hipstr 0.8.0

Yet another string for Rust: zero-cost borrow and slicing, inline representation for small strings, (atomic) reference counting
Documentation 
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//! Representation for borrowed slice.

use core::mem::{offset_of, size_of, MaybeUninit};
use core::num::NonZeroU8;

use super::TAG_BORROWED;

#[cfg(test)]
mod tests;

const TAG_NZ: NonZeroU8 = NonZeroU8::new(TAG_BORROWED).unwrap();

/// Borrowed slice representation.
///
/// # Warning!
///
/// For big-endian platform, the reserved word is **after** the slice.
/// For little-endian platform, the reserved word is **before** the slice.
#[derive(Clone, Copy)]
#[repr(C)]
pub struct Borrowed<'borrow> {
    #[cfg(target_endian = "little")]
    tag: NonZeroU8,

    #[cfg(target_endian = "little")]
    reserved: MaybeUninit<[u8; size_of::<usize>() - 1]>,

    slice: &'borrow [u8],

    #[cfg(target_endian = "big")]
    reserved: MaybeUninit<[u8; size_of::<usize>() - 1]>,

    #[cfg(target_endian = "big")]
    tag: NonZeroU8,
}

impl<'borrow> Borrowed<'borrow> {
    const ASSERTS: () = {
        if cfg!(target_endian = "little") {
            assert!(offset_of!(Self, tag) == 0);
        } else {
            assert!(offset_of!(Self, tag) == size_of::<Self>() - 1);
        }
    };

    /// Creates a new borrowed representation.
    #[inline]
    pub const fn new(slice: &'borrow [u8]) -> Self {
        let () = Self::ASSERTS; // HACK to actually do the check

        let this = Self {
            slice,
            tag: TAG_NZ,
            reserved: MaybeUninit::uninit(),
        };
        debug_assert!(this.is_valid());
        this
    }

    /// Returns the length of the underlying data.
    #[inline]
    pub const fn len(&self) -> usize {
        debug_assert!(self.is_valid());
        self.slice.len()
    }

    /// Returns the underlying data as a slice
    #[inline]
    pub const fn as_slice(&self) -> &'borrow [u8] {
        debug_assert!(self.is_valid());
        self.slice
    }

    /// Returns the underlying data as a slice
    #[inline]
    pub const fn as_ptr(&self) -> *const u8 {
        debug_assert!(self.is_valid());
        self.slice.as_ptr()
    }

    /// Return `true` iff this representation is valid.
    #[inline]
    pub const fn is_valid(&self) -> bool {
        self.tag.get() == TAG_BORROWED
    }

    /// Sets the length
    ///
    /// # Safety
    ///
    /// `new_len` must be less than or equal to the current length.
    pub unsafe fn set_len(&mut self, new_len: usize) {
        debug_assert!(self.len() >= new_len, "set_len on borrowed slice");

        // SAFETY: function's safety precondition
        self.slice = unsafe { self.slice.get_unchecked(0..new_len) };
    }
}