Add tools that help with making code `const`

There are various challenges when making code `const`: making it
conditional, awkward copying of slices... This change adds tools that
help dealing with these challenges.
This commit is contained in:
Martin Habovstiak 2023-12-14 17:21:37 +01:00
parent 3d6151b9e1
commit f17c0402b7
2 changed files with 93 additions and 0 deletions

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@ -0,0 +1,92 @@
//! Contains tools (workarounds) to make implementing `const fn`s easier.
/// Copies first `$len` bytes from `$slice` and returns them as an array.
///
/// Returns `None` if `$len > $slice.len()`. `$len` must be (obviously) statically known.
/// Calling from non-const context doesn't affect performance.
#[macro_export]
macro_rules! copy_byte_array_from_slice {
($slice:expr, $len:expr) => {
if $len > $slice.len() {
None
} else {
let mut array = [0u8; $len];
// Note: produces same assemble as copy_from_slice
let mut i = 0;
while i < $len {
array[i] = $slice[i];
i += 1;
}
Some(array)
}
};
}
pub use copy_byte_array_from_slice;
/// Concatenates two byte slices or byte arrays (or combination) to a single array.
///
/// # Panics
///
/// This macro panics if `$len` is not equal to the sum of `$a.len()` and `$b.len()`.
#[macro_export]
macro_rules! concat_bytes_to_arr {
($a:expr, $b:expr, $len:expr) => {{
// avoid repeated eval
let a = $a;
let b = $b;
#[allow(unconditional_panic)]
let _ = [(); 1][($len != a.len() + b.len()) as usize];
let mut output = [0u8; $len];
let mut i = 0;
while i < a.len() {
output[i] = $a[i];
i += 1;
}
while i < a.len() + b.len() {
output[i] = b[i - a.len()];
i += 1;
}
output
}};
}
pub use concat_bytes_to_arr;
#[macro_export]
/// Enables const fn in specified Rust version
macro_rules! cond_const {
($($(#[$attr:meta])* $vis:vis const(in $ver:ident $(= $human_ver:literal)?) fn $name:ident$(<$($gen:tt)*>)?($($args:tt)*) $(-> $ret:ty)? $body:block)+ ) => {
$(
#[cfg($ver)]
$(#[$attr])*
$(
#[doc = "\nNote: the function is only `const` in Rust "]
#[doc = $human_ver]
#[doc = "."]
)?
$vis const fn $name$(<$($gen)*>)?($($args)*) $(-> $ret)? $body
#[cfg(not($ver))]
$(#[$attr])*
$vis fn $name$(<$($gen)*>)?($($args)*) $(-> $ret)? $body
)+
};
($($(#[$attr:meta])* $vis:vis const(in $ver:ident $(= $human_ver:literal)?) unsafe fn $name:ident$(<$($gen:tt)*>)?($($args:tt)*) $(-> $ret:ty)? $body:block)+ ) => {
$(
#[cfg($ver)]
$(#[$attr])*
$(
#[doc = "\nNote: the function is only `const` in Rust "]
#[doc = $human_ver]
#[doc = " and newer."]
)?
$vis const unsafe fn $name$(<$($gen)*>)?($($args)*) $(-> $ret)? $body
#[cfg(not($ver))]
$(#[$attr])*
$vis unsafe fn $name$(<$($gen)*>)?($($args)*) $(-> $ret)? $body
)+
};
}
pub use cond_const;

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@ -20,6 +20,7 @@ extern crate alloc;
#[cfg(feature = "std")] #[cfg(feature = "std")]
extern crate std; extern crate std;
pub mod const_tools;
pub mod error; pub mod error;
pub mod macros; pub mod macros;
mod parse; mod parse;