Provide structured suggestion for removal of &mut

compiler/rustc_mir/src/borrow_check/diagnostics/mutability_errors.rs

@@ -12,7 +12,7 @@ use rustc_middle::{
 };
 use rustc_span::source_map::DesugaringKind;
 use rustc_span::symbol::{kw, Symbol};
-use rustc_span::Span;
+use rustc_span::{BytePos, Span};
 
 use crate::borrow_check::diagnostics::BorrowedContentSource;
 use crate::borrow_check::MirBorrowckCtxt;
@@ -278,7 +278,25 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                             );
                         }
                     }
-                    err.span_help(source_info.span, "try removing `&mut` here");
+                    if let Ok(snippet) =
+                        self.infcx.tcx.sess.source_map().span_to_snippet(source_info.span)
+                    {
+                        if snippet.starts_with("&mut ") {
+                            // We don't have access to the HIR to get accurate spans, but we can
+                            // give a best effort structured suggestion.
+                            err.span_suggestion_verbose(
+                                source_info.span.with_hi(source_info.span.lo() + BytePos(5)),
+                                "try removing `&mut` here",
+                                String::new(),
+                                Applicability::MachineApplicable,
+                            );
+                        } else {
+                            // This can occur with things like `(&mut self).foo()`.
+                            err.span_help(source_info.span, "try removing `&mut` here");
+                        }
+                    } else {
+                        err.span_help(source_info.span, "try removing `&mut` here");
+                    }
                 } else if decl.mutability == Mutability::Not
                     && !matches!(
                         decl.local_info,

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

@@ -10,10 +10,9 @@ note: the binding is already a mutable borrow
 LL | pub fn f(b: &mut i32) {
    |             ^^^^^^^^
 help: try removing `&mut` here
-  --> $DIR/mut-borrow-of-mut-ref.rs:7:7
    |
-LL |     h(&mut b);
+LL |     h(b);
-   |       ^^^^^^
+   |      --
 
 error[E0596]: cannot borrow `b` as mutable, as it is not declared as mutable
   --> $DIR/mut-borrow-of-mut-ref.rs:11:12
@@ -27,10 +26,9 @@ note: the binding is already a mutable borrow
 LL | pub fn f(b: &mut i32) {
    |             ^^^^^^^^
 help: try removing `&mut` here
-  --> $DIR/mut-borrow-of-mut-ref.rs:11:12
    |
-LL |     g(&mut &mut b);
+LL |     g(&mut b);
-   |            ^^^^^^
+   |           --
 
 error[E0596]: cannot borrow `b` as mutable, as it is not declared as mutable
   --> $DIR/mut-borrow-of-mut-ref.rs:18:12
@@ -44,10 +42,9 @@ note: the binding is already a mutable borrow
 LL | pub fn g(b: &mut i32) {
    |             ^^^^^^^^
 help: try removing `&mut` here
-  --> $DIR/mut-borrow-of-mut-ref.rs:18:12
    |
-LL |     h(&mut &mut b);
+LL |     h(&mut b);
-   |            ^^^^^^
+   |           --
 
 error[E0596]: cannot borrow `f` as mutable, as it is not declared as mutable
   --> $DIR/mut-borrow-of-mut-ref.rs:35:5

src/test/ui/did_you_mean/issue-34126.stderr

@@ -10,10 +10,9 @@ note: the binding is already a mutable borrow
 LL |     fn start(&mut self) {
    |              ^^^^^^^^^
 help: try removing `&mut` here
-  --> $DIR/issue-34126.rs:6:18
    |
-LL |         self.run(&mut self);
+LL |         self.run(self);
-   |                  ^^^^^^^^^
+   |                 --
 
 error[E0502]: cannot borrow `self` as mutable because it is also borrowed as immutable
   --> $DIR/issue-34126.rs:6:18