Macro static_assertions::assert_fields

macro_rules! assert_fields {
    ($($xs:tt)+) => { ... };
}

Asserts that the type has the given fields.

Examples

One common use case is when types have fields defined multiple times as a result of #[cfg]. This can be an issue when exposing a public API.

pub struct Ty {
    #[cfg(windows)]
    pub val1: u8,
    #[cfg(not(windows))]
    pub val1: usize,

    #[cfg(unix)]
    pub val2: u32,
    #[cfg(not(unix))]
    pub val2: usize,
}

// Requires a unique label in module scope
assert_fields!(windows; Ty, val1);

fn main() {
    // Always have `val2` regardless of OS
    assert_fields!(Ty, val2);
}

The labeling limitation is not necessary if compiling on nightly Rust with the nightly feature enabled:

#![feature(underscore_const_names)]

use std::ops::Range;

assert_fields!(Range<u32>, start, end);

Range does not have a field named middle:

assert_fields!(Range<u32>, middle);