use structured suggestion for "missing mut" label

Fixes #54133.

src/librustc_borrowck/borrowck/mod.rs

@@ -45,7 +45,7 @@ use rustc_data_structures::sync::Lrc;
 use std::hash::{Hash, Hasher};
 use syntax::ast;
 use syntax_pos::{MultiSpan, Span};
-use errors::{DiagnosticBuilder, DiagnosticId};
+use errors::{Applicability, DiagnosticBuilder, DiagnosticId};
 
 use rustc::hir;
 use rustc::hir::intravisit::{self, Visitor};
@@ -1299,9 +1299,11 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                         snippet
                     );
                 } else {
-                    db.span_label(
+                    db.span_suggestion_with_applicability(
                         let_span,
-                        format!("consider changing this to `mut {}`", snippet),
+                        "make this binding mutable",
+                        format!("mut {}", snippet),
+                        Applicability::MachineApplicable,
                     );
                 }
             }

src/librustc_mir/borrow_check/error_reporting.rs

@@ -17,7 +17,7 @@ use rustc::mir::{ProjectionElem, Rvalue, Statement, StatementKind};
 use rustc::ty;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_data_structures::sync::Lrc;
-use rustc_errors::DiagnosticBuilder;
+use rustc_errors::{Applicability, DiagnosticBuilder};
 use syntax_pos::Span;
 
 use super::borrow_set::BorrowData;
@@ -690,9 +690,11 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
         if let Some(decl) = local_decl {
             if let Some(name) = decl.name {
                 if decl.can_be_made_mutable() {
-                    err.span_label(
+                    err.span_suggestion_with_applicability(
                         decl.source_info.span,
-                        format!("consider changing this to `mut {}`", name),
+                        "make this binding mutable",
+                        format!("mut {}", name),
+                        Applicability::MachineApplicable,
                     );
                 }
             }

src/test/ui/E0596.ast.stderr

@@ -2,7 +2,7 @@ error[E0596]: cannot borrow immutable local variable `x` as mutable
   --> $DIR/E0596.rs:16:18
    |
 LL |     let x = 1;
-   |         - consider changing this to `mut x`
+   |         - help: make this binding mutable: `mut x`
 LL |     let y = &mut x; //[ast]~ ERROR [E0596]
    |                  ^ cannot borrow mutably
 

src/test/ui/asm/asm-out-assign-imm.nll.stderr

@@ -2,7 +2,7 @@ error[E0384]: cannot assign twice to immutable variable `x`
   --> $DIR/asm-out-assign-imm.rs:34:9
    |
 LL |     let x: isize;
-   |         - consider changing this to `mut x`
+   |         - help: make this binding mutable: `mut x`
 LL |     x = 1;
    |     ----- first assignment to `x`
 ...

src/test/ui/assign-imm-local-twice.ast.nll.stderr

@@ -1,9 +1,9 @@
 error[E0384]: cannot assign twice to immutable variable `v`
-  --> $DIR/assign-imm-local-twice.rs:20:5
+  --> $DIR/assign-imm-local-twice.rs:21:5
    |
 LL |     let v: isize;
-   |         - consider changing this to `mut v`
-LL |     //[mir]~^ NOTE consider changing this to `mut v`
+   |         - help: make this binding mutable: `mut v`
+...
 LL |     v = 1; //[ast]~ NOTE first assignment
    |     ----- first assignment to `v`
 ...

src/test/ui/assign-imm-local-twice.ast.stderr

@@ -1,5 +1,5 @@
 error[E0384]: cannot assign twice to immutable variable `v`
-  --> $DIR/assign-imm-local-twice.rs:20:5
+  --> $DIR/assign-imm-local-twice.rs:21:5
    |
 LL |     v = 1; //[ast]~ NOTE first assignment
    |     ----- first assignment to `v`

src/test/ui/assign-imm-local-twice.mir.stderr

@@ -1,9 +1,9 @@
 error[E0384]: cannot assign twice to immutable variable `v`
-  --> $DIR/assign-imm-local-twice.rs:20:5
+  --> $DIR/assign-imm-local-twice.rs:21:5
    |
 LL |     let v: isize;
-   |         - consider changing this to `mut v`
-LL |     //[mir]~^ NOTE consider changing this to `mut v`
+   |         - help: make this binding mutable: `mut v`
+...
 LL |     v = 1; //[ast]~ NOTE first assignment
    |     ----- first assignment to `v`
 ...

