Macro compile_warning 

macro_rules! compile_warning {
    ($expr:expr) => { ... };
}

compile_warning macro is a brother of std::compile_error, which emits a compile-time warning with a provided message.

This implemented through an existing dead_code warning, thus the output for the following example:

compile_warning!("Sample user-defined warning!");

may look as follows:

warning: constant item is never used: `WARNING`
  --> src/lib.rs:7:9
  |
7 |         const WARNING: &str = $expr;
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
...
11 | compile_warning!("Sample user-defined warning!");
   | ------------------------------------------------- in this macro invocation

Once proc_macro_diagnostics feature is stabilized, this macro will be replaced with a proper proc-macro-based implementation.

This macro is intended to be used in the development process, as an alternative to the unimplemented macro which doesn’t cause code to panic.