src/doc/unstable-book/src/language-features/rustc-attrs.md

   1 # `rustc_attrs`
   2
   3 This feature has no tracking issue, and is therefore internal to
   4 the compiler, not being intended for general use.
   5
   6 Note: `rustc_attrs` enables many rustc-internal attributes and this page
   7 only discuss a few of them.
   8
   9 ------------------------
  10
  11 The `rustc_attrs` feature allows debugging rustc type layouts by using
  12 `#[rustc_layout(...)]` to debug layout at compile time (it even works
  13 with `cargo check`) as an alternative to `rustc -Z print-type-sizes`
  14 that is way more verbose.
  15
  16 Options provided by `#[rustc_layout(...)]` are `debug`, `size`, `align`,
  17 `abi`. Note that it only works on sized types without generics.
  18
  19 ## Examples
  20
  21 ```rust,ignore
  22 #![feature(rustc_attrs)]
  23
  24 #[rustc_layout(abi, size)]
  25 pub enum X {
  26     Y(u8, u8, u8),
  27     Z(isize),
  28 }
  29 ```
  30
  31 When that is compiled, the compiler will error with something like
  32
  33 ```text
  34 error: abi: Aggregate { sized: true }
  35  --> src/lib.rs:4:1
  36   |
  37 4 | / pub enum T {
  38 5 | |     Y(u8, u8, u8),
  39 6 | |     Z(isize),
  40 7 | | }
  41   | |_^
  42
  43 error: size: Size { raw: 16 }
  44  --> src/lib.rs:4:1
  45   |
  46 4 | / pub enum T {
  47 5 | |     Y(u8, u8, u8),
  48 6 | |     Z(isize),
  49 7 | | }
  50   | |_^
  51
  52 error: aborting due to 2 previous errors
  53 ```