trait selection: check & before suggest remove deref

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

@@ -1516,6 +1516,12 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 } else {
                     expr.span.with_hi(expr.span.lo() + BytePos(1))
                 };
+
+                match self.tcx.sess.source_map().span_to_snippet(span) {
+                    Ok(snippet) if snippet.starts_with("&") => {}
+                    _ => break 'outer,
+                }
+
                 suggestions.push((span, String::new()));
 
                 let ty::Ref(_, inner_ty, _) = suggested_ty.kind() else {

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

@@ -0,0 +1,18 @@
+trait Trait {}
+
+struct Chars;
+impl Trait for Chars {}
+
+struct FlatMap<T>(T);
+impl<T: Trait> std::fmt::Debug for FlatMap<T> {
+    fn fmt(&self, _: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        unimplemented!()
+    }
+}
+
+fn lol() {
+    format_args!("{:?}", FlatMap(&Chars));
+    //~^ ERROR the trait bound `&Chars: Trait` is not satisfied [E0277]
+}
+
+fn main() {}

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

@@ -0,0 +1,22 @@
+error[E0277]: the trait bound `&Chars: Trait` is not satisfied
+  --> $DIR/suggest-remove-deref-issue-140166.rs:14:26
+   |
+LL |     format_args!("{:?}", FlatMap(&Chars));
+   |                   ----   ^^^^^^^^^^^^^^^ the trait `Trait` is not implemented for `&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
+   |
+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
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.