rustc_attrs - The Rust Unstable Book

# `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`, `abi`. Note that it only work best with sized type without generics.

## Examples

    #![feature(rustc_attrs)]

    #[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