fix: Extract Function produces duplicate fn names

This change fixes issue #10037, in more or less the most naive fashion
possible.

We continue to start with the hardcoded default of "fun_name", and now append a
counter to the end of it if that name is already in scope.

In the future, we can probably apply more heuristics here to wind up with more
useful names by default, but for now this resolves the immediate problem.

crates/ide-assists/src/handlers/extract_function.rs

@@ -81,7 +81,8 @@ pub(crate) fn extract_function(acc: &mut Assists, ctx: &AssistContext) -> Option
 
     let anchor = if self_param.is_some() { Anchor::Method } else { Anchor::Freestanding };
     let insert_after = node_to_insert_after(&body, anchor)?;
-    let module = ctx.sema.scope(&insert_after)?.module();
+    let semantics_scope = ctx.sema.scope(&insert_after)?;
+    let module = semantics_scope.module();
 
     let ret_ty = body.return_ty(ctx)?;
     let control_flow = body.external_control_flow(ctx, &container_info)?;
@@ -105,8 +106,10 @@ pub(crate) fn extract_function(acc: &mut Assists, ctx: &AssistContext) -> Option
             let params =
                 body.extracted_function_params(ctx, &container_info, locals_used.iter().copied());
 
+            let name = make_function_name(&semantics_scope);
+
             let fun = Function {
-                name: make::name_ref("fun_name"),
+                name,
                 self_param,
                 params,
                 control_flow,
@@ -155,6 +158,21 @@ pub(crate) fn extract_function(acc: &mut Assists, ctx: &AssistContext) -> Option
     )
 }
 
+fn make_function_name(semantics_scope: &hir::SemanticsScope) -> ast::NameRef {
+    let mut names_in_scope = vec![];
+    semantics_scope.process_all_names(&mut |name, _| names_in_scope.push(name.to_string()));
+
+    let default_name = "fun_name";
+
+    let mut name = default_name.to_string();
+    let mut counter = 0;
+    while names_in_scope.contains(&name) {
+        counter += 1;
+        name = format!("{}{}", &default_name, counter)
+    }
+    make::name_ref(&name)
+}
+
 /// Try to guess what user wants to extract
 ///
 /// We have basically have two cases:
@@ -4709,6 +4727,56 @@ fn $0fun_name() {
     /* a comment */
     let x = 0;
 }
+"#,
+        );
+    }
+
+    #[test]
+    fn it_should_not_generate_duplicate_function_names() {
+        check_assist(
+            extract_function,
+            r#"
+fn fun_name() {
+    $0let x = 0;$0
+}
+"#,
+            r#"
+fn fun_name() {
+    fun_name1();
+}
+
+fn $0fun_name1() {
+    let x = 0;
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn should_increment_suffix_until_it_finds_space() {
+        check_assist(
+            extract_function,
+            r#"
+fn fun_name1() {
+    let y = 0;
+}
+
+fn fun_name() {
+    $0let x = 0;$0
+}
+"#,
+            r#"
+fn fun_name1() {
+    let y = 0;
+}
+
+fn fun_name() {
+    fun_name2();
+}
+
+fn $0fun_name2() {
+    let x = 0;
+}
 "#,
         );
     }