README.md

qualifier_attr

Procedural macro attributes for adding "qualifiers" to various items.

At the moment, the crate supports the following "qualifiers":

• pub, pub(crate), etc. - visibility and restriction
• default - default implementations (a feature of specialization)
• async - asynchronous code, e.g. async fn
• unsafe - unsafe code, e.g. unsafe fn, unsafe trait
• const - code that may run at compile time, e.g. const fn
• extern "ABI" - specifying an ABI, e.g. extern "C" fn

Limitations

• Non-inline modules in proc macro input are unstable but can be enabled with unstable #[feature(proc_macro_hygiene)]. Screenshot of a compile error.

Examples

#[macro_use]
extern crate qualifier_attr;

// We can add a qualifier to a function
// with an attribute.
#[qualifiers(const)]
fn const_fn() -> u32 {
    42
}

const CONST_RES: u32 = const_fn();

// It's not so impressive on its own,
// but with `cfg_attr`, it can be conditional.
#[cfg_attr(feature = "extern_c", no_mangle, qualifiers(pub, extern "C"))]
fn extern_c_fn() -> u32 {
    42
}

// It even works with types, imports, and more!
mod foo {
    #[qualifiers(pub)]
    struct Foo {
        x: i32,
        y: i32,
    }
}

#[qualifiers(pub)]
use foo::Foo;

// Traits and implementations too!?
#[cfg_attr(feature = "unsafe_quux", qualifiers(unsafe))]
trait Quux {
    fn quux_the_thing();
}

#[cfg_attr(feature = "unsafe_quux", qualifiers(unsafe))]
impl Quux for Foo {
    fn quux_the_thing() {
        println!("The thing was quuxed.");
    }
}

// You can add qualifiers to the fields of a
// struct as well with this special attribute.
#[field_qualifiers(x(pub), y(pub))]
struct Point2 {
    x: i32,
    y: i32,
}

#[field_qualifiers(_0(pub), _1(pub), _2(pub))]
struct Point3(i32, i32, i32);

Learn more about cfg_attr here.

Note on legacy attributes

Before version 0.2.0, this crate provided a separate attribute for each kind of item. While this should generally be a small win for compile times, it is generally not justified by the additional complexity. The legacy attributes are still available behind the default legacy_attrs feature flag, but their use is currently discouraged. In order to disable the legacy attributes, add the following to your Cargo.toml:

[dependencies]
qualifier_attr = { version = "0.2", default-features = false }

Similar crates

• const_fn.

License

^{Licensed under either of Apache License, Version 2.0 or MIT license at your option.}
_{Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this crate by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.}