generic-upper-bound

Provides functionality to get a const generic usize that is that is a reasonable upper bound for a specified associated const usize for the purpose of intermediate const calculations, as a workaround for generic_const_exprs.

The API of this crate is structed as follows:

• AcceptUpperBound is the heart of this crate. Implementors use it to specify which generic const they want to be passed and what to do with any given upper bound for it.
• eval_with_upper_bound is used to get the result of evaluating an upper bound acceptor with the best-effort upper bound that this crate can offer.

While you cannot use this to write a function with a signature that returns e.g. [T; M + N] with generic M and N, you can use it to temporarily get an array of size M + N, use it to do something useful, then return the result of that computation. For example, you can concatenate two strings at compile time, even if their value is dependent on generic paramters:

use generic_upper_bound as gub;
pub trait MyTrait {
    const SOME_STR: &'static str;
}
impl<A, B> MyTrait for (A, B)
where
    A: MyTrait,
    B: MyTrait,
{
    // evaluate our upper bound acceptor to implement concatenation
    const SOME_STR: &'static str = {
        let slice: &'static [u8] = gub::eval_with_upper_bound::<Concat<A, B>>();

        // take subslice without trailing zeros and convert to string
        let total_length = gub::desired_generic::<Concat<A, B>>();
        match core::str::from_utf8(slice.split_at(total_length).0) {
            Ok(s) => s,
            _ => unreachable!(),
        }
    };
}

struct Concat<A, B>(A, B);
impl<A: MyTrait, B: MyTrait> gub::AcceptUpperBound for Concat<A, B> {
    type Output = &'static [u8];
    // Want to be passed at least the total length of the strings
    const DESIRED_GENERIC: usize = A::SOME_STR.len() + B::SOME_STR.len();
    // Decide on what to do with each generic const
    type Eval<const UPPER: usize> = ConcatImpl<A, B, UPPER>;
}

struct ConcatImpl<A, B, const N: usize>(A, B);
impl<A, B, const N: usize> gub::Const for ConcatImpl<A, B, N>
where
    A: MyTrait,
    B: MyTrait,
{
    type Type = &'static [u8];
    // Write the bytes into `[u8; N]` and promote the result
    const VALUE: Self::Type = &{
        let l = A::SOME_STR.as_bytes();
        let r = B::SOME_STR.as_bytes();
        let mut out = [0; N];
        let mut off = 0;
        let mut i = 0; // in >=1.86, you can use split_at_mut and copy_from_slice
        while i < l.len() {
            out[off] = l[i];
            off += 1;
            i += 1;
        }
        i = 0;
        while i < r.len() {
            out[off] = r[i];
            off += 1;
            i += 1;
        }
        out
    };
}

impl MyTrait for () {
    const SOME_STR: &'static str = "ABC";
}
impl MyTrait for i32 {
    const SOME_STR: &'static str = "123";
}
let concatenated: &'static str = <((), i32)>::SOME_STR;
assert_eq!(concatenated, "ABC123");

Note that this example can be generalized and optimized. For instance, it is possible to accept any &'a [&'b str] where 'b: 'a as input and this will also be more efficient (most of the time) due to the overhead from the inexact upper bound used for each concatenation (which will likely affect the final binary size).

MSRV

MSRV is 1.78. This is to allow this crate to be used as a workaround the breaking change to const promotion that was introduced by that version.