Re: [PATCH v2 2/4] rust: add `Alignment` type

["Alexandre Courbot" <acourbot@nvidia.com>]

On Tue Aug 5, 2025 at 12:47 AM JST, Daniel Almeida wrote:
<snip>
>> +/// [`align_down!`] macros.
>> +///
>> +/// Heavily inspired by the [`Alignment`] nightly feature from the Rust standard library, and
>> +/// hopefully to be eventually replaced by it.
>
> It’s a bit hard to parse this.
>
> Also, I wonder if we should standardize some syntax for TODOs so we can parse
> them using a script? This way we can actually keep track and perhaps pipe them
> to our GitHub page as “good first issues” or just regular issues.
>
> I guess a simple "// TODO: “ here will do, for example.

FWIW, in Nova we tag each TODO items with a 4-letter identifier (i.e.
`TODO[ABCD]:` that is defined in our `todo.rst` file. This makes
grepping all the sites relevant to a given item easy.

>
>> +///
>> +/// [`Alignment`]: https://github.com/rust-lang/rust/issues/102070
>> +///
>> +/// # Invariants
>> +///
>> +/// An alignment is always a power of two.
>> +#[repr(transparent)]
>> +#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
>> +pub struct Alignment(NonZero<usize>);
>> +
>> +impl Alignment {
>> +    /// Validates that `align` is a power of two at build-time, and returns an [`Alignment`] of the
>> +    /// same value.
>> +    ///
>> +    /// A build error is triggered if `align` cannot be asserted to be a power of two.
>> +    ///
>> +    /// # Examples
>> +    ///
>> +    /// ```
>> +    /// use kernel::ptr::Alignment;
>> +    ///
>> +    /// let v = Alignment::new(16);
>> +    /// assert_eq!(v.as_usize(), 16);
>> +    /// ```
>> +    #[inline(always)]
>> +    pub const fn new(align: usize) -> Self {
>> +        build_assert!(align.is_power_of_two());
>> +
>> +        // INVARIANT: `align` is a power of two.
>> +        // SAFETY: `align` is a power of two, and thus non-zero.
>> +        Self(unsafe { NonZero::new_unchecked(align) })
>> +    }
>> +
>> +    /// Validates that `align` is a power of two at runtime, and returns an
>> +    /// [`Alignment`] of the same value.
>> +    ///
>> +    /// [`None`] is returned if `align` was not a power of two.
>> +    ///
>> +    /// # Examples
>> +    ///
>> +    /// ```
>> +    /// use kernel::ptr::Alignment;
>> +    ///
>> +    /// assert_eq!(Alignment::new_checked(16), Some(Alignment::new(16)));
>> +    /// assert_eq!(Alignment::new_checked(15), None);
>> +    /// assert_eq!(Alignment::new_checked(1), Some(Alignment::new(1)));
>> +    /// assert_eq!(Alignment::new_checked(0), None);
>> +    /// ```
>> +    #[inline(always)]
>> +    pub const fn new_checked(align: usize) -> Option<Self> {
>> +        if align.is_power_of_two() {
>> +            // INVARIANT: `align` is a power of two.
>> +            // SAFETY: `align` is a power of two, and thus non-zero.
>> +            Some(Self(unsafe { NonZero::new_unchecked(align) }))
>> +        } else {
>> +            None
>> +        }
>> +    }
>> +
>> +    /// Returns the alignment of `T`.
>> +    #[inline(always)]
>> +    pub const fn of<T>() -> Self {
>> +        // INVARIANT: `align_of` always returns a power of 2.
>> +        Self(unsafe { NonZero::new_unchecked(align_of::<T>()) })
>> +    }
>
>> +
>> +    /// Returns the base-2 logarithm of the alignment.
>> +    ///
>> +    /// # Examples
>> +    ///
>> +    /// ```
>> +    /// use kernel::ptr::Alignment;
>> +    ///
>> +    /// assert_eq!(Alignment::of::<u8>().log2(), 0);
>> +    /// assert_eq!(Alignment::new(16).log2(), 4);
>> +    /// ```
>> +    #[inline(always)]
>> +    pub const fn log2(self) -> u32 {
>> +        self.0.ilog2()
>> +    }
>> +
>> +    /// Returns this alignment as a [`NonZero`].
>> +    ///
>> +    /// It is guaranteed to be a power of two.
>> +    ///
>> +    /// # Examples
>> +    ///
>> +    /// ```
>> +    /// use kernel::ptr::Alignment;
>> +    ///
>> +    /// assert_eq!(Alignment::new(16).as_nonzero().get(), 16);
>> +    /// ```
>> +    #[inline(always)]
>> +    pub const fn as_nonzero(self) -> NonZero<usize> {
>> +        if !self.0.is_power_of_two() {
>> +            // SAFETY: per the invariants, `self.0` is always a power of two so this block will
>> +            // never be reached.
>> +            unsafe { core::hint::unreachable_unchecked() }
>> +        }
>> +        self.0
>> +    }
>> +
>> +    /// Returns this alignment as a `usize`.
>> +    ///
>> +    /// It is guaranteed to be a power of two.
>> +    ///
>> +    /// # Examples
>> +    ///
>> +    /// ```
>> +    /// use kernel::ptr::Alignment;
>> +    ///
>> +    /// assert_eq!(Alignment::new(16).as_usize(), 16);
>> +    /// ```
>> +    #[inline(always)]
>> +    pub const fn as_usize(self) -> usize {
>> +        self.as_nonzero().get()
>> +    }
>> +
>> +    /// Returns the mask corresponding to `self.as_usize() - 1`.
>> +    ///
>> +    /// # Examples
>> +    ///
>> +    /// ```
>> +    /// use kernel::ptr::Alignment;
>> +    ///
>> +    /// assert_eq!(Alignment::new(0x10).mask(), 0xf);
>> +    /// ```
>> +    #[inline(always)]
>> +    pub const fn mask(self) -> usize {
>> +        // INVARIANT: `self.as_usize()` is guaranteed to be a power of two (i.e. non-zero), thus
>> +        // `1` can safely be substracted from it.
>> +        self.as_usize() - 1
>> +    }
>> +
>> +    /// Aligns `value` down to this alignment.
>> +    ///
>> +    /// If the alignment contained in `self` is too large for `T`, then `0` is returned, which
>> +    /// is correct as it is also the result that would have been returned if it did.
>
> I half get this, but still: If it did what?

I also stumbled while re-reading this sentence. :) Fixed.