Auto merge of #12991 - TiddoLangerak:extract-method-from-trait-into-impl-root, r=Veykril

Feat: extracted method from trait impl is placed in existing impl

**Before**

https://user-images.githubusercontent.com/1759192/183872883-3b0eafd2-d1dc-440e-9e66-38e3372f8b64.mp4

**After**

https://user-images.githubusercontent.com/1759192/183875769-87f34c7d-52f0-4dfc-9766-f591ee738ebb.mp4

Previously, when triggering a method extraction from within an impl trait block, then this would always create a new impl block for
the struct, even if there already is one. Now, if there is already an existing trait-less impl block, then it'll put the extracted method in there.

**Caveats**:
- It currently requires the target impl block to be non-empty. This limitation is because the current architecture takes a `node_to_insert_after` as reference for where to insert the extracted function. An empty impl block doesn't have such a reference node, since it's empty. It seems that supporting this requires a much larger and more complex change.
- This is my first contribution in rust, so apologies for any beginner mistakes.

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

@@ -109,8 +109,6 @@ pub(crate) fn extract_function(acc: &mut Assists, ctx: &AssistContext<'_>) -> Op
             let params =
                 body.extracted_function_params(ctx, &container_info, locals_used.iter().copied());
 
-            let extracted_from_trait_impl = body.extracted_from_trait_impl();
-
             let name = make_function_name(&semantics_scope);
 
             let fun = Function {
@@ -129,8 +127,11 @@ pub(crate) fn extract_function(acc: &mut Assists, ctx: &AssistContext<'_>) -> Op
 
             builder.replace(target_range, make_call(ctx, &fun, old_indent));
 
+            let has_impl_wrapper =
+                insert_after.ancestors().any(|a| a.kind() == SyntaxKind::IMPL && a != insert_after);
+
             let fn_def = match fun.self_param_adt(ctx) {
-                Some(adt) if extracted_from_trait_impl => {
+                Some(adt) if anchor == Anchor::Method && !has_impl_wrapper => {
                     let fn_def = format_function(ctx, module, &fun, old_indent, new_indent + 1);
                     generate_impl_text(&adt, &fn_def).replace("{\n\n", "{")
                 }
@@ -272,7 +273,7 @@ enum FunType {
 }
 
 /// Where to put extracted function definition
-#[derive(Debug)]
+#[derive(Debug, Eq, PartialEq, Clone, Copy)]
 enum Anchor {
     /// Extract free function and put right after current top-level function
     Freestanding,
@@ -1245,6 +1246,14 @@ fn node_to_insert_after(body: &FunctionBody, anchor: Anchor) -> Option<SyntaxNod
     while let Some(next_ancestor) = ancestors.next() {
         match next_ancestor.kind() {
             SyntaxKind::SOURCE_FILE => break,
+            SyntaxKind::IMPL => {
+                if body.extracted_from_trait_impl() && matches!(anchor, Anchor::Method) {
+                    let impl_node = find_non_trait_impl(&next_ancestor);
+                    if let target_node @ Some(_) = impl_node.as_ref().and_then(last_impl_member) {
+                        return target_node;
+                    }
+                }
+            }
             SyntaxKind::ITEM_LIST if !matches!(anchor, Anchor::Freestanding) => continue,
             SyntaxKind::ITEM_LIST => {
                 if ancestors.peek().map(SyntaxNode::kind) == Some(SyntaxKind::MODULE) {
@@ -1265,6 +1274,29 @@ fn node_to_insert_after(body: &FunctionBody, anchor: Anchor) -> Option<SyntaxNod
     last_ancestor
 }
 
+fn find_non_trait_impl(trait_impl: &SyntaxNode) -> Option<ast::Impl> {
+    let as_impl = ast::Impl::cast(trait_impl.clone())?;
+    let impl_type = Some(impl_type_name(&as_impl)?);
+
+    let sibblings = trait_impl.parent()?.children();
+    sibblings
+        .filter_map(ast::Impl::cast)
+        .find(|s| impl_type_name(s) == impl_type && !is_trait_impl(s))
+}
+
+fn last_impl_member(impl_node: &ast::Impl) -> Option<SyntaxNode> {
+    let last_child = impl_node.assoc_item_list()?.assoc_items().last()?;
+    Some(last_child.syntax().clone())
+}
+
+fn is_trait_impl(node: &ast::Impl) -> bool {
+    node.trait_().is_some()
+}
+
+fn impl_type_name(impl_node: &ast::Impl) -> Option<String> {
+    Some(impl_node.self_ty()?.to_string())
+}
+
 fn make_call(ctx: &AssistContext<'_>, fun: &Function, indent: IndentLevel) -> String {
     let ret_ty = fun.return_type(ctx);
 
