"consider removing the borrow" suggests to only remove `&` and not `mut`

[m-ou-se]

In the following snippet, rustc suggests to remove just the & instead of removing the &mut, resulting in an invalid suggestion: mut format!("").

fn main() {
    let _: String = &mut format!("");
}

error[E0308]: mismatched types
 --> src/main.rs:2:21
  |
2 |     let _: String = &mut format!("");
  |            ------   ^^^^^^^^^^^^^^^^
  |            |        |
  |            |        expected struct `String`, found `&mut String`
  |            |        help: consider removing the borrow: `mut format!("")`
  |            expected due to this

[m-ou-se]

The issue lies in the strip_prefix('&') here:

rust/compiler/rustc_typeck/src/check/demand.rs

Lines 579 to 584 in 1e13a9b

	if sm.is_imported(expr.span) {
	if let Ok(src) = sm.span_to_snippet(sp) {
	if let Some(src) = src.strip_prefix('&') {
	return Some((
	sp,
	"consider removing the borrow",

Not sure how to easily fix it though, because & and mut are separate tokens (and unlike & is usually followed by whitespace), so strip_prefix("&mut") won't do.