Improve mutable-binding suggestion to include name

Author: reddevilmidzy

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -3940,12 +3940,29 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
         if let Some(decl) = local_decl
             && decl.can_be_made_mutable()
         {
-            err.span_suggestion_verbose(
-                decl.source_info.span.shrink_to_lo(),
-                "consider making this binding mutable",
-                "mut ".to_string(),
-                Applicability::MachineApplicable,
-            );
+            if matches!(
+                decl.local_info(),
+                LocalInfo::User(BindingForm::Var(VarBindingForm {
+                    opt_match_place: Some((_, match_span)),
+                    ..
+                })) if matches!(match_span.desugaring_kind(), Some(DesugaringKind::ForLoop))
+            ) && let Ok(binding_name) =
+                self.infcx.tcx.sess.source_map().span_to_snippet(decl.source_info.span)
+            {
+                err.span_suggestion_verbose(
+                    decl.source_info.span.shrink_to_lo(),
+                    "consider making this binding mutable",
+                    format!("(mut {}) ", binding_name),
+                    Applicability::MachineApplicable,
+                );
+            } else {
+                err.span_suggestion_verbose(
+                    decl.source_info.span.shrink_to_lo(),
+                    "consider making this binding mutable",
+                    "mut ".to_string(),
+                    Applicability::MachineApplicable,
+                );
+            }
             if !from_arg
                 && matches!(
                     decl.local_info(),

tests/ui/borrowck/borrowck-for-loop-deref-pattern-assignment.rs

@@ -0,0 +1,7 @@
+fn main() {
+    let nums: &[u32] = &[1, 2, 3];
+    for &num in nums {
+        num *= 2; //~ ERROR cannot assign twice to immutable variable `num`
+        println!("{num}");
+    }
+}

tests/ui/borrowck/borrowck-for-loop-deref-pattern-assignment.stderr

@@ -0,0 +1,20 @@
+error[E0384]: cannot assign twice to immutable variable `num`
+  --> $DIR/borrowck-for-loop-deref-pattern-assignment.rs:4:9
+   |
+LL |     for &num in nums {
+   |          --- first assignment to `num`
+LL |         num *= 2;
+   |         ^^^^^^^^ cannot assign twice to immutable variable
+   |
+help: consider making this binding mutable
+   |
+LL |     for &(mut num) num in nums {
+   |          +++++++++
+help: to modify the original value, take a borrow instead
+   |
+LL |     for &ref mut num in nums {
+   |          +++++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0384`.