Typo suggestion to change bindings with leading underscore

When encountering a binding that isn't found but has a typo suggestion
for a binding with a leading underscore, suggest changing the binding
definition instead of the use place.

Fix #60164.

Author: estebank

compiler/rustc_resolve/src/diagnostics.rs

@@ -1511,17 +1511,30 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                 ),
             );
         }
+
+        let (span, sugg, post) = if let SuggestionTarget::SimilarlyNamed = suggestion.target
+            && let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span)
+            && let Some(span) = suggestion.span
+            && let Some(candidate) = suggestion.candidate.as_str().strip_prefix("_")
+            && snippet == candidate
+        {
+            // When the suggested binding change would be from `x` to `_x`, suggest changing the
+            // original binding definition instead. (#60164)
+            (span, snippet, ", consider changing it")
+        } else {
+            (span, suggestion.candidate.to_string(), "")
+        };
         let msg = match suggestion.target {
             SuggestionTarget::SimilarlyNamed => format!(
-                "{} {} with a similar name exists",
+                "{} {} with a similar name exists{post}",
                 suggestion.res.article(),
                 suggestion.res.descr()
             ),
             SuggestionTarget::SingleItem => {
                 format!("maybe you meant this {}", suggestion.res.descr())
             }
         };
-        err.span_suggestion(span, msg, suggestion.candidate, Applicability::MaybeIncorrect);
+        err.span_suggestion(span, msg, sugg, Applicability::MaybeIncorrect);
         true
     }
 

tests/ui/suggestions/silenced-binding-typo.fixed

@@ -0,0 +1,5 @@
+// run-rustfix
+fn main() {
+    let x = 42; //~ HELP
+    let _y = x; //~ ERROR
+}

tests/ui/suggestions/silenced-binding-typo.rs

@@ -0,0 +1,5 @@
+// run-rustfix
+fn main() {
+    let _x = 42; //~ HELP
+    let _y = x; //~ ERROR
+}

tests/ui/suggestions/silenced-binding-typo.stderr

@@ -0,0 +1,14 @@
+error[E0425]: cannot find value `x` in this scope
+  --> $DIR/silenced-binding-typo.rs:4:14
+   |
+LL |     let _y = x;
+   |              ^
+   |
+help: a local variable with a similar name exists, consider changing it
+   |
+LL |     let x = 42;
+   |         ~
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0425`.