src/test/ui/assign-imm-local-twice.rs

@@ -13,7 +13,8 @@
 
 fn test() {
     let v: isize;
-    //[mir]~^ NOTE consider changing this to `mut v`
+    //[mir]~^ HELP make this binding mutable
+    //[mir]~| SUGGESTION mut v
     v = 1; //[ast]~ NOTE first assignment
            //[mir]~^ NOTE first assignment
     println!("v={}", v);

src/test/ui/augmented-assignments.nll.stderr

@@ -15,11 +15,11 @@ LL | |     x;  //~ value moved here
    |       borrow later used here
 
 error[E0596]: cannot borrow `y` as mutable, as it is not declared as mutable
-  --> $DIR/augmented-assignments.rs:30:5
+  --> $DIR/augmented-assignments.rs:31:5
    |
 LL |     let y = Int(2);
    |         - help: consider changing this to be mutable: `mut y`
-LL |     //~^ consider changing this to `mut y`
+...
 LL |     y   //~ error: cannot borrow immutable local variable `y` as mutable
    |     ^ cannot borrow as mutable
 

src/test/ui/augmented-assignments.rs

@@ -26,7 +26,8 @@ fn main() {
     x;  //~ value moved here
 
     let y = Int(2);
-    //~^ consider changing this to `mut y`
+    //~^ HELP make this binding mutable
+    //~| SUGGESTION mut y
     y   //~ error: cannot borrow immutable local variable `y` as mutable
         //~| cannot borrow
     +=

src/test/ui/augmented-assignments.stderr

@@ -1,9 +1,9 @@
 error[E0596]: cannot borrow immutable local variable `y` as mutable
-  --> $DIR/augmented-assignments.rs:30:5
+  --> $DIR/augmented-assignments.rs:31:5
    |
 LL |     let y = Int(2);
-   |         - consider changing this to `mut y`
-LL |     //~^ consider changing this to `mut y`
+   |         - help: make this binding mutable: `mut y`
+...
 LL |     y   //~ error: cannot borrow immutable local variable `y` as mutable
    |     ^ cannot borrow mutably
 

src/test/ui/borrowck/borrowck-access-permissions.ast.stderr

@@ -2,7 +2,7 @@ error[E0596]: cannot borrow immutable local variable `x` as mutable
   --> $DIR/borrowck-access-permissions.rs:22:24
    |
 LL |     let x = 1;
-   |         - consider changing this to `mut x`
+   |         - help: make this binding mutable: `mut x`
 ...
 LL |         let _y1 = &mut x; //[ast]~ ERROR [E0596]
    |                        ^ cannot borrow mutably
@@ -17,7 +17,7 @@ error[E0596]: cannot borrow immutable `Box` content `*box_x` as mutable
   --> $DIR/borrowck-access-permissions.rs:37:24
    |
 LL |         let box_x = Box::new(1);
-   |             ----- consider changing this to `mut box_x`
+   |             ----- help: make this binding mutable: `mut box_x`
 ...
 LL |         let _y1 = &mut *box_x; //[ast]~ ERROR [E0596]
    |                        ^^^^^^ cannot borrow as mutable

src/test/ui/borrowck/borrowck-argument.stderr

@@ -2,23 +2,23 @@ error[E0596]: cannot borrow immutable argument `arg` as mutable
   --> $DIR/borrowck-argument.rs:20:5
    |
 LL | fn func(arg: S) {
-   |         --- consider changing this to `mut arg`
+   |         --- help: make this binding mutable: `mut arg`
 LL |     arg.mutate(); //~ ERROR: cannot borrow immutable argument
    |     ^^^ cannot borrow mutably
 
