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Use repeat_packed when calculating layouts in RawVec #151389

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Merged
rust-bors merged 1 commit into from
Jan 23, 2026
Merged

Use repeat_packed when calculating layouts in RawVec #151389

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8 changes: 6 additions & 2 deletions library/alloc/src/raw_vec/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -865,9 +865,13 @@ const fn handle_error(e: TryReserveError) -> ! {
#[inline]
#[rustc_const_unstable(feature = "const_heap", issue = "79597")]
const fn layout_array(cap: usize, elem_layout: Layout) -> Result<Layout, TryReserveError> {
// This is only used with `elem_layout`s which are those of real rust types,
// which lets us use the much-simpler `repeat_packed`.
debug_assert!(elem_layout.size() == elem_layout.pad_to_align().size());

// FIXME(const-hack) return to using `map` and `map_err` once `const_closures` is implemented
match elem_layout.repeat(cap) {
Ok((layout, _pad)) => Ok(layout),
match elem_layout.repeat_packed(cap) {
Ok(layout) => Ok(layout),
Err(_) => Err(CapacityOverflow.into()),
}
}
2 changes: 1 addition & 1 deletion tests/codegen-llvm/iter-repeat-n-trivial-drop.rs
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Expand Up @@ -47,7 +47,7 @@ pub fn iter_repeat_n_next(it: &mut std::iter::RepeatN<NotCopy>) -> Option<NotCop
#[no_mangle]
// CHECK-LABEL: @vec_extend_via_iter_repeat_n
pub fn vec_extend_via_iter_repeat_n() -> Vec<u8> {
// CHECK: %[[ADDR:.+]] = tail call {{(noalias )?}}noundef dereferenceable_or_null(1234) ptr @{{.*}}__rust_alloc(i64 noundef {{(range\(i64 1, 0\) )?}}1234, i64 noundef {{(range\(i64 1, -9223372036854775807\) )?}}1)
// CHECK: %[[ADDR:.+]] = tail call {{(noalias )?}}noundef dereferenceable_or_null(1234) ptr @{{.*}}__rust_alloc(i64 noundef {{(range\(i64 0, -9223372036854775808\) )?}}1234, i64 noundef {{(range\(i64 1, -9223372036854775807\) )?}}1)
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@scottmcm scottmcm Jan 21, 2026

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Interesting. LLVM now knows it's 0..=isize::MAX instead of 1..=usize::MAX.

Arguably it ought to know that it's 1..=isize::MAX, but investigating that is for a future PR.

// CHECK: tail call void @llvm.memset.p0.i64(ptr noundef nonnull align 1 dereferenceable(1234) %[[ADDR]], i8 42, i64 1234,

let n = 1234_usize;
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