Static rustc_lint::macro_expr_fragment_specifier_2024_migration::EDITION_2024_EXPR_FRAGMENT_SPECIFIER

pub static EDITION_2024_EXPR_FRAGMENT_SPECIFIER: &Lint

The edition_2024_expr_fragment_specifier lint detects the use of expr fragments in macros during migration to the 2024 edition.

The expr fragment specifier will accept more expressions in the 2024 edition. To maintain the behavior from the 2021 edition and earlier, use the expr_2021 fragment specifier.

Example

#![deny(edition_2024_expr_fragment_specifier)]
macro_rules! m {
  ($e:expr) => {
      $e
  }
}

fn main() {
   m!(1);
}

{{produces}}

Explanation

Rust editions allow the language to evolve without breaking backwards compatibility. This lint catches code that uses macro matcher fragment specifiers that have changed meaning in the 2024 edition. If you switch to the new edition without updating the code, your macros may behave differently.

In the 2024 edition, the expr fragment specifier expr will also match const { ... } blocks. This means if a macro had a pattern that matched $e:expr and another that matches const { $e: expr }, for example, that under the 2024 edition the first pattern would match while in the 2021 and earlier editions the second pattern would match. To keep the old behavior, use the expr_2021 fragment specifier.

This lint detects macros whose behavior might change due to the changing meaning of the expr fragment specifier. It is “allow” by default because the code is perfectly valid in older editions. The cargo fix tool with the --edition flag will switch this lint to “warn” and automatically apply the suggested fix from the compiler. This provides a completely automated way to update old code for a new edition.

Using cargo fix --edition with this lint will ensure that your code retains the same behavior. This may not be the desired, as macro authors often will want their macros to use the latest grammar for matching expressions. Be sure to carefully review changes introduced by this lint to ensure the macros implement the desired behavior.