Or patterns in macro-rules

Summary

• How patterns work in macro_rules macros changes slightly:
  • $_:pat in macro_rules now matches usage of | too: e.g. A | B.
  • The new $_:pat_param behaves like $_:pat did before; it does not match (top level) |.
  • $_:pat_param is available in all editions.

Details

Starting in Rust 1.53.0, patterns are extended to support | nested anywhere in the pattern. This enables you to write Some(1 | 2) instead of Some(1) | Some(2). Since this was simply not allowed before, this is not a breaking change.

However, this change also affects macro_rules macros. Such macros can accept patterns using the :pat fragment specifier. Currently, :pat does not match top level |, since before Rust 1.53, not all patterns (at all nested levels) could contain a |. Macros that accept patterns like A | B, such as matches!() use something like $($_:pat)|+.

Because this would potentially break existing macros, the meaning of :pat did not change in Rust 1.53.0 to include |. Instead, that change happens in Rust 2021. In the new edition, the :pat fragment specifier will match A | B.

$_:pat fragments in Rust 2021 cannot be followed by an explicit |. Since there are times that one still wishes to match pattern fragments followed by a |, the fragment specified :pat_param has been added to retain the older behavior.

It's important to remember that editions are per crate, so the only relevant edition is the edition of the crate where the macro is defined. The edition of the crate where the macro is used does not change how the macro works.

Migration

A lint, rust_2021_incompatible_or_patterns, gets triggered whenever there is a use $_:pat which will change meaning in Rust 2021.

You can automatically migrate your code to be Rust 2021 Edition compatible or ensure it is already compatible by running:

cargo fix --edition

If you have a macro which relies on $_:pat not matching the top level use of | in patterns, you'll need to change each occurrence of $_:pat to $_:pat_param.

For example:

#![allow(unused)]
fn main() {
macro_rules! my_macro { 
	($x:pat | $y:pat) => {
		// TODO: implementation
	} 
}

// This macro works in Rust 2018 since `$x:pat` does not match against `|`:
my_macro!(1 | 2);

// In Rust 2021 however, the `$_:pat` fragment matches `|` and is not allowed
// to be followed by a `|`. To make sure this macro still works in Rust 2021
// change the macro to the following:
macro_rules! my_macro { 
	($x:pat_param | $y:pat) => { // <- this line is different
		// TODO: implementation
	} 
}
}