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It's unfortunate because it disincentivizes using it in interface boundaries. Introduce a macro to make it nicer to use. The macro `cv!` (for constant value) takes a const integer expression and widens it to i128 (at build time only) before passing it as a const generic value to a new trait `FromConst`. The value is then converted and appears in the associated constant `FromConst::VALUE`. The trait is implemented by NonZero, Bounded, and Alignment and lets values of each be constructed from constants without a verbose turbofish syntax. For example, `const { NonZero::new(1).unwrap() }` can be written as `cv!(1)`. Add the `cv!` macro to the prelude so it doesn't need to be imported explicitly to use it. Suggested-by: Gary Guo Reviewed-by: Gary Guo Signed-off-by: Eliot Courtney --- rust/kernel/num.rs | 135 +++++++++++++++++++++++++++++++++++++++++++++ rust/kernel/num/bounded.rs | 16 ++++++ rust/kernel/prelude.rs | 1 + rust/kernel/ptr.rs | 12 +++- 4 files changed, 163 insertions(+), 1 deletion(-) diff --git a/rust/kernel/num.rs b/rust/kernel/num.rs index dbe848e30efe..6a4cb8d3ec95 100644 --- a/rust/kernel/num.rs +++ b/rust/kernel/num.rs @@ -79,3 +79,138 @@ impl Integer for $type { i128: Signed, isize: Signed ); + +/// Creates a value from an integer constant expression, with validity checked at build time. +/// +/// This works for any type that implements [`FromConst`], with the target type inferred from +/// the context, or named explicitly with `cv!(value => Type)`. +/// +/// # Examples +/// +/// ``` +/// use core::num::NonZero; +/// use kernel::num::Bounded; +/// use kernel::ptr::Alignment; +/// +/// let v: NonZero = cv!(8); +/// assert_eq!(v.get(), 8); +/// +/// // Any integer constant expression works, not only literals. +/// let m: NonZero = cv!(usize::MAX); +/// assert_eq!(m.get(), usize::MAX); +/// +/// let b: Bounded = cv!(15); +/// assert_eq!(b.get(), 15); +/// +/// let a: Alignment = cv!(4096); +/// assert_eq!(a.as_usize(), 4096); +/// +/// // Checked narrowing of integer constants, including in `const` items. +/// const SMALL: u8 = cv!(200u32); +/// assert_eq!(SMALL, 200); +/// +/// const N: NonZero = cv!(5); +/// assert_eq!(N.get(), 5); +/// +/// // The target type can be given explicitly. +/// let e = cv!(200u32 => u8); +/// assert_eq!(e, 200); +/// +/// // With an explicit primitive target, the expression can use generic parameters. +/// const fn as_u64() -> u64 { +/// cv!(KEY => u64) +/// } +/// assert_eq!(as_u64::<0x40>(), 0x40); +/// ``` +#[macro_export] +#[doc(hidden)] +macro_rules! cv { + (@cast $v:expr => $t:ty) => { + const { + #[allow(unused_comparisons, unused_assignments, clippy::as_underscore)] + { + let v = $v; + let r = v as $t; + // Pin `back` to `v`'s type so `as _` casts back to the source type. + let mut back = v; + back = r as _; + + ::core::assert!( + back == v && (v < 0) == (r < 0), + "value does not fit into the target type" + ); + + r + } + } + }; + // Using `FromConst::VALUE` means const generic expressions can't be used, since it requires + // the `generic_const_exprs` feature. Provide a special path for each primitive type that allows + // const generic expressions. + ($v:expr => u8) => { $crate::cv!(@cast $v => u8) }; + ($v:expr => u16) => { $crate::cv!(@cast $v => u16) }; + ($v:expr => u32) => { $crate::cv!(@cast $v => u32) }; + ($v:expr => u64) => { $crate::cv!(@cast $v => u64) }; + ($v:expr => u128) => { $crate::cv!(@cast $v => u128) }; + ($v:expr => usize) => { $crate::cv!(@cast $v => usize) }; + ($v:expr => i8) => { $crate::cv!(@cast $v => i8) }; + ($v:expr => i16) => { $crate::cv!