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Document why layout.align() + layout.size() doesn't overflow #155255

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Document why layout.align() + layout.size() doesn't overflow #155255

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3 changes: 3 additions & 0 deletions library/std/src/sys/alloc/windows.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -118,6 +118,9 @@ unsafe fn allocate(layout: Layout, zeroed: bool) -> *mut u8 {
process_heap_alloc(MaybeUninit::uninit(), flags, layout.size()) as *mut u8
} else {
// Allocate extra padding in order to be able to satisfy the alignment.
// This addition does not overflow due to `Layout` type invariants,
// `size()` is at most `isize::MAX` while
// `align()` is at most `1 << (bits in usize - 2)` if `size()` is non-zero.
let total = layout.align() + layout.size();

let ptr = process_heap_alloc(MaybeUninit::uninit(), flags, total) as *mut u8;
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