`parenthesized_params_in_types_and_modules` future-compatibility warnings

[qnighy]

This is the summary issue for the PARENTHESIZED_PARAMS_IN_TYPES_AND_MODULES future-compatibility warning and other related errors. The goal of this page is describe why this change was made and how you can fix code that is affected by it. It also provides a place to ask questions or register a complaint if you feel the change should not be made.

What is the warning for?

The parenthesized_params_in_types_and_modules warning is issued when you use parenthesized generic parameters in modules, primitive types, type parameters, or Self. For structs, enums, enum variants, unions, and type aliases, the syntax has already issued an error. Typical example of this warning is a code like this:

fn something(&self) -> i32 () {
  //                       ^^ parenthesized params

  ..
  let x = self.foo() as usize();
  //                         ^^ parenthesized params
  ..
}

This warning can always be fixed by removing the parentheses.

When will this warning become a hard error?

At the beginning of each 6-week release cycle, the Rust compiler team will review the set of outstanding future compatibility warnings and nominate some of them for Final Comment Period. Toward the end of
the cycle, we will review any comments and make a final determination whether to convert the warning into a hard error or remove it entirely.

Current status

• Prohibit parenthesized params in more types. #41856 introduces the parenthesized_params_in_types_and_modules lint as warn-by-default
• Make sufficiently old or low-impact compatibility lints deny-by-default #42894 makes the parenthesized_params_in_types_and_modules lint deny-by-default
• PR ? makes the parenthesized_params_in_types_and_modules lint a hard error

[qnighy]

I noticed there is a case where () cannot just be removed.

fn main() {
    0 as usize() < 1; // `()` can't just be removed.
}

The cause is the well-known ambiguity between "less-than" and "open angle bracket". In this case, you can fix the problem like this:

fn main() {
    0 as (usize) < 1; // OK
    (0 as usize) < 1; // OK
}