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//! This crate provides macros that look just like `panic!()` but instead of panicking, they cause
//! linking error if their calls are not optimized-out. This can be used to ensure the compiler
//! optimizes away some code.
//!
//! # Example
//!
//! ```no_compile
//! #[macro_use]
//! extern crate dont_panic;
//!
//! fn main() {
//! /*
//!     let x = 6 * 9;
//!     if x == 42 {
//!         dont_panic!("6 * 9 == 42");
//!     }
//! */
//! let x = false;
//! if x {
//!     dont_panic!("42");
//! }
//! }
//! ```
//!
//! Compile with `--release` or `--features=panic`

#![no_std]

extern "C" {
    /// This function doesn't actually exist. It ensures linking error if it isn't optimized-out.
    pub fn rust_panic_called_where_shouldnt() -> !;
}

/// This macro doesn't panic. Instead it tries to call non-existing function. If the compiler can
/// prove it can't be called an optimizes it away, the code will compile just fine. Else you get
/// linking error.
///
/// This should be used only in cases you are absolutely sure are OK and optimizable by compiler.
#[cfg(not(feature = "panic"))]
#[macro_export]
macro_rules! dont_panic {
    () => ({
        unsafe { $crate::rust_panic_called_where_shouldnt(); }
    });
    ($msg:expr) => ({
        unsafe { $crate::rust_panic_called_where_shouldnt(); }
    });
    ($fmt:expr, $($arg:tt)+) => ({
        unsafe { $crate::rust_panic_called_where_shouldnt(); }
    });
}

/// This macro is active only with `panic` feature turned on and it will really panic, instead of
/// linking error. The purpose is to make development easier. (E.g. in debug mode.)
#[cfg(feature = "panic")]
#[macro_export]
macro_rules! dont_panic {
    () => ({
        panic!();
    });
    ($msg:expr) => ({
        panic!($msg);
    });
    ($fmt:expr, $($arg:tt)+) => ({
        panic!($fmt, $($arg)+);
    });
}

/// Like asser but calls `dont_panic!()` instead of `panic!()`
#[macro_export]
macro_rules! dp_assert {
    ($cond:expr) => (
        if !$cond {
            dont_panic!(concat!("assertion failed: ", stringify!($cond)))
        }
    );

    ($cond:expr, $($arg:tt)+) => (
        if !$cond {
            dont_panic!($($arg)+)
        }
    );
}

#[cfg(test)]
mod tests {
    #[test]
    fn it_works() {
        let should_panic = false;
        if should_panic {
            dont_panic!();
        }
    }

    #[cfg(feature = "panic")]
    #[test]
    #[should_panic]
    fn panic() {
        let should_panic = true;
        if should_panic {
            dont_panic!();
        }
    }

    #[cfg(feature = "panic")]
    #[test]
    fn no_panic() {
        let should_panic = false;
        if should_panic {
            dont_panic!();
        }
    }
}