 error[E0596]: cannot borrow immutable argument `arg` as mutable
   --> $DIR/borrowck-argument.rs:25:9
    |
 LL |     fn method(&self, arg: S) {
-   |                      --- consider changing this to `mut arg`
+   |                      --- help: make this binding mutable: `mut arg`
 LL |         arg.mutate(); //~ ERROR: cannot borrow immutable argument
    |         ^^^ cannot borrow mutably
 
 error[E0596]: cannot borrow immutable argument `arg` as mutable
   --> $DIR/borrowck-argument.rs:31:9
    |
 LL |     fn default(&self, arg: S) {
-   |                       --- consider changing this to `mut arg`
+   |                       --- help: make this binding mutable: `mut arg`
 LL |         arg.mutate(); //~ ERROR: cannot borrow immutable argument
    |         ^^^ cannot borrow mutably
 
@@ -28,7 +28,7 @@ error[E0596]: cannot borrow immutable argument `arg` as mutable
 LL |     (|arg: S| { arg.mutate() })(s); //~ ERROR: cannot borrow immutable argument
    |       ---       ^^^ cannot borrow mutably
    |       |
-   |       consider changing this to `mut arg`
+   |       help: make this binding mutable: `mut arg`
 
 error: aborting due to 4 previous errors
 

src/test/ui/borrowck/borrowck-asm.ast.nll.stderr

@@ -28,7 +28,7 @@ LL |         let x = 3;
    |             -
    |             |
    |             first assignment to `x`
-   |             consider changing this to `mut x`
+   |             help: make this binding mutable: `mut x`
 LL |         unsafe {
 LL |             asm!("nop" : "=r"(x));  //[ast]~ ERROR cannot assign twice
    |             ^^^^^^^^^^^^^^^^^^^^^^ cannot assign twice to immutable variable
@@ -40,7 +40,7 @@ LL |         let x = 3;
    |             -
    |             |
    |             first assignment to `x`
-   |             consider changing this to `mut x`
+   |             help: make this binding mutable: `mut x`
 LL |         unsafe {
 LL |             asm!("nop" : "+r"(x));  //[ast]~ ERROR cannot assign twice
    |             ^^^^^^^^^^^^^^^^^^^^^^ cannot assign twice to immutable variable

src/test/ui/borrowck/borrowck-asm.mir.stderr

@@ -28,7 +28,7 @@ LL |         let x = 3;
    |             -
    |             |
    |             first assignment to `x`
-   |             consider changing this to `mut x`
+   |             help: make this binding mutable: `mut x`
 LL |         unsafe {
 LL |             asm!("nop" : "=r"(x));  //[ast]~ ERROR cannot assign twice
    |             ^^^^^^^^^^^^^^^^^^^^^^ cannot assign twice to immutable variable
@@ -40,7 +40,7 @@ LL |         let x = 3;
    |             -
    |             |
    |             first assignment to `x`
-   |             consider changing this to `mut x`
+   |             help: make this binding mutable: `mut x`
 LL |         unsafe {
 LL |             asm!("nop" : "+r"(x));  //[ast]~ ERROR cannot assign twice
    |             ^^^^^^^^^^^^^^^^^^^^^^ cannot assign twice to immutable variable

src/test/ui/borrowck/borrowck-auto-mut-ref-to-immut-var.stderr

@@ -2,7 +2,7 @@ error[E0596]: cannot borrow immutable local variable `x` as mutable
   --> $DIR/borrowck-auto-mut-ref-to-immut-var.rs:25:5
    |
 LL |     let x = Foo { x: 3 };
-   |         - consider changing this to `mut x`
+   |         - help: make this binding mutable: `mut x`
 LL |     x.printme();    //~ ERROR cannot borrow
    |     ^ cannot borrow mutably
 

