Explicit Hygiene Markers

Macro hygiene is not only there to avoid identifier name clashes; it affects language semantics in at least one other way: editions. When the behavior of a program depends on the edition, the edition to use is taken from an appropriate token; for example for the if let rescope changes I believe the relevant edition is that of the if token.

When a crate in edition N calls a macro from a crate in edition M, each token recalls which edition it comes from. So the following code may or may not panic depending on the edition of the crate that defines the if_let macro.

use core::cell::Cell;

// Possibly defined in another crate.
macro_rules! if_let {
    (($pat:pat = $expr:expr) { $($then:tt)* } else  { $($else:tt)* }) => {
        if let $pat = $expr { $($then)* } else  { $($else)* }
    }
}

fn main() {
    let x = Cell::new(0);
    x.set(42);
    if_let!((None = Some(&ZeroOnDrop(&x))) {
        assert_eq!(x.get(), 42);
        // In edition 2024, value is dropped here
    } else {
        assert_eq!(x.get(), 0); // Fails in edition 2021
    });
    // In edition 2021, value is dropped here
    assert_eq!(x.get(), 0);
}

/// Sets the value to `0` on drop.
struct ZeroOnDrop<'b>(&'b Cell<u32>);
impl<'b> Drop for ZeroOnDrop<'b> {
    fn drop(&mut self) {
        self.0.set(0);
    }
}

Naively expanding macros could therefore change program semantics. To be able to expand the macro in a semantics-preserving way, this feature adds a #[edition = "..."] attribute that changes the edition of the following token. So the macro above would expand to #[edition = "2021"] if let ... or #[edition = "2024"] if let ... depending on the edition of the crate that declares it.

Discussion

I do not know if there are other hygiene considerations that can affect program semantics. If so we'll need to add similar attributes.

This attribute proposal is actually incomplete: attributes are not allowed between any two tokens. Maybe the edition-relevant tokens can all be preceded by an attribute? Otherwise we'll need to invent more syntax like x#edition#2024 or such horrors.

Backlinks

• Name Resolution & Macro Expansion