@@ -5051,6 +5083,236 @@ fn $0fun_name(&self) -> i32 {
         );
     }
 
+    #[test]
+    fn extract_method_from_trait_with_existing_non_empty_impl_block() {
+        check_assist(
+            extract_function,
+            r#"
+struct Struct(i32);
+trait Trait {
+    fn bar(&self) -> i32;
+}
+
+impl Struct {
+    fn foo() {}
+}
+
+impl Trait for Struct {
+    fn bar(&self) -> i32 {
+        $0self.0 + 2$0
+    }
+}
+"#,
+            r#"
+struct Struct(i32);
+trait Trait {
+    fn bar(&self) -> i32;
+}
+
+impl Struct {
+    fn foo() {}
+
+    fn $0fun_name(&self) -> i32 {
+        self.0 + 2
+    }
+}
+
+impl Trait for Struct {
+    fn bar(&self) -> i32 {
+        self.fun_name()
+    }
+}
+"#,
+        )
+    }
+
+    #[test]
+    fn extract_function_from_trait_with_existing_non_empty_impl_block() {
+        check_assist(
+            extract_function,
+            r#"
+struct Struct(i32);
+trait Trait {
+    fn bar(&self) -> i32;
+}
+
+impl Struct {
+    fn foo() {}
+}
+
+impl Trait for Struct {
+    fn bar(&self) -> i32 {
+        let three_squared = $03 * 3$0;
+        self.0 + three_squared
+    }
+}
+"#,
+            r#"
+struct Struct(i32);
+trait Trait {
+    fn bar(&self) -> i32;
+}
+
+impl Struct {
+    fn foo() {}
+}
+
+impl Trait for Struct {
+    fn bar(&self) -> i32 {
+        let three_squared = fun_name();
+        self.0 + three_squared
+    }
+}
+
+fn $0fun_name() -> i32 {
+    3 * 3
+}
+"#,
+        )
+    }
+
+    #[test]
+    fn extract_method_from_trait_with_multiple_existing_impl_blocks() {
+        check_assist(
+            extract_function,
+            r#"
+struct Struct(i32);
+struct StructBefore(i32);
+struct StructAfter(i32);
+trait Trait {
+    fn bar(&self) -> i32;
+}
+
+impl StructBefore {
+    fn foo(){}
+}
+
+impl Struct {
+    fn foo(){}
+}
+
+impl StructAfter {
+    fn foo(){}
+}
+
+impl Trait for Struct {
+    fn bar(&self) -> i32 {
+        $0self.0 + 2$0
+    }
+}
+"#,
+            r#"
+struct Struct(i32);
+struct StructBefore(i32);
+struct StructAfter(i32);
+trait Trait {
+    fn bar(&self) -> i32;
+}
+
+impl StructBefore {
+    fn foo(){}
+}
+
+impl Struct {
+    fn foo(){}
+
+    fn $0fun_name(&self) -> i32 {
+        self.0 + 2
+    }
+}
+
+impl StructAfter {
+    fn foo(){}
+}
+
+impl Trait for Struct {
+    fn bar(&self) -> i32 {
+        self.fun_name()
+    }
+}
+"#,
+        )
+    }
+
+    #[test]
+    fn extract_method_from_trait_with_multiple_existing_trait_impl_blocks() {
+        check_assist(
+            extract_function,
+            r#"
+struct Struct(i32);
+trait Trait {
+    fn bar(&self) -> i32;
+}
+trait TraitBefore {
+    fn before(&self) -> i32;
+}
+trait TraitAfter {
+    fn after(&self) -> i32;
+}
+
+impl TraitBefore for Struct {
+    fn before(&self) -> i32 {
+        42
+    }
+}
+
+impl Struct {
+    fn foo(){}
+}
+
+impl TraitAfter for Struct {
+    fn after(&self) -> i32 {
+        42
+    }
+}
+
+impl Trait for Struct {
+    fn bar(&self) -> i32 {
+        $0self.0 + 2$0
+    }
+}
+"#,
+            r#"
+struct Struct(i32);
+trait Trait {
+    fn bar(&self) -> i32;
+}
+trait TraitBefore {
+    fn before(&self) -> i32;
+}
+trait TraitAfter {
+    fn after(&self) -> i32;
+}
+
+impl TraitBefore for Struct {
+    fn before(&self) -> i32 {
+        42
+    }
+}
+
+impl Struct {
+    fn foo(){}
+
+    fn $0fun_name(&self) -> i32 {
+        self.0 + 2
+    }
+}
+
+impl TraitAfter for Struct {
+    fn after(&self) -> i32 {
+        42
+    }
+}
+
+impl Trait for Struct {
+    fn bar(&self) -> i32 {
+        self.fun_name()
+    }
+}
+"#,
+        )
+    }
+
     #[test]
     fn closure_arguments() {
         check_assist(