fix: don't remove parentheses for calls of function-like pointers that are members of a struct or union

Author: davidsemakula

src/tools/rust-analyzer/crates/ide-assists/src/handlers/remove_parentheses.rs

@@ -239,4 +239,33 @@ mod tests {
 
         check_assist_not_applicable(remove_parentheses, r#"fn f() { $0(return 2) + 2 }"#);
     }
+
+    #[test]
+    fn remove_parens_indirect_calls() {
+        check_assist(
+            remove_parentheses,
+            r#"fn f(call: fn(usize), arg: usize) { $0(call)(arg); }"#,
+            r#"fn f(call: fn(usize), arg: usize) { call(arg); }"#,
+        );
+        check_assist(
+            remove_parentheses,
+            r#"fn f<F>(call: F, arg: usize) where F: Fn(usize) { $0(call)(arg); }"#,
+            r#"fn f<F>(call: F, arg: usize) where F: Fn(usize) { call(arg); }"#,
+        );
+
+        // Parentheses are necessary when calling a function-like pointer that is a member of a struct or union.
+        check_assist_not_applicable(
+            remove_parentheses,
+            r#"
+struct Foo<T> {
+    t: T,
+}
+
+impl Foo<fn(usize)> {
+    fn foo(&self, arg: usize) {
+        $0(self.t)(arg);
+    }
+}"#,
+        );
+    }
 }

src/tools/rust-analyzer/crates/syntax/src/ast/prec.rs

@@ -27,6 +27,14 @@ impl Expr {
     }
 
     fn needs_parens_in_expr(&self, parent: &Expr) -> bool {
+        // Parentheses are necessary when calling a function-like pointer that is a member of a struct or union
+        // (e.g. `(a.f)()`).
+        let is_parent_call_expr = matches!(parent, ast::Expr::CallExpr(_));
+        let is_field_expr = matches!(self, ast::Expr::FieldExpr(_));
+        if is_parent_call_expr && is_field_expr {
+            return true;
+        }
+
         // Special-case block weirdness
         if parent.child_is_followed_by_a_block() {
             use Expr::*;