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| -rw-r--r-- | src/doc/unstable-book/src/language-features/rustc-attrs.md | 50 |
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diff --git a/src/doc/unstable-book/src/language-features/rustc-attrs.md b/src/doc/unstable-book/src/language-features/rustc-attrs.md new file mode 100644 index 00000000000..500ae61b1dd --- /dev/null +++ b/src/doc/unstable-book/src/language-features/rustc-attrs.md @@ -0,0 +1,50 @@ +# `rustc_attrs` + +This feature has no tracking issue, and is therefore likely internal to +the compiler, not being intended for general use. + +------------------------ + +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 + +```rust +#![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 +``` |