Support #[derive(KnownLayout)] for unsized repr(transparent) structs
#2841
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Previously, `#[derive(KnownLayout)]` only supported unsized types if they were `repr(C)`. `repr(transparent)` structs with unsized fields were not supported, even though they are valid if the single non-ZST field is unsized. This change updates the derive macro to include `repr(transparent)` structs in the "complex" layout logic path. It also ensures that the shadow struct generated for layout analysis is emitted as `#[repr(C)]` instead of `#[repr(transparent)]`. This avoids `E0690` errors (strict transparent alignment rules) when fields are wrapped in `MaybeUninit` or `ManuallyDrop`, while still guaranteeing the correct layout offsets. Tests have been added to `zerocopy-derive/tests/struct_known_layout.rs`.
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Support
#[derive(KnownLayout)]for unsizedrepr(transparent)structs.Updates the
KnownLayoutderive macro to handlerepr(transparent)structs similarly torepr(C)structs, enabling support for unsized types. To ensure valid compilation of the generated shadow struct (used for layout analysis), the shadow struct is emitted with#[repr(C)]when the original struct is#[repr(transparent)]. This works around strictrepr(transparent)validation rules that can be triggered byMaybeUninitwrappers while maintaining layout correctness.Fixes an issue where
#[derive(KnownLayout)]would fail or fallback to requiringSizedfor valid unsizedrepr(transparent)structs.PR created automatically by Jules for task 13518082163380290330 started by @joshlf