Attribute rustc_dump_layout 

Dumps the layout of the annotated item.

This ends up calling the layout_of query to get the Layout of the annotated item. If used with the debug modifier, it will print the entirety of Layout. Other modifiers will print only parts of it.

See AttributeKind::RustcDumpLayout for the internal representation of this attribute.

Example: debug

//@ dont-require-annotations: ERROR
//@ compile-flags: --crate-type lib -Z ui-testing=no
//@ normalize-stderr: "randomization_seed: \d+" -> "randomization_seed: $$SEED"

#![feature(rustc_attrs)]

#[rustc_dump_layout(debug)]
pub union Union {
    Float: f32,
    Int: u32,
}

produces:

error: layout_of(Union) = Layout {
          size: Size(4 bytes),
          align: AbiAlign {
              abi: Align(4 bytes),
          },
          backend_repr: Memory {
              sized: true,
          },
          fields: Union(
              2,
          ),
          largest_niche: None,
          uninhabited: false,
          variants: Single {
              index: 0,
          },
          max_repr_align: None,
          unadjusted_abi_align: Align(4 bytes),
          randomization_seed: $SEED,
      }
--> $DIR/rustc_dump_layout_debug.rs:8:1
 |
8 | pub union Union {
 | ^^^^^^^^^^^^^^^

error: aborting due to 1 previous error

Example: largest_niche

//@ dont-require-annotations: ERROR
//@ compile-flags: --crate-type lib -Z ui-testing=no

#![feature(rustc_attrs)]

#[rustc_dump_layout(largest_niche)]
type Alias = Option<char>;

produces:

error: largest_niche: Some(Niche { offset: Size(0 bytes), value: u32, valid_range: (..=1114111) | (4294967295..) })
--> $DIR/rustc_dump_layout_largest_niche.rs:7:1
 |
7 | type Alias = Option<char>;
 | ^^^^^^^^^^

error: aborting due to 1 previous error

Example: size

//@ dont-require-annotations: ERROR
//@ compile-flags: --crate-type lib -Z ui-testing=no

#![feature(rustc_attrs)]

#[rustc_dump_layout(size)]
type ID = std::any::TypeId;

produces:

error: size: Size(16 bytes)
--> $DIR/rustc_dump_layout_size.rs:7:1
 |
7 | type ID = std::any::TypeId;
 | ^^^^^^^

error: aborting due to 1 previous error

Example: align

//@ dont-require-annotations: ERROR
//@ compile-flags: --crate-type lib -Z ui-testing=no

#![feature(rustc_attrs)]

#[rustc_dump_layout(align)]
enum Enum {
    Bytes([u8; 4]),
    Int(u32),
}

produces:

error: align: Align(4 bytes)
--> $DIR/rustc_dump_layout_align.rs:7:1
 |
7 | enum Enum {
 | ^^^^^^^^^

error: aborting due to 1 previous error

Example: backend_repr

//@ dont-require-annotations: ERROR
//@ compile-flags: --crate-type lib -Z ui-testing=no

#![feature(rustc_attrs)]
#![feature(portable_simd)]

#[rustc_dump_layout(backend_repr)]
type Simd = std::simd::u32x4;

produces:

error: backend_repr: SimdVector { element: u32 is .., count: BackendLaneCount(4) }
--> $DIR/rustc_dump_layout_backend_repr.rs:8:1
 |
8 | type Simd = std::simd::u32x4;
 | ^^^^^^^^^

error: aborting due to 1 previous error

Example: homogeneous_aggregate

//@ dont-require-annotations: ERROR
//@ compile-flags: --crate-type lib -Z ui-testing=no

#![feature(rustc_attrs)]

#[rustc_dump_layout(homogeneous_aggregate)]
#[repr(C)]
struct Struct {
    field: [u8; 32],
    unit: (),
}

produces:

error: homogeneous_aggregate: Ok(Homogeneous(Reg { kind: Integer, size: Size(1 bytes) }))
--> $DIR/rustc_dump_layout_homogeneous_aggregate.rs:8:1
 |
8 | struct Struct {
 | ^^^^^^^^^^^^^

error: aborting due to 1 previous error