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Strengthen soundness proof for size_of_val_raw #1574

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merged 2 commits into from
Aug 6, 2024
Merged

Strengthen soundness proof for size_of_val_raw #1574

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987c34a
Strengthen soundness proof for size_of_val_raw
joshlf Aug 6, 2024
d5ab566
Merge branch 'main' into size-of-val-raw-zero-dst
joshlf Aug 6, 2024
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17 changes: 7 additions & 10 deletions src/macro_util.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -132,7 +132,8 @@ macro_rules! trailing_field_offset {
let min_size = {
let zero_elems: *const [()] =
$crate::macro_util::core_reexport::ptr::slice_from_raw_parts(
#[allow(clippy::incompatible_msrv)] // Work around https://github.com/rust-lang/rust-clippy/issues/12280
// Work around https://github.com/rust-lang/rust-clippy/issues/12280
#[allow(clippy::incompatible_msrv)]
$crate::macro_util::core_reexport::ptr::NonNull::<()>::dangling()
.as_ptr()
.cast_const(),
Expand All @@ -144,16 +145,12 @@ macro_rules! trailing_field_offset {
// - If `$ty` is not a slice DST, this pointer conversion will
// fail due to "mismatched vtable kinds", and compilation will
// fail.
// - If `$ty` is a slice DST, the safety requirement is that "the
// length of the slice tail must be an initialized integer, and
// the size of the entire value (dynamic tail length +
// statically sized prefix) must fit in isize." The length is
// initialized to 0 above, and Rust guarantees that no type's
// minimum size may overflow `isize`. [1]
// - If `$ty` is a slice DST, we have constructed `zero_elems` to
// have zero trailing slice elements. Per the `size_of_val_raw`
// docs, "For the special case where the dynamic tail length is
// 0, this function is safe to call." [1]
//
// [1] TODO(#429),
// TODO(https://github.com/rust-lang/unsafe-code-guidelines/issues/465#issuecomment-1782206516):
// Citation for this?
// [1] https://doc.rust-lang.org/nightly/std/mem/fn.size_of_val_raw.html
unsafe {
#[allow(clippy::as_conversions)]
$crate::macro_util::core_reexport::mem::size_of_val_raw(zero_elems as *const $ty)
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