How to choose between macros at compile time? - macros

Question

I have different versions of the same macro and I want to be able to choose one of them at compile time.
Here is the code I have:
macro_rules! macro_a {
($identifier:ident) => {
println!("A: {}", stringify!($identifier));
}
}
macro_rules! macro_b {
($identifier:ident) => {
println!("B: {}", stringify!($identifier));
}
}
macro_rules! macro_c {
($identifier:ident) => {
println!("C: {}", stringify!($identifier));
}
}
macro_rules! choose_macro {
(a) => {
const CHOSEN_MACRO: u32 = 1;
};
(b) => {
const CHOSEN_MACRO: u32 = 2;
};
(c) => {
const CHOSEN_MACRO: u32 = 3;
};
}
choose_macro!(c);
macro_rules! use_macro {
($identifier:ident) => {
match CHOSEN_MACRO {
1 => macro_a!($identifier),
2 => macro_b!($identifier),
3 => macro_c!($identifier),
_ => unreachable!(),
}
}
}
fn main() {
use_macro!(test);
}
This will print, as expected:
C: test
I wonder if there is a better way to doing this (with macro or attribute or anything else).
It is not clear if the macro is chosen at compile time here. Will Rust remove the match because it is on a constant?
Update: I prefer to choose the macro in the code, not using compiler flags. Also, I do not want to hide the macros that are not chosen: I want to be able to use them using their real name.

Answer 1

I would recommend using conditional compilation flags for something like this. See https://doc.rust-lang.org/book/conditional-compilation.html
In your case, it might look something like this:
#[cfg(feature = "feature_a")]
macro_rules! use_macro {
($identifier:ident) => {
println!("A: {}", stringify!($identifier));
}
}
#[cfg(feature = "feature_b")]
macro_rules! use_macro {
($identifier:ident) => {
println!("B: {}", stringify!($identifier));
}
}
#[cfg(feature = "feature_c")]
macro_rules! use_macro {
($identifier:ident) => {
println!("C: {}", stringify!($identifier));
}
}
fn main() {
use_macro!(test);
}
Then add the following to your Cargo.toml file:
[features]
feature_a = []
feature_b = []
feature_c = []
If you want it to print out "C: test" for example, then run the following:
cargo run --features feature_c