lexe-std 0.1.7

//! Utilities for use in `const` fns and expressions.

use std::mem::MaybeUninit;

/// Assert at compile that that a boolean expression evaluates to true.
/// Implementation copied from the static_assertions crate.
#[macro_export]
macro_rules! const_assert {
    ($x:expr $(,)?) => {
        #[allow(clippy::const_is_empty, clippy::eq_op, unknown_lints)]
        const _: [(); 0 - !{
            const CONST_ASSERT: bool = $x;
            CONST_ASSERT
        } as usize] = [];
    };
}

/// Assert at compile time that two `usize` values are equal. This assert has a
/// nice benefit where there compiler error will actually _print out_ the
/// two values.
#[macro_export]
macro_rules! const_assert_usize_eq {
    ($x:expr, $y:expr $(,)?) => {
        const _: [(); $x] = [(); $y];
    };
}

/// Assert at compile time that a type has a specific in-memory size in bytes.
///
/// The assertion is only effective on 64-bit archs since we don't care about
/// 32-bit Android ARM (the only 32-bit arch we compile to, and making the
/// assertion more complicated just for that is not worth it).
///
/// Usage: `const_assert_mem_size!(u64, 8);`
#[macro_export]
macro_rules! const_assert_mem_size {
    ($type:ty, $size:expr $(,)?) => {
        #[cfg(target_pointer_width = "64")]
        $crate::const_assert_usize_eq!(::core::mem::size_of::<$type>(), $size);
    };
}

/// [`Result::unwrap`] but works in `const fn`.
// TODO(phlip9): remove this when const unwrap stabilizes
pub const fn const_result_unwrap<T: Copy, E: Copy>(result: Result<T, E>) -> T {
    match result {
        Ok(result) => result,
        Err(_) => panic!("unwrap on Err"),
    }
}

/// Compile-time cast from `&T::From` to `&T`, where `T` is just a struct with
/// a single field of type `T::From` and `T` is `#[repr(transparent)]`.
///
/// Useful for casting a new-type's inner struct reference to a new-type
/// reference.
///
/// See [`ref_cast`] for more details. Just use `T::ref_cast` if you don't need
/// `const`.
///
/// ## Example
///
/// ```rust
/// use lexe_std::const_utils;
/// use ref_cast::RefCast;
///
/// #[derive(RefCast)]
/// #[repr(transparent)]
/// struct Id(u32);
///
/// // Safe, const cast from `&123` to `&Id(123)`
/// const MY_ID: &'static Id = const_utils::const_ref_cast(&123);
/// ```
#[cfg(feature = "ref-cast")]
pub const fn const_ref_cast<T: ref_cast::RefCast>(from: &T::From) -> &T {
    // SAFETY: we require that `T: RefCast`, which guarantees that this cast is
    // safe. Unfortunately we need this extra method as `T::ref_cast` is not
    // currently const (Rust doesn't support const traits yet).
    unsafe { &*(from as *const T::From as *const T) }
}

/// Easily concatenate multiple const `&str` slices into a single const `&str`.
///
/// Unlike the `concat!` macro, which only works with string literals, this
/// works with any const `&str` expressions.
///
/// ### Example
///
/// ```rust
/// use lexe_std::const_concat_str;
///
/// const NAME: &str = const_concat_str!("HUGE", " ", "MAN");
/// const GREETING: &str = const_concat_str!("Hello ", NAME, "!!");
/// assert_eq!(GREETING, "Hello HUGE MAN!!");
/// ```
#[macro_export]
macro_rules! const_concat_str {
    ($($s:expr),* $(,)?) => {{
        use $crate::std::primitive::{str, u8};
        // ensure all inputs are &str
        $(const _: &str = $s;)*
        const LEN: usize = 0 $(+ $s.len())*;
        const ARR: [u8; LEN] = $crate::const_utils::const_concat_inner::<LEN, _>(
            &[$($s.as_bytes()),*]
        );
        // SAFETY: all bytes are valid UTF-8 as they come from &str slices.
        unsafe { $crate::std::str::from_utf8_unchecked(&ARR) }
    }};
}

/// Easily concatenate multiple const `&[u8]` slices into a single const
/// `&[u8]`.
//
// TODO(phlip9): replace with `concat_bytes!` when it stabilizes.
#[macro_export]
macro_rules! const_concat_bytes {
    ($($slice:expr),* $(,)?) => {{
        use $crate::std::primitive::u8;
        // ensure all inputs are &[u8]
        $(const _: &[u8] = $slice;)*
        const LEN: usize = 0 $(+ $slice.len())*;
        const ARR: [u8; LEN] = $crate::const_utils::const_concat_inner::<LEN, _>(
            &[$($slice),*]
        );
        &ARR
    }};
}

/// Internal helper for `const_concat_str!` and `const_concat_bytes!`.
#[doc(hidden)]
pub const fn const_concat_inner<const LEN: usize, T: Copy>(
    slices: &[&[T]],
) -> [T; LEN] {
    let mut arr: [MaybeUninit<T>; LEN] = [MaybeUninit::uninit(); LEN];
    let mut base = 0;
    let mut i = 0;
    while i < slices.len() {
        let slice = slices[i];
        let mut j = 0;
        while j < slice.len() {
            arr[base + j] = MaybeUninit::new(slice[j]);
            j += 1;
        }
        base += slice.len();
        i += 1;
    }
    if base != LEN {
        panic!("invalid length");
    }
    // SAFETY: all elements have been initialized.
    // TODO(phlip9): replace with `MaybeUninit::array_assume_init` when stable.
    unsafe { std::mem::transmute_copy::<[MaybeUninit<T>; LEN], [T; LEN]>(&arr) }
}