72 lines
1.8 KiB
Rust
72 lines
1.8 KiB
Rust
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// Taken from https://github.com/Vurich/const-concat/issues/13
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#![no_std]
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use core::mem::ManuallyDrop;
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const unsafe fn transmute_prefix<From, To>(from: From) -> To {
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union Transmute<From, To> {
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from: ManuallyDrop<From>,
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to: ManuallyDrop<To>,
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}
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ManuallyDrop::into_inner(
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Transmute {
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from: ManuallyDrop::new(from),
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}
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.to,
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)
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}
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/// # Safety
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///
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/// `Len1 + Len2 >= Len3`
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#[doc(hidden)]
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#[allow(non_upper_case_globals)]
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pub const unsafe fn concat<const Len1: usize, const Len2: usize, const Len3: usize>(
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arr1: [u8; Len1],
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arr2: [u8; Len2],
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) -> [u8; Len3] {
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#[repr(C)]
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struct Concat<A, B>(A, B);
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transmute_prefix(Concat(arr1, arr2))
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}
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#[macro_export]
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macro_rules! const_concat {
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() => ("");
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($a:expr) => ($a);
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($a:expr, $b:expr $(,)?) => {{
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const A: &str = $a;
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const B: &str = $b;
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const BYTES: [u8; { A.len() + B.len() }] = unsafe {
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$crate::concat::<
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{ A.len() },
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{ B.len() },
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{ A.len() + B.len() }
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>(
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*A.as_ptr().cast(),
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*B.as_ptr().cast(),
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)
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};
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unsafe { ::core::str::from_utf8_unchecked(&BYTES) }
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}};
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($a:expr, $b:expr, $($rest:expr),+ $(,)?) => {{
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const TAIL: &str = $crate::const_concat!($b, $($rest),+);
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$crate::const_concat!($a, TAIL)
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}}
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}
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#[test]
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fn tests() {
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const SALUTATION: &str = "Hello";
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const TARGET: &str = "world";
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const GREETING: &str = const_concat!(SALUTATION, ", ", TARGET, "!");
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const GREETING_TRAILING_COMMA: &str = const_concat!(SALUTATION, ", ", TARGET, "!",);
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assert_eq!(GREETING, "Hello, world!");
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assert_eq!(GREETING_TRAILING_COMMA, "Hello, world!");
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}
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