(@cast $v => i16) }; + ($v:expr => i32) => { $crate::cv!(@cast $v => i32) }; + ($v:expr => i64) => { $crate::cv!(@cast $v => i64) }; + ($v:expr => i128) => { $crate::cv!(@cast $v => i128) }; + ($v:expr => isize) => { $crate::cv!(@cast $v => isize) }; + ($v:expr => $t:ty) => { + <$t as $crate::num::FromConst<{ $crate::cv!(@cast $v => i128) }>>::VALUE + }; + ($v:expr) => { + <_ as $crate::num::FromConst<{ $crate::cv!(@cast $v => i128) }>>::VALUE + }; +} +#[doc(inline)] +pub use cv; + +/// Types that can be created from an integer constant expression validated at build time. +/// +/// Implement this trait to make a type usable with [`cv!`]. Use the [`cv`] macro, not this trait +/// directly, for creating values. +#[diagnostic::on_unimplemented(message = "`{Self}` cannot be converted from a constant")] +pub trait FromConst: Sized { + /// The value that corresponds to the constant `V`. + /// + /// Fails the build if `V` is not a valid value for `Self`. + const VALUE: Self; +} + +/// Implements [`FromConst`] for primitive integer types and their [`NonZero`](core::num::NonZero) +/// versions. +macro_rules! impl_from_const { + ($($type:ty)*) => { + $( + impl FromConst for $type { + const VALUE: Self = { + // CAST: the macro is only used for types up to 64 bits wide, so `MIN` and `MAX` + // widen to `i128` losslessly. + assert!( + V >= <$type>::MIN as i128 && V <= <$type>::MAX as i128, + "constant cannot be represented by the target type" + ); + + // CAST: the assert above confirmed that `V` fits in `$type`. + V as $type + }; + } + + impl FromConst for core::num::NonZero<$type> { + const VALUE: Self = match core::num::NonZero::new(<$type as FromConst>::VALUE) { + Some(value) => value, + None => panic!("constant cannot be zero"), + }; + } + )* + }; +} + +impl_from_const!( + u8 u16 u32 u64 usize + i8 i16 i32 i64 isize +); diff --git a/rust/kernel/num/bounded.rs b/rust/kernel/num/bounded.rs index 2a2b0a4bca5e..1d5a151478a7 100644 --- a/rust/kernel/num/bounded.rs +++ b/rust/kernel/num/bounded.rs @@ -14,6 +14,7 @@ use kernel::{ num::{ + FromConst, Integer, Unsigned, // }, @@ -272,6 +273,21 @@ pub const fn new() -> Self { unsafe { Self::__new(VALUE) } } } + + impl FromConst for Bounded<$type, N> { + const VALUE: Self = { + let value = <$type as FromConst>::VALUE; + // Statically assert that `value` fits within the set number of bits. + assert!( + fits_within!(value, $type, N), + "constant cannot be represented within the given number of bits" + ); + + // SAFETY: the assert above confirmed that `value` can be represented within `N` + // bits. + unsafe { Self::__new(value) } + }; + } )* }; } diff --git a/rust/kernel/prelude.rs b/rust/kernel/prelude.rs index ca396f1f78a6..5facf8f7af90 100644 --- a/rust/kernel/prelude.rs +++ b/rust/kernel/prelude.rs @@ -106,6 +106,7 @@ Result, // }, init::InPlaceInit, + num::cv, pr_alert, pr_crit, pr_debug, diff --git a/rust/kernel/ptr.rs b/rust/kernel/ptr.rs index 82acb531b17b..2c12e4fd4572 100644 --- a/rust/kernel/ptr.rs +++ b/rust/kernel/ptr.rs @@ -11,7 +11,10 @@ }; use core::num::NonZero; -use crate::const_assert; +use crate::{ + const_assert, + num::FromConst, // +}; /// Type representing an alignment, which is always a power of two. /// @@ -166,6 +169,13 @@ pub const fn mask(self) -> usize { } } +impl FromConst for Alignment { + const VALUE: Self = match Alignment::new_checked(>::VALUE) { + Some(alignment) => alignment, + None => panic!("constant is not a power of two"), + }; +} + /// Trait for items that can be aligned against an [`Alignment`]. pub trait Alignable: Sized { /// Aligns `self` down to `alignment`. -- 2.55.0