Improve keyword completion for 'let' and 'let mut'

Author: Natural-selection1

src/tools/rust-analyzer/crates/ide-completion/src/completions/expr.rs

@@ -365,7 +365,8 @@ pub(crate) fn complete_expr_path(
                     add_keyword("false", "false");
 
                     if in_condition || in_block_expr {
-                        add_keyword("let", "let");
+                        add_keyword("let mut", "let mut $0");
+                        add_keyword("let", "let $0");
                     }
 
                     if after_if_expr {

src/tools/rust-analyzer/crates/ide-completion/src/completions/keyword.rs

@@ -330,4 +330,34 @@ fn main() {
 ",
         )
     }
+
+    #[test]
+    fn completes_let_with_space() {
+        check_edit(
+            "let",
+            r#"
+fn main() {
+    $0
+}
+"#,
+            r#"
+fn main() {
+    let $0
+}
+"#,
+        );
+        check_edit(
+            "let mut",
+            r#"
+fn main() {
+    $0
+}
+"#,
+            r#"
+fn main() {
+    let mut $0
+}
+"#,
+        );
+    }
 }