feat: add fix for unused_variables

crates/ide-diagnostics/src/handlers/unused_variables.rs

@@ -1,3 +1,11 @@
+use ide_db::{
+    assists::{Assist, AssistId, AssistKind},
+    base_db::FileRange,
+    label::Label,
+    source_change::SourceChange,
+};
+use text_edit::TextEdit;
+
 use crate::{Diagnostic, DiagnosticCode, DiagnosticsContext};
 
 // Diagnostic: unused-variables
@@ -8,15 +16,35 @@ pub(crate) fn unused_variables(
     d: &hir::UnusedVariable,
 ) -> Diagnostic {
     let ast = d.local.primary_source(ctx.sema.db).syntax_ptr();
+    let diagnostic_range = ctx.sema.diagnostics_display_range(ast);
+    let var_name = d.local.primary_source(ctx.sema.db).syntax().to_string();
     Diagnostic::new_with_syntax_node_ptr(
         ctx,
         DiagnosticCode::RustcLint("unused_variables"),
         "unused variable",
         ast,
     )
+    .with_fixes(fixes(&var_name, diagnostic_range, ast.file_id.is_macro()))
     .experimental()
 }
 
+fn fixes(var_name: &String, diagnostic_range: FileRange, is_in_marco: bool) -> Option<Vec<Assist>> {
+    if is_in_marco {
+        return None;
+    }
+    Some(vec![Assist {
+        id: AssistId("unscore_unused_variable_name", AssistKind::QuickFix),
+        label: Label::new(format!("Rename unused {} to _{}", var_name, var_name)),
+        group: None,
+        target: diagnostic_range.range,
+        source_change: Some(SourceChange::from_text_edit(
+            diagnostic_range.file_id,
+            TextEdit::replace(diagnostic_range.range, format!("_{}", var_name)),
+        )),
+        trigger_signature_help: false,
+    }])
+}
+
 #[cfg(test)]
 mod tests {
     use crate::tests::check_diagnostics;