diagnostic_on_unmatch_args

The tracking issue for this feature is: #155642

The diagnostic_on_unmatch_args feature adds the #[diagnostic::on_unmatch_args(...)] attribute for declarative macros. It lets a macro definition customize diagnostics for matcher failures after all arms have been tried, such as incomplete invocations or trailing extra arguments.

This attribute currently applies to declarative macros such as macro_rules! and pub macro. It is currently used for errors emitted by declarative macro matching itself; fragment parser errors still use their existing diagnostics.

#![allow(unused)]
#![feature(diagnostic_on_unmatch_args)]

fn main() {
#[diagnostic::on_unmatch_args(
    message = "invalid arguments to {This} macro invocation",
    label = "expected a type and value here",
    note = "this macro expects a type and a value, like `pair!(u8, 0)`",
    note = "see <link/to/docs>",
)]
macro_rules! pair {
    ($ty:ty, $value:expr) => {};
}

pair!(u8);
}

This emits output like:

error: invalid arguments to pair macro invocation
  --> example.rs:13:9
   |
9  | macro_rules! pair {
   | ----------------- when calling this macro
...
13 | pair!(u8);
   |         ^ expected a type and value here
   |
note: while trying to match `,`
  --> example.rs:10:12
   |
10 |     ($ty:ty, $value:expr) => {};
   |            ^
   = note: this macro expects a type and a value, like `pair!(u8, 0)`
   = note: see <link/to/docs>