fix: consider indentation in the "Generate impl" and "Generate trait impl" assists

This makes the generated impl's indentation match the ADT it targets, improving formatting when
using nested modules inside of the same file or when defining types inside of a function.

Author: IvarWithoutBones

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

@@ -1,10 +1,22 @@
 use syntax::{
-    ast::{self, make, AstNode, HasName},
+    ast::{self, edit_in_place::Indent, make, AstNode, HasName},
     ted,
 };
 
 use crate::{utils, AssistContext, AssistId, AssistKind, Assists};
 
+fn insert_impl(impl_: ast::Impl, nominal: &ast::Adt) {
+    let indent = nominal.indent_level();
+    ted::insert_all_raw(
+        ted::Position::after(nominal.syntax()),
+        vec![
+            // Add a blank line after the ADT, and indentation for the impl to match the ADT
+            make::tokens::whitespace(&format!("\n\n{indent}")).into(),
+            impl_.syntax().clone().into(),
+        ],
+    );
+}
+
 // Assist: generate_impl
 //
 // Adds a new inherent impl for a type.
@@ -46,12 +58,7 @@ pub(crate) fn generate_impl(acc: &mut Assists, ctx: &AssistContext<'_>) -> Optio
                 }
             }
 
-            // Add the impl after the adt
-            let nominal = edit.make_mut(nominal);
-            ted::insert_all_raw(
-                ted::Position::after(nominal.syntax()),
-                vec![make::tokens::blank_line().into(), impl_.syntax().clone().into()],
-            );
+            insert_impl(impl_, &edit.make_mut(nominal));
         },
     )
 }
@@ -97,12 +104,7 @@ pub(crate) fn generate_trait_impl(acc: &mut Assists, ctx: &AssistContext<'_>) ->
                 }
             }
 
-            // Add the impl after the adt
-            let nominal = edit.make_mut(nominal);
-            ted::insert_all_raw(
-                ted::Position::after(nominal.syntax()),
-                vec![make::tokens::blank_line().into(), impl_.syntax().clone().into()],
-            );
+            insert_impl(impl_, &edit.make_mut(nominal));
         },
     )
 }
@@ -418,4 +420,65 @@ mod tests {
             "/// Has a lifetime parameter\nstruct Foo<'a, T: Foo<'a>> {}",
         );
     }
+
+    #[test]
+    fn add_impl_with_indent() {
+        check_assist(
+            generate_impl,
+            r#"
+                mod foo {
+                    struct Bar$0 {}
+                }
+            "#,
+            r#"
+                mod foo {
+                    struct Bar {}
+
+                    impl Bar {$0}
+                }
+            "#,
+        );
+    }
+
+    #[test]
+    fn add_impl_with_multiple_indent() {
+        check_assist(
+            generate_impl,
+            r#"
+                mod foo {
+                    fn bar() {
+                        struct Baz$0 {}
+                    }
+                }
+            "#,
+            r#"
+                mod foo {
+                    fn bar() {
+                        struct Baz {}
+
+                        impl Baz {$0}
+                    }
+                }
+            "#,
+        );
+    }
+
+    #[test]
+    fn add_trait_impl_with_indent() {
+        check_assist(
+            generate_trait_impl,
+            r#"
+                mod foo {
+                    struct Bar$0 {}
+                }
+            "#,
+            r#"
+                mod foo {
+                    struct Bar {}
+
+                    impl ${0:_} for Bar {}
+                }
+            "#,
+        );
+    }
 }