use structured suggestion for "missing mut" label

Fixes #54133.

Author: euclio

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
     +=