Fix ICE in borrowck mutability suggestion with multi-byte ref sigil

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -1410,9 +1410,20 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
             (span, " mut".to_owned(), true)
         // If there is already a binding, we modify it to be `mut`.
         } else if binding_exists {
-            // Shrink the span to just after the `&` in `&variable`.
-            let span = span.with_lo(span.lo() + BytePos(1)).shrink_to_lo();
-            (span, "mut ".to_owned(), true)
+            // Replace the sigil with the mutable version. We may be dealing
+            // with parser recovery here and cannot assume the user actually
+            // typed `&` or `*const`, so we compute the prefix from the snippet.
+            let Ok(src) = self.infcx.tcx.sess.source_map().span_to_snippet(span) else {
+                return;
+            };
+            let (prefix_len, replacement) = if local_decl.ty.is_ref() {
+                (src.chars().next().map_or(0, char::len_utf8), "&mut ")
+            } else {
+                (src.find("const").map_or(1, |i| i + "const".len()), "*mut ")
+            };
+            let ws_len = src[prefix_len..].len() - src[prefix_len..].trim_start().len();
+            let span = span.with_hi(span.lo() + BytePos((prefix_len + ws_len) as u32));
+            (span, replacement.to_owned(), true)
         } else {
             // Otherwise, suggest that the user annotates the binding; We provide the
             // type of the local.

tests/ui/borrowck/borrowck-assign-to-andmut-in-aliasable-loc.stderr

@@ -17,7 +17,7 @@ LL |     *s.pointer += 1;
    |
 help: consider changing this to be a mutable reference
    |
-LL | fn c(s: &mut  &mut S) {
+LL | fn c(s: &mut &mut S) {
    |          +++
 
 error: aborting due to 2 previous errors

tests/ui/borrowck/borrowck-borrow-mut-base-ptr-in-aliasable-loc.stderr

@@ -27,7 +27,7 @@ LL |     let x:  &mut isize = &mut **t0;
    |
 help: consider changing this to be a mutable reference
    |
-LL | fn foo4(t0: &mut  &mut isize) {
+LL | fn foo4(t0: &mut &mut isize) {
    |              +++
 
 error: aborting due to 3 previous errors

tests/ui/borrowck/mutability-errors.stderr

@@ -74,8 +74,9 @@ LL |     *x = (1,);
    |
 help: consider changing this to be a mutable pointer
    |
-LL | unsafe fn named_ptr(x: *mut const (i32,)) {
-   |                         +++
+LL - unsafe fn named_ptr(x: *const (i32,)) {
+LL + unsafe fn named_ptr(x: *mut (i32,)) {
+   |
 
 error[E0594]: cannot assign to `x.0`, which is behind a `*const` pointer
   --> $DIR/mutability-errors.rs:24:5
@@ -85,8 +86,9 @@ LL |     (*x).0 = 1;
    |
 help: consider changing this to be a mutable pointer
    |
-LL | unsafe fn named_ptr(x: *mut const (i32,)) {
-   |                         +++
+LL - unsafe fn named_ptr(x: *const (i32,)) {
+LL + unsafe fn named_ptr(x: *mut (i32,)) {
+   |
 
 error[E0596]: cannot borrow `*x` as mutable, as it is behind a `*const` pointer
   --> $DIR/mutability-errors.rs:25:5
@@ -96,8 +98,9 @@ LL |     &mut *x;
    |
 help: consider changing this to be a mutable pointer
    |
-LL | unsafe fn named_ptr(x: *mut const (i32,)) {
-   |                         +++
+LL - unsafe fn named_ptr(x: *const (i32,)) {
+LL + unsafe fn named_ptr(x: *mut (i32,)) {
+   |
 
 error[E0596]: cannot borrow `x.0` as mutable, as it is behind a `*const` pointer
   --> $DIR/mutability-errors.rs:26:5
@@ -107,8 +110,9 @@ LL |     &mut (*x).0;
    |
 help: consider changing this to be a mutable pointer
    |
-LL | unsafe fn named_ptr(x: *mut const (i32,)) {
-   |                         +++
+LL - unsafe fn named_ptr(x: *const (i32,)) {
+LL + unsafe fn named_ptr(x: *mut (i32,)) {
+   |
 
 error[E0594]: cannot assign to data in a `*const` pointer
   --> $DIR/mutability-errors.rs:30:5

tests/ui/span/mutability-suggestion-fullwidth-ampersand.rs

@@ -0,0 +1,9 @@
+// Regression test for https://github.com/rust-lang/rust/issues/139089
+
+fn foo(x: ＆Vec<u8>) {
+    //~^ ERROR unknown start of token
+    x.push(0);
+    //~^ ERROR cannot borrow `*x` as mutable
+}
+
+fn main() {}

tests/ui/span/mutability-suggestion-fullwidth-ampersand.stderr

@@ -0,0 +1,27 @@
+error: unknown start of token: \u{ff06}
+  --> $DIR/mutability-suggestion-fullwidth-ampersand.rs:3:11
+   |
+LL | fn foo(x: ＆Vec<u8>) {
+   |           ^^
+   |
+help: Unicode character '＆' (Fullwidth Ampersand) looks like '&' (Ampersand), but it is not
+   |
+LL - fn foo(x: ＆Vec<u8>) {
+LL + fn foo(x: &Vec<u8>) {
+   |
+
+error[E0596]: cannot borrow `*x` as mutable, as it is behind a `&` reference
+  --> $DIR/mutability-suggestion-fullwidth-ampersand.rs:5:5
+   |
+LL |     x.push(0);
+   |     ^ `x` is a `&` reference, so it cannot be borrowed as mutable
+   |
+help: consider changing this to be a mutable reference
+   |
+LL - fn foo(x: ＆Vec<u8>) {
+LL + fn foo(x: &mut Vec<u8>) {
+   |
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0596`.