Code and feature conditional on Rust version or specific compiler feature - rust

Question

Rust 1.63 stabilized const std::sync::Mutex::new(). Till date to have global flag preventing parallel execution would look like (example):
use once_cell::sync::Lazy;
static GLOBAL_FLAG: Lazy<Mutex<()>> = Lazy::new(Default::default);
while in since 1.63 it can look like this and the dependency isn't required anymore in Cargo.toml:
static GLOBAL_FLAG: Mutex<()> = Mutex::new(());
Now, the question is how to make use of both conditionally, depending on either:
which version of the compiler is used to build the crate
or
whether the feature is stable (I struggle to find it's correct name at the moment)?
I was looking at cfg_rust_features crate, but the part where dependency isn't required by Cargo.toml remains mystery. Any hints welcome.

Answer 1

This is typically accomplished by:
Detecting the Rust version (using rustversion/rustc_version) or the required feature (using autocfg),
Making the fallback dependency (once_cell) optional, and
Emitting an error if the required Rust feature is missing and the fallback dependency is also missing.
For example, using rustversion:
#[rustversion::since(1.63)]
mod private {
use std::sync::Mutex;
pub static GLOBAL_FLAG: Mutex<()> = Mutex::new(());
}
#[rustversion::before(1.63)]
#[cfg(feature = "once_cell")]
mod private {
use once_cell::sync::Lazy;
use std::sync::Mutex;
pub static GLOBAL_FLAG: Lazy<Mutex<()>> = Lazy::new(Default::default);
}
#[rustversion::before(1.63)]
#[cfg(not(feature = "once_cell"))]
compile_error!(
"The \"once_cell\" feature must be enabled for Rust compiler versions prior to 1.63."
);
When compiling on 1.62 without once_cell, this emits
error: The "once_cell" feature must be enabled for Rust compiler versions prior to 1.63.
--> src/main.rs:18:1
|
18 | / compile_error!(
19 | | "The \"once_cell\" feature must be enabled for Rust compiler versions prior to 1.63."
20 | | );
| |_^
Note: the use of mod private seen above is not strictly necessary, but it reduces the number of attributes and ensures that the relevant compile error is the only error emitted. You can pub use private::GLOBAL_FLAG to put it back in scope.