Merge #4945

4945: do not suggest assist for return type to result in bad case  r=matklad a=bnjjj

close #4826

Co-authored-by: Benjamin Coenen <5719034+bnjjj@users.noreply.github.com>

crates/ra_assists/src/handlers/change_return_type_to_result.rs

@@ -4,6 +4,7 @@ use ra_syntax::{
 };
 
 use crate::{AssistContext, AssistId, Assists};
+use test_utils::mark;
 
 // Assist: change_return_type_to_result
 //
@@ -22,8 +23,13 @@ pub(crate) fn change_return_type_to_result(acc: &mut Assists, ctx: &AssistContex
     let fn_def = ret_type.syntax().parent().and_then(ast::FnDef::cast)?;
 
     let type_ref = &ret_type.type_ref()?;
-    if type_ref.syntax().text().to_string().starts_with("Result<") {
-        return None;
+    let ret_type_str = type_ref.syntax().text().to_string();
+    let first_part_ret_type = ret_type_str.splitn(2, '<').next();
+    if let Some(ret_type_first_part) = first_part_ret_type {
+        if ret_type_first_part.ends_with("Result") {
+            mark::hit!(change_return_type_to_result_simple_return_type_already_result);
+            return None;
+        }
     }
 
     let block_expr = &fn_def.body()?;
@@ -296,6 +302,29 @@ mod tests {
         );
     }
 
+    #[test]
+    fn change_return_type_to_result_simple_return_type_already_result_std() {
+        check_assist_not_applicable(
+            change_return_type_to_result,
+            r#"fn foo() -> std::result::Result<i32<|>, String> {
+                let test = "test";
+                return 42i32;
+            }"#,
+        );
+    }
+
+    #[test]
+    fn change_return_type_to_result_simple_return_type_already_result() {
+        mark::check!(change_return_type_to_result_simple_return_type_already_result);
+        check_assist_not_applicable(
+            change_return_type_to_result,
+            r#"fn foo() -> Result<i32<|>, String> {
+                let test = "test";
+                return 42i32;
+            }"#,
+        );
+    }
+
     #[test]
     fn change_return_type_to_result_simple_with_cursor() {
         check_assist(