How do I write a custom attribute for const fields in my Rust library?

Question

I'd like to write a custom attribute for a const field, which will later be accessed throughout my entire library.

Example

// default declaration in `my_lib`...
pub const INITIAL_VEC_CAPACITY: usize = 10;

//...but can be overriden by dependent crates...
#[mylib_initial_vec_capacity]
pub const INITIAL_VEC_CAPACITY: usize = 5;

//...then can be accessed within my crate:
pub fn do_something() {
    let mut vec = Vec::with_capacity(macros::INITIAL_VEC_CAPACITY);
    /* do stuff with vec */
}

How would I go about achieving this?

Why do you need an attribute for that, just `const` works great. — Chayim Friedman, Jun 22 '22 at 22:16
> which will later be accessed throughout my entire library. — ImajinDevon, Jun 22 '22 at 22:26
I don't understand? I asked a clear question, and I want an answer for my question. What if others were to see my post, and search for an answer that not only answers mine, but others' question(s)? — ImajinDevon, Jun 22 '22 at 22:29
Oops, I had typos :E I meant "What **if** two **crates** create incompatible values?" — Chayim Friedman, Jun 22 '22 at 22:32
Whatever Rust chooses. You don't experience that issue because Rust supposedly handles all of that. — ImajinDevon, Jun 22 '22 at 22:39
What do you mean "whatever Rust chooses"? It should be random? — Chayim Friedman, Jun 22 '22 at 22:39
I'd assume that Rust would choose whichever is creating the binary..? — ImajinDevon, Jun 22 '22 at 22:42
Here's an answer: there's nothing like that in Rust. Is it enough for you? — Chayim Friedman, Jun 22 '22 at 23:20
If you want to provide an answer, it's probably best to provide a formal, well-written one, using the "Answer Question" button below so others can find it more easily. — ImajinDevon, Jun 22 '22 at 23:34
But I don't want to post that as an answer, because it probably isn't useful. If you will elaborate more on your problem, or at least answer my questions, then I may be able to help you (and others), or at least provide more details. — Chayim Friedman, Jun 22 '22 at 23:37

score 0 · Answer 1 · answered Jun 23 '22 at 05:02

You can use features to allow the user the compilation process of your library.

https://doc.rust-lang.org/cargo/reference/features-examples.html

Other than that I think you should use some configuration object the user passes into your code or is able to configure from outside using one of the following crates

How do I write a custom attribute for const fields in my Rust library?

1 Answers1