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

Author: lapla-cogito

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -1411,7 +1411,16 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
         // 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();
+            let amp_len = self
+                .infcx
+                .tcx
+                .sess
+                .source_map()
+                .span_to_snippet(span)
+                .ok()
+                .and_then(|s| s.chars().next().map(|c| c.len_utf8() as u32))
+                .unwrap_or(1);
+            let span = span.with_lo(span.lo() + BytePos(amp_len)).shrink_to_lo();
             (span, "mut ".to_owned(), true)
         } else {
             // Otherwise, suggest that the user annotates the binding; We provide the

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 +1,26 @@
+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: ＆mut Vec<u8>) {
+   |             +++
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0596`.