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54 lines
1.5 KiB
Rust
54 lines
1.5 KiB
Rust
#![allow(dead_code)]
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mod ref_with_flag {
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use std::marker::PhantomData;
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use std::mem::align_of;
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/// A `&T` and a `bool`, wrapped up in a single word.
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/// The type `T` must require at least two-byte alignment.
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///
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/// If you're the kind of programmer who's never met a pointer whose
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/// 2⁰-bit you didn't want to steal, well, now you can do it safely!
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/// ("But it's not nearly as exciting this way...")
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pub struct RefWithFlag<'a, T> {
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ptr_and_bit: usize,
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behaves_like: PhantomData<&'a T> // occupies no space
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}
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impl<'a, T: 'a> RefWithFlag<'a, T> {
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pub fn new(ptr: &'a T, flag: bool) -> RefWithFlag<T> {
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assert!(align_of::<T>() % 2 == 0);
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RefWithFlag {
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ptr_and_bit: ptr as *const T as usize | flag as usize,
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behaves_like: PhantomData
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}
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}
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pub fn get_ref(&self) -> &'a T {
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unsafe {
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let ptr = (self.ptr_and_bit & !1) as *const T;
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&*ptr
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}
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}
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pub fn get_flag(&self) -> bool {
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self.ptr_and_bit & 1 != 0
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}
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}
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}
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#[cfg(test)]
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mod ref_with_flag_tests {
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use super::ref_with_flag;
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#[test]
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fn use_ref_with_flag() {
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use ref_with_flag::RefWithFlag;
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let vec = vec![10, 20, 30];
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let flagged = RefWithFlag::new(&vec, true);
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assert_eq!(flagged.get_ref()[1], 20);
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assert_eq!(flagged.get_flag(), true);
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}
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}
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