Macro hygiene change breaks Firefox builds.

[emilio]

The following program with paste = "0.1.12" compiles on stable (rustc 1.46.0 (04488afe3 2020-08-24)), but doesn't on Nightly (rustc 1.48.0-nightly (0e2c1281e 2020-09-07)).

This is minimized from Firefox source code, which doesn't build anymore with Rust Nightly.

To be fair, I'm surprised it built in the first place...

#[macro_use] extern crate paste;

#[derive(Clone, Copy)]
struct Bar {
    member: i32,
}

impl Bar {
    fn get_member(&self, _: i32, _: i32) -> i32 {
        self.member
    }
}

macro_rules! foo {
    ($fn_name:ident $(, $arg:ident)*) => {
        paste::expr! {
            Bar::[<get_ $fn_name>](&self.bar $(, $arg)* ,callback)
        }
    };
}

struct Foo {
    bar: Bar,
}

impl Foo {
    fn member(&self, n: i32, callback: i32) -> i32 {
        foo!(member, n)
    }
}

fn main() {}

[emilio]

The Firefox source for reference: https://searchfox.org/mozilla-central/rev/76a83d0a218837ba6937d6a0fac51cb0008c2334/services/fxaccounts/rust-bridge/firefox-accounts-bridge/src/bridge.rs#26-38,111-113

[sugar700]

This also happens on 1.47.0-beta.2 (84b047b 2020-08-28) making it regression-from-stable-to-beta.

[Mark-Simulacrum]

cc @Aaron1011 @petrochenkov

[emilio]

I'm fixing this in Firefox in https://bugzilla.mozilla.org/show_bug.cgi?id=1663677. There's another macro-related regression that I'm trying to reduce and that seems much harder to work around, but I'll file that separately.

[Aaron1011]

This is intentional - before PR #73084, we were using re-parsed tokens for the call to paste::expr! due to the use of a metavariable. We now use the original tokens, which preserves the hygiene information on self and callback.

[emilio]

I'm ok with wontfix, as I noted in my report I was surprised that this compiled at all. But it seems odd to see breaking changes like that...

[emilio]

Chances are that #76482 is the same too (just with a bit of indirection because of syn), not sure.

[Aaron1011]

@emilio: The breaking changes are unfortunate, but are an unavoidable part of making progress towards fixing #43081. As you noted, the broken code should have never compiled in the first place due to the hygiene rules of macro_rules! macros.

Several Crater runs were performed before landing #73084. Unfortunately, Crater cannot test all Rust code - in this case, it looks like firefox-accounts-bridge could have been tested, but Crater did not know about it (it's on neither crates.io nor GitHub).