src/test/ui/borrowck/borrowck-borrow-from-owned-ptr.stderr

@@ -147,7 +147,7 @@ error[E0596]: cannot borrow field `foo.bar1` of immutable binding as mutable
   --> $DIR/borrowck-borrow-from-owned-ptr.rs:132:21
    |
 LL |     let foo = make_foo();
-   |         --- consider changing this to `mut foo`
+   |         --- help: make this binding mutable: `mut foo`
 LL |     let bar1 = &mut foo.bar1; //~ ERROR cannot borrow
    |                     ^^^^^^^^ cannot mutably borrow field of immutable binding
 

src/test/ui/borrowck/borrowck-borrow-from-stack-variable.stderr

@@ -114,7 +114,7 @@ error[E0596]: cannot borrow field `foo.bar1` of immutable binding as mutable
   --> $DIR/borrowck-borrow-from-stack-variable.rs:130:21
    |
 LL |     let foo = make_foo();
-   |         --- consider changing this to `mut foo`
+   |         --- help: make this binding mutable: `mut foo`
 LL |     let bar1 = &mut foo.bar1; //~ ERROR cannot borrow
    |                     ^^^^^^^^ cannot mutably borrow field of immutable binding
 

src/test/ui/borrowck/borrowck-borrow-immut-deref-of-box-as-mut.stderr

@@ -2,7 +2,7 @@ error[E0596]: cannot borrow immutable `Box` content `*a` as mutable
   --> $DIR/borrowck-borrow-immut-deref-of-box-as-mut.rs:22:5
    |
 LL |     let a: Box<_> = box A;
-   |         - consider changing this to `mut a`
+   |         - help: make this binding mutable: `mut a`
 LL |     a.foo();
    |     ^ cannot borrow as mutable
 

src/test/ui/borrowck/borrowck-match-binding-is-assignment.ast.nll.stderr

@@ -5,7 +5,7 @@ LL |         x => {
    |         -
    |         |
    |         first assignment to `x`
-   |         consider changing this to `mut x`
+   |         help: make this binding mutable: `mut x`
 LL |             x += 1; //[ast]~ ERROR cannot assign twice to immutable variable `x`
    |             ^^^^^^ cannot assign twice to immutable variable
 
@@ -16,7 +16,7 @@ LL |         E::Foo(x) => {
    |                -
    |                |
    |                first assignment to `x`
-   |                consider changing this to `mut x`
+   |                help: make this binding mutable: `mut x`
 LL |             x += 1; //[ast]~ ERROR cannot assign twice to immutable variable `x`
    |             ^^^^^^ cannot assign twice to immutable variable
 
@@ -27,7 +27,7 @@ LL |         S { bar: x } => {
    |                  -
    |                  |
    |                  first assignment to `x`
-   |                  consider changing this to `mut x`
+   |                  help: make this binding mutable: `mut x`
 LL |             x += 1; //[ast]~ ERROR cannot assign twice to immutable variable `x`
    |             ^^^^^^ cannot assign twice to immutable variable
 
@@ -38,7 +38,7 @@ LL |         (x,) => {
    |          -
    |          |
    |          first assignment to `x`
-   |          consider changing this to `mut x`
+   |          help: make this binding mutable: `mut x`
 LL |             x += 1; //[ast]~ ERROR cannot assign twice to immutable variable `x`
    |             ^^^^^^ cannot assign twice to immutable variable
 
@@ -49,7 +49,7 @@ LL |         [x,_,_] => {
    |          -
    |          |
    |          first assignment to `x`
-   |          consider changing this to `mut x`
+   |          help: make this binding mutable: `mut x`
 LL |             x += 1; //[ast]~ ERROR cannot assign twice to immutable variable `x`
    |             ^^^^^^ cannot assign twice to immutable variable
 

src/test/ui/borrowck/borrowck-match-binding-is-assignment.mir.stderr

@@ -5,7 +5,7 @@ LL |         x => {
    |         -
    |         |
    |         first assignment to `x`
-   |         consider changing this to `mut x`
+   |         help: make this binding mutable: `mut x`
 LL |             x += 1; //[ast]~ ERROR cannot assign twice to immutable variable `x`
    |             ^^^^^^ cannot assign twice to immutable variable
 
