Re: [PATCH v7 16/31] rust: ptr: add const_align_up()

[David Rheinsberg]

Hi!

On Fri, Mar 20, 2026, at 10:03 AM, Alice Ryhl wrote:
> The Alignment type can only hold values that are a power of two.

That solves my concern!

Kinda off-topic: why doesn't `Alignment` store a u8 that represents the exponent, rather than the power? The left-shift when needing the power should be effectively free, shouldn't it? It would avoid all the unsafety in the impl.

>> >> FYI, `core` provides `usize::checked_next_multiple_of()` ((const-)stable since 1.73). So an alternative would be:
>> >>
>> >> pub const fn const_align_up(value: usize, align: Alignment) -> Option<usize> {
>> >>     value.checked_next_multiple_of(align.as_usize())
>> >> }
>> >
>> > That would return value+align when value is already aligned, which is wrong.
>>
>> You sure? (emphasis mine:)
>>
>> "Calculates the smallest value greater than or **EQUAL TO** self that is a multiple of rhs."
>>
>> assert_eq!(16_u64.next_multiple_of(8), 16);
>
> Okay, well, weird naming then.

Do you need this helper then at all? I assume it is added because `Alignable` cannot be used in const. But it hard-codes `usize` as type, yet does not reflect that in the name. It comes down to which one is more readable, I guess:

    const_align_up(value, align)
vs
    value.checked_next_multiple_of(align.as_usize())

Meh, I don't mind too much. Just wanted to point out that the standard library provides this exactly.

David