diff options
| -rw-r--r-- | library/alloc/src/vec/in_place_collect.rs | 13 |
1 files changed, 8 insertions, 5 deletions
diff --git a/library/alloc/src/vec/in_place_collect.rs b/library/alloc/src/vec/in_place_collect.rs index ec5f32539f2..5a783e66752 100644 --- a/library/alloc/src/vec/in_place_collect.rs +++ b/library/alloc/src/vec/in_place_collect.rs @@ -168,7 +168,9 @@ const fn in_place_collectible<DEST, SRC>( step_merge: Option<NonZeroUsize>, step_expand: Option<NonZeroUsize>, ) -> bool { - if const { SRC::IS_ZST || DEST::IS_ZST || mem::align_of::<SRC>() < mem::align_of::<DEST>() } { + // Require matching alignments because an alignment-changing realloc is inefficient on many + // system allocators and better implementations would require the unstable Allocator trait. + if const { SRC::IS_ZST || DEST::IS_ZST || mem::align_of::<SRC>() != mem::align_of::<DEST>() } { return false; } @@ -188,7 +190,8 @@ const fn in_place_collectible<DEST, SRC>( const fn needs_realloc<SRC, DEST>(src_cap: usize, dst_cap: usize) -> bool { if const { mem::align_of::<SRC>() != mem::align_of::<DEST>() } { - return src_cap > 0; + // FIXME: use unreachable! once that works in const + panic!("in_place_collectible() prevents this"); } // If src type size is an integer multiple of the destination type size then @@ -276,8 +279,8 @@ where let dst_guard = InPlaceDstBufDrop { ptr: dst_buf, len, cap: dst_cap }; src.forget_allocation_drop_remaining(); - // Adjust the allocation if the alignment didn't match or the source had a capacity in bytes - // that wasn't a multiple of the destination type size. + // Adjust the allocation if the source had a capacity in bytes that wasn't a multiple + // of the destination type size. // Since the discrepancy should generally be small this should only result in some // bookkeeping updates and no memmove. if needs_realloc::<I::Src, T>(src_cap, dst_cap) { @@ -290,7 +293,7 @@ where let src_size = mem::size_of::<I::Src>().unchecked_mul(src_cap); let old_layout = Layout::from_size_align_unchecked(src_size, src_align); - // The must be equal or smaller for in-place iteration to be possible + // The allocation must be equal or smaller for in-place iteration to be possible // therefore the new layout must be ≤ the old one and therefore valid. let dst_align = mem::align_of::<T>(); let dst_size = mem::size_of::<T>().unchecked_mul(dst_cap); |