bp3d-util 2.5.0

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//! Simple memory tools.

use std::process::abort;

struct AbortUnwind;

impl Drop for AbortUnwind {
    fn drop(&mut self) {
        eprintln!("Attempt to replace memory with panic!");
        abort()
    }
}

/// Replaces content of `dest` by value mapped with `f`.
/// This function aborts if `f` panics.
///
/// # Arguments
///
/// * `dest`: the destination to replace.
/// * `f`: the replace function.
///
/// returns: ()
pub fn replace_with<T, F: FnOnce(T) -> T>(dest: &mut T, f: F) {
    unsafe {
        let old = std::ptr::read(dest);
        if cfg!(panic = "abort") {
            std::ptr::write(dest, f(old));
        } else {
            let p = AbortUnwind;
            let new = f(old);
            std::mem::forget(p);
            std::ptr::write(dest, new);
        }
    }
}

/// Replaces content of `dest` by value mapped with `f`.
///
/// # Arguments
///
/// * `dest`: the destination to replace.
/// * `f`: the replace function.
///
/// returns: ()
///
/// # Safety
///
/// If `f` panics, then this function is UB. Use this variant when absolutely sure `f` cannot panic.
#[inline]
pub unsafe fn replace_with_unchecked<T, F: FnOnce(T) -> T>(dest: &mut T, f: F) {
    let old = std::ptr::read(dest);
    std::ptr::write(dest, f(old));
}

#[cfg(test)]
mod tests {
    use crate::mem::replace_with;

    #[test]
    fn basic() {
        let mut value: Vec<String> = Vec::with_capacity(12);
        assert_eq!(value.len(), 0);
        assert_eq!(value.capacity(), 12);
        replace_with(&mut value, |mut value| {
            value.push("test".into());
            value
        });
        assert_eq!(value.len(), 1);
        assert_eq!(value.capacity(), 12);
    }
}