@@ -16,7 +16,7 @@ LL |         E::Foo(x) => {
    |                -
    |                |
    |                first assignment to `x`
-   |                consider changing this to `mut x`
+   |                help: make this binding mutable: `mut x`
 LL |             x += 1; //[ast]~ ERROR cannot assign twice to immutable variable `x`
    |             ^^^^^^ cannot assign twice to immutable variable
 
@@ -27,7 +27,7 @@ LL |         S { bar: x } => {
    |                  -
    |                  |
    |                  first assignment to `x`
-   |                  consider changing this to `mut x`
+   |                  help: make this binding mutable: `mut x`
 LL |             x += 1; //[ast]~ ERROR cannot assign twice to immutable variable `x`
    |             ^^^^^^ cannot assign twice to immutable variable
 
@@ -38,7 +38,7 @@ LL |         (x,) => {
    |          -
    |          |
    |          first assignment to `x`
-   |          consider changing this to `mut x`
+   |          help: make this binding mutable: `mut x`
 LL |             x += 1; //[ast]~ ERROR cannot assign twice to immutable variable `x`
    |             ^^^^^^ cannot assign twice to immutable variable
 
@@ -49,7 +49,7 @@ LL |         [x,_,_] => {
    |          -
    |          |
    |          first assignment to `x`
-   |          consider changing this to `mut x`
+   |          help: make this binding mutable: `mut x`
 LL |             x += 1; //[ast]~ ERROR cannot assign twice to immutable variable `x`
    |             ^^^^^^ cannot assign twice to immutable variable
 

src/test/ui/borrowck/borrowck-mut-addr-of-imm-var.stderr

@@ -2,7 +2,7 @@ error[E0596]: cannot borrow immutable local variable `x` as mutable
   --> $DIR/borrowck-mut-addr-of-imm-var.rs:13:30
    |
 LL |     let x: isize = 3;
-   |         - consider changing this to `mut x`
+   |         - help: make this binding mutable: `mut x`
 LL |     let y: &mut isize = &mut x; //~ ERROR cannot borrow
    |                              ^ cannot borrow mutably
 

src/test/ui/borrowck/borrowck-mut-slice-of-imm-vec.stderr

@@ -2,7 +2,7 @@ error[E0596]: cannot borrow immutable local variable `v` as mutable
   --> $DIR/borrowck-mut-slice-of-imm-vec.rs:17:16
    |
 LL |     let v = vec![1, 2, 3];
-   |         - consider changing this to `mut v`
+   |         - help: make this binding mutable: `mut v`
 LL |     write(&mut v); //~ ERROR cannot borrow
    |                ^ cannot borrow mutably
 

src/test/ui/borrowck/borrowck-overloaded-call.stderr

@@ -12,7 +12,7 @@ error[E0596]: cannot borrow immutable local variable `s` as mutable
   --> $DIR/borrowck-overloaded-call.rs:77:5
    |
 LL |     let s = SFnMut {
-   |         - consider changing this to `mut s`
+   |         - help: make this binding mutable: `mut s`
 ...
 LL |     s(3);   //~ ERROR cannot borrow immutable local variable `s` as mutable
    |     ^ cannot borrow mutably

src/test/ui/borrowck/borrowck-ref-mut-of-imm.stderr

@@ -2,7 +2,7 @@ error[E0596]: cannot borrow field `(x as std::prelude::v1::Some).0` of immutable
   --> $DIR/borrowck-ref-mut-of-imm.rs:14:12
    |
 LL | fn destructure(x: Option<isize>) -> isize {
-   |                - consider changing this to `mut x`
+   |                - help: make this binding mutable: `mut x`
 ...
 LL |       Some(ref mut v) => *v //~ ERROR cannot borrow
    |            ^^^^^^^^^ cannot mutably borrow field of immutable binding