rust/kernel/num/casts.rs

// SPDX-License-Identifier: GPL-2.0

//! Helpers for performing lossless integer casts.
//!
//! The `as` keyword can be used to perform casts between integer types, but it unfortunately makes
//! no distinction between casts that are lossless, and casts from a larger type into a smaller one
//! that might silently strip data away. Thus, its use in the kernel is discouraged in favor of
//! [`From`] implementations.
//!
//! Conversely, there are casts that are lossless depending on the build architecture (such as
//! casting [`usize`] to [`u64`] on 32 or 64 bit archs), but not supported by [`From`]
//! implementations in the standard library because they are not portable. It does however make
//! sense for the kernel to support these, if only for code that is architecture-specific.
//!
//! This module provides ways to perform such conversions safely:
//!
//! - A series of const functions (e.g. [`usize_as_u64`]) supporting safe conversions in const
//!   context. Conversions supported by [`From`] implementations in the standard library are also
//!   covered as the [`From`] trait cannot be used in const context.
//! - Two extension traits, [`FromSafeCast`] and [`IntoSafeCast`], providing conversion methods
//!   similar to [`From`] and [`Into`] for conversions that are safe to perform in the kernel, but
//!   not supported by the standard library.
//! - Another series of const functions (e.g. [`u64_into_u8`]) supporting the conversion of a const
//!   value from a larger type into a smaller one, provided the value fits into the destination
//!   type. This is useful if a constant is defined as a larger type, but needs to be used as a
//!   smaller one.
//! - An [`arch`] sub-module, defining more conversion functions that are only guaranteed to be
//!   lossless for a given pointer size. These can only be used in code that is specific to a
//!   given pointer size.
//!
//! # Examples
//!
//! ```
//! use kernel::num::casts::{self, FromSafeCast, IntoSafeCast};
//!
//! // Conversion from const context.
//! const USIZED_CONST: usize = casts::u8_as_usize(255u8);
//!
//! // Non-const conversions.
//! let a = u64::from_safe_cast(4096usize);
//! let b: u64 = 4096usize.into_safe_cast();
//! ```

use crate::prelude::*;

/// Implements safe `as` conversion functions from a given type into a series of target types.
///
/// These functions can be used in place of `as`, with the guarantee that they will be lossless.
macro_rules! impl_safe_as {
    ($from:ty as { $($into:ty),* }) => {
        $(
        $crate::macros::paste! {
            #[doc = ::core::concat!(
                "Losslessly converts a [`",
                ::core::stringify!($from),
                "`] into a [`",
                ::core::stringify!($into),
                "`].")]
            ///
            /// This conversion is allowed as it is always lossless. Prefer this over the `as`
            /// keyword to ensure no lossy casts are performed.
            ///
            /// This is for use from a `const` context. For non `const` use, prefer the
            /// [`FromSafeCast`] and [`IntoSafeCast`] traits.
            ///
            /// # Examples
            ///
            /// ```
            /// use kernel::num::casts;
            ///
            #[doc = ::core::concat!(
                "assert_eq!(casts::",
                ::core::stringify!($from),
                "_as_",
                ::core::stringify!($into),
                "(1",
                ::core::stringify!($from),
                "), 1",
                ::core::stringify!($into),
                ");")]
            /// ```
            #[inline]
            pub const fn [<$from _as_ $into>](value: $from) -> $into {
                $crate::static_assert!(size_of::<$into>() >= size_of::<$from>());

                value as $into
            }
        }
        )*
    };
}

// Valid `Into` transformations.
impl_safe_as!(u8 as { u16, u32, u64, usize });
impl_safe_as!(u16 as { u32, u64, usize });
impl_safe_as!(u32 as { u64 });
// A `usize` fits into a `u64` on all supported platforms.
impl_safe_as!(usize as { u64 });
// A `u32` fits into a `usize` on all supported platforms.
impl_safe_as!(u32 as { usize });

/// Extension trait providing guaranteed lossless cast to [`Self`] from `T`.
///
/// The standard library's [`From`] implementations do not cover conversions that are not portable
/// or future-proof. For instance, even though it is safe today, [`From<usize>`] is not implemented
/// for [`u64`] because of the possibility of needing to support larger-than-64bit architectures in
/// the future.
///
/// The workaround is to either deal with the error handling of [`TryFrom`] for an operation that
/// technically cannot fail, or to use the `as` keyword, which can silently strip data if the
/// destination type is smaller than the source.
///
/// Both options are hardly acceptable for the kernel. It is also a much more architecture
/// dependent environment, supporting only 32 and 64 bit architectures, with some modules
/// explicitly depending on a specific bus width that could greatly benefit from infallible
/// conversion operations.
///
/// Thus this extension trait that provides, for all architectures supported by the kernel,
/// conversion methods between types for which such a cast is lossless.
///
/// In other words, this trait is implemented if, for all supported targets and with `t: T`, the
/// `t as Self` operation is completely lossless.
///
/// Prefer this over the `as` keyword to guarantee that no lossy casts are performed.
///
/// If you need to perform a conversion in `const` context, use [`u32_as_usize`], [`usize_as_u64`],
/// etc.
///
/// # Examples
///
/// ```
/// use kernel::num::casts::FromSafeCast;
///
/// assert_eq!(usize::from_safe_cast(0xf00u32), 0xf00usize);
/// ```
pub trait FromSafeCast<T> {
    /// Create a [`Self`] from `value`. This operation is guaranteed to be lossless.
    fn from_safe_cast(value: T) -> Self;
}

