Check & before suggest remove deref when trait_selection

Signed-off-by: xizheyin <[email protected]>

Author: xizheyin

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

@@ -1510,13 +1510,11 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         let mut suggested_ty = trait_pred.self_ty().skip_binder();
         'outer: loop {
             while let hir::ExprKind::AddrOf(_, _, borrowed) = expr.kind {
-                count += 1;
                 let span = if expr.span.eq_ctxt(borrowed.span) {
                     expr.span.until(borrowed.span)
                 } else {
                     expr.span.with_hi(expr.span.lo() + BytePos(1))
                 };
-                suggestions.push((span, String::new()));
 
                 let ty::Ref(_, inner_ty, _) = suggested_ty.kind() else {
                     break 'outer;
@@ -1525,8 +1523,16 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
 
                 expr = borrowed;
 
-                if maybe_suggest(suggested_ty, count, suggestions.clone()) {
-                    return true;
+                if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span)
+                    && snippet.starts_with("&")
+                {
+                    // Only when the expr has leading `&`-reference,
+                    // we record it and suggest removing it.
+                    count += 1;
+                    suggestions.push((span, String::new()));
+                    if maybe_suggest(suggested_ty, count, suggestions.clone()) {
+                        return true;
+                    }
                 }
             }
             if let hir::ExprKind::Path(hir::QPath::Resolved(None, path)) = expr.kind

tests/ui/traits/suggest-remove-deref-issue-140166.stderr

@@ -6,6 +6,7 @@ LL |     format_args!("{:?}", FlatMap(&Chars));
    |                   |
    |                   required by a bound introduced by this call
    |
+   = help: the trait `Trait` is implemented for `Chars`
 note: required for `FlatMap<&Chars>` to implement `Debug`
   --> $DIR/suggest-remove-deref-issue-140166.rs:7:16
    |
@@ -15,11 +16,6 @@ LL | impl<T: Trait> std::fmt::Debug for FlatMap<T> {
    |         unsatisfied trait bound introduced here
 note: required by a bound in `core::fmt::rt::Argument::<'_>::new_debug`
   --> $SRC_DIR/core/src/fmt/rt.rs:LL:COL
-help: consider removing the leading `&`-reference
-   |
-LL -     format_args!("{:?}", FlatMap(&Chars));
-LL +     format_args!("{:?}", latMap(&Chars));
-   |
 
 error: aborting due to 1 previous error