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

@@ -378,9 +378,26 @@ impl ast::UseTreeList {
 
     /// Remove the unnecessary braces in current `UseTreeList`
     pub fn remove_unnecessary_braces(mut self) {
+        // Returns true iff there is a single subtree and it is not the self keyword. The braces in
+        // `use x::{self};` are necessary and so we should not remove them.
+        let has_single_subtree_that_is_not_self = |u: &ast::UseTreeList| {
+            if let Some((single_subtree,)) = u.use_trees().collect_tuple() {
+                // We have a single subtree, check whether it is self.
+
+                let is_self = single_subtree.path().as_ref().map_or(false, |path| {
+                    path.segment().and_then(|seg| seg.self_token()).is_some()
+                        && path.qualifier().is_none()
+                });
+
+                !is_self
+            } else {
+                // Not a single subtree
+                false
+            }
+        };
+
         let remove_brace_in_use_tree_list = |u: &ast::UseTreeList| {
-            let use_tree_count = u.use_trees().count();
-            if use_tree_count == 1 {
+            if has_single_subtree_that_is_not_self(u) {
                 if let Some(a) = u.l_curly_token() {
                     ted::remove(a)
                 }