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Field Layout

FieldLayout

#![allow(unused)]
fn main() {
pub struct FieldLayout {
    pub name: &'static str,
    pub offset: usize,
    pub size: usize,
    pub alignment: usize,
    pub pad_after: usize,
}
}

Each entry in Layout::FIELDS describes one field of the struct.

FieldMeaning
nameField identifier as written in Rust.
offsetByte offset of the field within the struct.
sizeWGSL size of the field’s type.
alignmentWGSL alignment of the field’s type.
pad_afterZero bytes to write after this field’s data.

pad_after Semantics

pad_after is the number of padding bytes between the end of this field’s data and the start of the next field (or the end of the struct). When writing a struct to a buffer byte-by-byte:

  1. Write the field’s size bytes.
  2. Write pad_after zero bytes.
  3. Proceed to the next field.

The final field’s pad_after accounts for struct-end padding so that the total equals SIZE.

RuntimeArray<T>

A runtime-sized array has no statically knowable size:

#![allow(unused)]
fn main() {
assert_eq!(<RuntimeArray<f32> as WgslLayout>::SIZE, 0);
}

SIZE is 0 because the array length is runtime-dependent. Such arrays may only appear as the last field of a storage-buffer struct.

Empty Structs

An empty struct is the identity element for layout:

#![allow(unused)]
fn main() {
assert_eq!(Empty::SIZE, 0);
assert_eq!(Empty::ALIGN, 1);
assert!(Empty::FIELDS.is_empty());
}

The WGSL spec does not define the empty-struct case; wgsl-rs-layout defines ALIGN = 1 so that roundUp(offset, 1) is a no-op and empty structs compose without disturbing surrounding layout.