// A `usize` fits into a `u64` on all supported platforms.
impl FromSafeCast<usize> for u64 {
    #[inline]
    fn from_safe_cast(value: usize) -> Self {
        usize_as_u64(value)
    }
}

// A `u32` fits into a `usize` on all supported platforms.
impl FromSafeCast<u32> for usize {
    #[inline]
    fn from_safe_cast(value: u32) -> Self {
        u32_as_usize(value)
    }
}

/// Counterpart to the [`FromSafeCast`] trait, i.e. this trait is to [`FromSafeCast`] what [`Into`]
/// is to [`From`].
///
/// See the documentation of [`FromSafeCast`] for the motivation.
///
/// # Examples
///
/// ```
/// use kernel::num::casts::IntoSafeCast;
///
/// assert_eq!(0xf00usize, 0xf00u32.into_safe_cast());
/// ```
pub trait IntoSafeCast<T> {
    /// Convert `self` into a `T`. This operation is guaranteed to be lossless.
    fn into_safe_cast(self) -> T;
}

/// Reverse operation for types implementing [`FromSafeCast`].
impl<S, T> IntoSafeCast<T> for S
where
    T: FromSafeCast<S>,
{
    #[inline]
    fn into_safe_cast(self) -> T {
        T::from_safe_cast(self)
    }
}

/// Implements lossless conversion of a constant from a larger type into a smaller one.
macro_rules! impl_const_into {
    ($from:ty => { $($into:ty),* }) => {
        $(
        $crate::macros::paste! {
            #[doc = ::core::concat!(
                "Performs a build-time safe conversion of a [`",
                ::core::stringify!($from),
                "`] constant value into a [`",
                ::core::stringify!($into),
                "`].")]
            ///
            /// This checks at compile-time that the conversion is lossless, and triggers a build
            /// error if it isn't.
            ///
            /// # Examples
            ///
            /// ```
            /// use kernel::num::casts;
            ///
            /// // Succeeds because the value of the source fits into the destination's type.
            #[doc = ::core::concat!(
                "assert_eq!(casts::",
                ::core::stringify!($from),
                "_into_",
                ::core::stringify!($into),
                "::<1",
                ::core::stringify!($from),
                ">(), 1",
                ::core::stringify!($into),
                ");")]
            /// ```
            #[inline]
            pub const fn [<$from _into_ $into>]<const N: $from>() -> $into {
                // Make sure that the target type is smaller than the source one.
                $crate::static_assert!($from::BITS >= $into::BITS);
                // CAST: we statically enforced above that `$from` is larger than `$into`, so the
                // `as` conversion will be lossless.
                $crate::const_assert!(N >= $into::MIN as $from && N <= $into::MAX as $from);

                N as $into
            }
        }
        )*
    };
}

impl_const_into!(usize => { u8, u16, u32 });
impl_const_into!(u64 => { u8, u16, u32 });
impl_const_into!(u32 => { u8, u16 });
impl_const_into!(u16 => { u8 });

/// Conversions that are only lossless for the current architecture.
///
/// # Portability
///
/// Callers of this module become dependent on the setting of `CONFIG_64BIT`. Use with caution, and
/// never in code that is portable across pointer sizes.
pub mod arch {
    /// Trait identical to [`FromSafeCast`](super::FromSafeCast), but for conversions that are not
    /// available on all architectures.
    pub trait FromSafeCastArch<T> {
        /// Create a [`Self`] from `value`. This operation is guaranteed to be lossless.
        fn from_safe_cast_arch(value: T) -> Self;
    }

    /// Trait identical to [`IntoSafeCast`](super::IntoSafeCast), but for conversions that are not
    /// available on all architectures.
    pub trait IntoSafeCastArch<T> {
        /// Convert `self` into a `T`. This operation is guaranteed to be lossless.
        fn into_safe_cast_arch(self) -> T;
    }

    /// Reverse operation for types implementing [`FromSafeCastArch`].
    impl<S, T> IntoSafeCastArch<T> for S
    where
        T: FromSafeCastArch<S>,
    {
        #[inline]
        fn into_safe_cast_arch(self) -> T {
            T::from_safe_cast_arch(self)
        }
    }

    /// A [`u64`] fits into a [`usize`] on 64-bit platforms.
    #[cfg(CONFIG_64BIT)]
    #[inline]
    pub const fn u64_as_usize(value: u64) -> usize {
        value as usize
    }

    #[cfg(CONFIG_64BIT)]
    impl FromSafeCastArch<u64> for usize {
        #[inline]
        fn from_safe_cast_arch(value: u64) -> Self {
            u64_as_usize(value)
        }
    }

    /// A [`usize`] fits into a [`u32`] on 32-bit platforms.
    #[cfg(not(CONFIG_64BIT))]
    #[inline]
    pub const fn usize_as_u32(value: usize) -> u32 {
        value as u32
    }

    #[cfg(not(CONFIG_64BIT))]
    impl FromSafeCastArch<usize> for u32 {
        #[inline]
        fn from_safe_cast_arch(value: usize) -> Self {
            usize_as_u32(value)
        }
    }
}