rustc_attrs

This feature has no tracking issue, and is therefore internal to the compiler, not being intended for general use.

Note: rustc_attrs enables many rustc-internal attributes and this page only discuss a few of them.

The rustc_attrs feature allows debugging rustc type layouts by using #[rustc_layout(...)] to debug layout at compile time (it even works with cargo check) as an alternative to rustc -Z print-type-sizes that is way more verbose.

Options provided by #[rustc_layout(...)] are debug, size, align, abi. Note that it only works on sized types without generics.

Examples

#![allow(unused)]
#![feature(rustc_attrs)]

fn main() {
#[rustc_layout(abi, size)]
pub enum X {
    Y(u8, u8, u8),
    Z(isize),
}
}

When that is compiled, the compiler will error with something like

error: abi: Aggregate { sized: true }
 --> src/lib.rs:4:1
  |
4 | / pub enum T {
5 | |     Y(u8, u8, u8),
6 | |     Z(isize),
7 | | }
  | |_^

error: size: Size { raw: 16 }
 --> src/lib.rs:4:1
  |
4 | / pub enum T {
5 | |     Y(u8, u8, u8),
6 | |     Z(isize),
7 | | }
  | |_^

error: aborting due to 2 previous errors