compile_warning!

[trueb2]

Related to #2902 and https://internals.rust-lang.org/t/pre-rfc-add-compile-warning-macro/9370, I want a compile_warning! macro that is similar to compile_error! except that it is not a hard error.

Here is a motivating use case. Say I have two barely compatible features, I would expect to be able to emit a warning based on my knowledge as a developer and experience with interface symmetry. However, it is not possible to do this:

#[cfg(all(feature = "feature_a", feature = "feature_b"))]
compile_warning!("Using 'feature_a' and 'feature_b' together is not recommended.");

There are many reasons why it may not be possible to have truly mutually exclusive features in Rust. I hit this a lot when trying to divvy up the limited static memory on microcontrollers.

I think the #[allow] and #[deny] points in the threads very relevant. It would be good feedback to someone building the crate to see the warning once, then be able to silence the warning. For example:
In a library crate:

#[cfg(all(feature = "feature_a", feature = "feature_b"))]
compile_warning!(foobar, "Using 'feature_a' and 'feature_b' together is not recommended.");

In the consuming crate:

// We checked we want this ill-advised configuration
#[allow(foobar)]

[trueb2]

My workaround to generate the warnings at the moment looks like this

// Emit a warning if neither 'feature_a' nor 'feature_b' is enabled
#[cfg(not(any(feature = "feature_a", feature = "feature_b")))]
fn emit_warning() {
    let warning = "Warning: Neither 'feature_a' nor 'feature_b' feature is set";
}

This gives me a warning (which is more important than silence). On the downside, I don't have a way to override that this is acceptable, and the function names conflict with each copy paste.

warning: unused variable: `warning`
  --> src/bin/asdf:27:9
   |
27 |     let warning = "Warning: Neither 'feature_a' nor 'feature_b' feature is set";
   |         ^^^^^^^ help: if this is intentional, prefix it with an underscore: `_warning`
   |
   = note: `#[warn(unused_variables)]` on by default

warning: function `emit_warning` is never used
  --> src/bin/asdf:26:4
   |
26 | fn emit_warning() {
   |    ^^^^^^^^^^^^
   |
   = note: `#[warn(dead_code)]` on by default