braces around {self} in UseTree are not unnecessary

Before this commit `UseTree::remove_unnecessary_braces` removed the braces
around `{self}` in `use x::y::{self};` but `use x::y::self;` is not valid
rust.

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

@@ -776,6 +776,40 @@ mod z {
         );
     }
 
+    #[test]
+    fn remove_unused_fixes_nested_self() {
+        check_assist(
+            remove_unused_imports,
+            r#"
+mod inner {
+    pub struct X();
+    pub struct Y();
+}
+
+mod z {
+    use super::inner::{self, X}$0;
+
+    fn f() {
+        let y = inner::Y();
+    }
+}
+"#,
+            r#"mod inner {
+    pub struct X();
+    pub struct Y();
+}
+
+mod z {
+    use super::inner::{self};
+
+    fn f() {
+        let y = inner::Y();
+    }
+}
+"#,
+        );
+    }
+
     #[test]
     fn dont_remove_used_glob() {
         check_assist_not_applicable(

crates/syntax/src/ast/node_ext.rs

@@ -376,11 +376,24 @@ impl ast::UseTreeList {
             .filter_map(|it| it.into_token().filter(|it| it.kind() == T![,]))
     }
 
+    pub fn is_just_self(&self) -> bool {
+        if let Some((single_subtree,)) = self.use_trees().collect_tuple() {
+            fn path_is_self(path: &ast::Path) -> bool {
+                path.segment().and_then(|seg| seg.self_token()).is_some()
+                    && path.qualifier().is_none()
+            }
+
+            single_subtree.path().as_ref().map_or(false, path_is_self)
+        } else {
+            false
+        }
+    }
+
     /// Remove the unnecessary braces in current `UseTreeList`
     pub fn remove_unnecessary_braces(mut self) {
         let remove_brace_in_use_tree_list = |u: &ast::UseTreeList| {
             let use_tree_count = u.use_trees().count();
-            if use_tree_count == 1 {
+            if use_tree_count == 1 && !u.is_just_self() {
                 if let Some(a) = u.l_curly_token() {
                     ted::remove(a)
                 }