Auto merge of rust-lang#12262 - jhgg:fix-whitespace-between-mut-self-in-macro-expansion, r=lnicola

ide: insert whitespace between 'mut' and 'self' in macro expansion

fixes rust-lang#12260

Author: bors

crates/ide-db/src/syntax_helpers/insert_whitespace_into_node.rs

@@ -88,6 +88,9 @@ pub fn insert_ws_into(syn: SyntaxNode) -> SyntaxNode {
             LIFETIME_IDENT if is_next(is_text, true) => {
                 mods.push(do_ws(after, tok));
             }
+            MUT_KW if is_next(|it| it == SELF_KW, false) => {
+                mods.push(do_ws(after, tok));
+            }
             AS_KW | DYN_KW | IMPL_KW | CONST_KW => {
                 mods.push(do_ws(after, tok));
             }