]
+}

src/test/ui/suggestions/issue-51244.stderr

@@ -0,0 +1,11 @@
+error[E0594]: cannot assign to immutable borrowed content `*my_ref`
+  --> $DIR/issue-51244.rs:13:5
+   |
+LL |     let ref my_ref @ _ = 0;
+   |         -------------- help: use a mutable reference instead: `ref mut my_ref @ _`
+LL |     *my_ref = 0; //~ ERROR cannot assign to immutable borrowed content `*my_ref` [E0594]
+   |     ^^^^^^^^^^^ cannot borrow as mutable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0594`.

src/test/ui/suggestions/suggest-ref-mut.rs

@@ -0,0 +1,42 @@
+// Copyright 2018 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 X(usize);
+
+impl X {
+    fn zap(&self) {
+        //~^ HELP
+        //~| SUGGESTION &mut self
+        self.0 = 32;
+        //~^ ERROR
+    }
+}
+
+fn main() {
+    let ref foo = 16;
+    //~^ HELP
+    //~| SUGGESTION ref mut foo
+    *foo = 32;
+    //~^ ERROR
+    if let Some(ref bar) = Some(16) {
+        //~^ HELP
+        //~| SUGGESTION ref mut bar
+        *bar = 32;
+        //~^ ERROR
+    }
+    match 16 {
+        ref quo => { *quo = 32; },
+        //~^ ERROR
+        //~| HELP
+        //~| SUGGESTION ref mut quo
+    }
+}

src/test/ui/suggestions/suggest-ref-mut.stderr

@@ -0,0 +1,38 @@
+error[E0594]: cannot assign to `self.0` which is behind a `&` reference
+  --> $DIR/suggest-ref-mut.rs:19:9
+   |
+LL |     fn zap(&self) {
+   |            ----- help: consider changing this to be a mutable reference: `&mut self`
+...
+LL |         self.0 = 32;
+   |         ^^^^^^^^^^^ `self` is a `&` reference, so the data it refers to cannot be written
+
+error[E0594]: cannot assign to `*foo` which is behind a `&` reference
+  --> $DIR/suggest-ref-mut.rs:28:5
+   |
+LL |     let ref foo = 16;
+   |         ------- help: consider changing this to be a mutable reference: `ref mut foo`
+...
+LL |     *foo = 32;
+   |     ^^^^^^^^^ `foo` is a `&` reference, so the data it refers to cannot be written
+
+error[E0594]: cannot assign to `*bar` which is behind a `&` reference
+  --> $DIR/suggest-ref-mut.rs:33:9
+   |
+LL |     if let Some(ref bar) = Some(16) {
+   |                 ------- help: consider changing this to be a mutable reference: `ref mut bar`
+...
+LL |         *bar = 32;
+   |         ^^^^^^^^^ `bar` is a `&` reference, so the data it refers to cannot be written
+
+error[E0594]: cannot assign to `*quo` which is behind a `&` reference
+  --> $DIR/suggest-ref-mut.rs:37:22
+   |
+LL |         ref quo => { *quo = 32; },
+   |         -------      ^^^^^^^^^ `quo` is a `&` reference, so the data it refers to cannot be written
+   |         |
+   |         help: consider changing this to be a mutable reference: `ref mut quo`
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0594`.