Tweak &mut self suggestion span

```
error[E0596]: cannot borrow `*self.s` as mutable, as it is behind a `&` reference
  --> $DIR/issue-38147-1.rs:17:9
   |
LL |         self.s.push('x');
   |         ^^^^^^ `self` is a `&` reference, so the data it refers to cannot be borrowed as mutable
   |
help: consider changing this to be a mutable reference
   |
LL |     fn f(&mut self) {
   |           +++
```

Note the suggestion to add `mut` instead of replacing the entire `&self` with `&mut self`.

Author: estebank

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -1140,10 +1140,9 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
 
         let amp_mut_sugg = match *local_decl.local_info() {
             LocalInfo::User(mir::BindingForm::ImplicitSelf(_)) => {
-                let suggestion = suggest_ampmut_self(self.infcx.tcx, decl_span);
-                let additional =
-                    local_trait.map(|span| (span, suggest_ampmut_self(self.infcx.tcx, span)));
-                Some(AmpMutSugg { has_sugg: true, span: decl_span, suggestion, additional })
+                let (span, suggestion) = suggest_ampmut_self(self.infcx.tcx, decl_span);
+                let additional = local_trait.map(|span| suggest_ampmut_self(self.infcx.tcx, span));
+                Some(AmpMutSugg { has_sugg: true, span, suggestion, additional })
             }
 
             LocalInfo::User(mir::BindingForm::Var(mir::VarBindingForm {
@@ -1202,10 +1201,11 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                                     opt_ty_info: None,
                                     ..
                                 })) => {
-                                    let sugg = suggest_ampmut_self(self.infcx.tcx, decl_span);
+                                    let (span, sugg) =
+                                        suggest_ampmut_self(self.infcx.tcx, decl_span);
                                     Some(AmpMutSugg {
                                         has_sugg: true,
-                                        span: decl_span,
+                                        span,
                                         suggestion: sugg,
                                         additional: None,
                                     })
@@ -1461,17 +1461,12 @@ fn mut_borrow_of_mutable_ref(local_decl: &LocalDecl<'_>, local_name: Option<Symb
     }
 }
 
-fn suggest_ampmut_self<'tcx>(tcx: TyCtxt<'tcx>, span: Span) -> String {
+fn suggest_ampmut_self(tcx: TyCtxt<'_>, span: Span) -> (Span, String) {
     match tcx.sess.source_map().span_to_snippet(span) {
-        Ok(snippet) => {
-            let lt_pos = snippet.find('\'');
-            if let Some(lt_pos) = lt_pos {
-                format!("&{}mut self", &snippet[lt_pos..snippet.len() - 4])
-            } else {
-                "&mut self".to_string()
-            }
+        Ok(snippet) if snippet.ends_with("self") => {
+            (span.with_hi(span.hi() - BytePos(4)).shrink_to_hi(), "mut ".to_string())
         }
-        _ => "&mut self".to_string(),
+        _ => (span, "&mut self".to_string()),
     }
 }
 

tests/ui/borrowck/issue-93093.rs

@@ -4,7 +4,7 @@ struct S {
 }
 impl S {
     async fn bar(&self) { //~ HELP consider changing this to be a mutable reference
-        //~| SUGGESTION &mut self
+        //~| SUGGESTION mut
         self.foo += 1; //~ ERROR cannot assign to `self.foo`, which is behind a `&` reference [E0594]
     }
 }

tests/ui/borrowck/issue-93093.stderr

@@ -7,7 +7,7 @@ LL |         self.foo += 1;
 help: consider changing this to be a mutable reference
    |
 LL |     async fn bar(&mut self) {
-   |                  ~~~~~~~~~
+   |                   +++
 
 error: aborting due to 1 previous error
 

tests/ui/borrowck/trait-impl-argument-difference-ice.stderr

@@ -41,7 +41,7 @@ LL |         let a16 = self.read_word() as u16;
 help: consider changing this to be a mutable reference in the `impl` method and the `trait` definition
    |
 LL |     extern "C" fn read_dword(&'_ mut self) -> u16 {
-   |                              ~~~~~~~~~~~~
+   |                                  +++
 
 error[E0596]: cannot borrow `*self` as mutable, as it is behind a `&` reference
   --> $DIR/trait-impl-argument-difference-ice.rs:18:19
@@ -52,7 +52,7 @@ LL |         let b16 = self.read_word() as u16;
 help: consider changing this to be a mutable reference in the `impl` method and the `trait` definition
    |
 LL |     extern "C" fn read_dword(&'_ mut self) -> u16 {
-   |                              ~~~~~~~~~~~~
+   |                                  +++
 
 error: aborting due to 5 previous errors; 1 warning emitted
 

tests/ui/did_you_mean/issue-38147-1.stderr

@@ -7,7 +7,7 @@ LL |         self.s.push('x');
 help: consider changing this to be a mutable reference
    |
 LL |     fn f(&mut self) {
-   |          ~~~~~~~~~
+   |           +++
 
 error: aborting due to 1 previous error
 

tests/ui/did_you_mean/issue-39544.stderr

@@ -18,7 +18,7 @@ LL |         let _ = &mut self.x;
 help: consider changing this to be a mutable reference
    |
 LL |     fn foo<'z>(&'z mut self) {
-   |                ~~~~~~~~~~~~
+   |                    +++
 
 error[E0596]: cannot borrow `self.x` as mutable, as it is behind a `&` reference
   --> $DIR/issue-39544.rs:20:17
@@ -29,7 +29,7 @@ LL |         let _ = &mut self.x;
 help: consider changing this to be a mutable reference
    |
 LL |     fn foo1(&mut self, other: &Z) {
-   |             ~~~~~~~~~
+   |              +++
 
 error[E0596]: cannot borrow `other.x` as mutable, as it is behind a `&` reference
   --> $DIR/issue-39544.rs:21:17
@@ -51,7 +51,7 @@ LL |         let _ = &mut self.x;
 help: consider changing this to be a mutable reference
    |
 LL |     fn foo2<'a>(&'a mut self, other: &Z) {
-   |                 ~~~~~~~~~~~~
+   |                     +++
 
 error[E0596]: cannot borrow `other.x` as mutable, as it is behind a `&` reference
   --> $DIR/issue-39544.rs:26:17

tests/ui/mut/mutable-class-fields-2.stderr

@@ -7,7 +7,7 @@ LL |     self.how_hungry -= 5;
 help: consider changing this to be a mutable reference
    |
 LL |   pub fn eat(&mut self) {
-   |              ~~~~~~~~~
+   |               +++
 
 error: aborting due to 1 previous error
 

tests/ui/suggestions/suggest-ref-mut.rs

@@ -3,7 +3,7 @@ struct X(usize);
 impl X {
     fn zap(&self) {
         //~^ HELP
-        //~| SUGGESTION &mut self
+        //~| SUGGESTION mut
         self.0 = 32;
         //~^ ERROR
     }

tests/ui/suggestions/suggest-ref-mut.stderr

@@ -7,7 +7,7 @@ LL |         self.0 = 32;
 help: consider changing this to be a mutable reference
    |
 LL |     fn zap(&mut self) {
-   |            ~~~~~~~~~
+   |             +++
 
 error[E0594]: cannot assign to `*foo`, which is behind a `&` reference
   --> $DIR/suggest-ref-mut.rs:15:5