2.2 KiB
- Start Date: 2014-03-20
- RFC PR: rust-lang/rfcs#49
- Rust Issue: rust-lang/rust#12812
Summary
Allow attributes on match arms.
Motivation
One sometimes wishes to annotate the arms of match statements with
attributes, for example with conditional compilation #[cfg]s or
with branch weights (the latter is the most important use).
For the conditional compilation, the work-around is duplicating the
whole containing function with a #[cfg]. A case study is
sfackler's bindings to OpenSSL,
where many distributions remove SSLv2 support, and so that portion of
Rust bindings needs to be conditionally disabled. The obvious way to
support the various different SSL versions is an enum
pub enum SslMethod {
#[cfg(sslv2)]
/// Only support the SSLv2 protocol
Sslv2,
/// Only support the SSLv3 protocol
Sslv3,
/// Only support the TLSv1 protocol
Tlsv1,
/// Support the SSLv2, SSLv3 and TLSv1 protocols
Sslv23,
}
However, all matchs can only mention Sslv2 when the cfg is
active, i.e. the following is invalid:
fn name(method: SslMethod) -> &'static str {
match method {
Sslv2 => "SSLv2",
Sslv3 => "SSLv3",
_ => "..."
}
}
A valid method would be to have two definitions: #[cfg(sslv2)] fn name(...) and #[cfg(not(sslv2)] fn name(...). The former has the
Sslv2 arm, the latter does not. Clearly, this explodes exponentially
for each additional cfg'd variant in an enum.
Branch weights would allow the careful micro-optimiser to inform the compiler that, for example, a certain match arm is rarely taken:
match foo {
Common => {}
#[cold]
Rare => {}
}
Detailed design
Normal attribute syntax, applied to a whole match arm.
match x {
#[attr]
Thing => {}
#[attr]
Foo | Bar => {}
#[attr]
_ => {}
}
Alternatives
There aren't really any general alternatives; one could probably hack around matching on conditional enum variants with some macros and helper functions to share as much code as possible; but in general this won't work.
Unresolved questions
Nothing particularly.