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use std::num::NonZeroU32; // Code from https://github.com/nvzqz/static-assertions-rs. macro_rules! assert_eq_size { ($x:ty, $($xs:ty),+ $(,)*) => { $(let _ = ::std::mem::transmute::<$x, $xs>;)+ }; ($label:ident; $($xs:tt)+) => { #[allow(dead_code, non_snake_case)] fn $label() { assert_eq_size!($($xs)+); } }; } mod ffi { extern "C" { pub fn overwrite_values(len: i32, values: *mut u32); } } #[inline] fn overwrite_values(values: &mut [Option<NonZeroU32>]) { assert_eq_size!(Option<NonZeroU32>, u32); unsafe { ffi::overwrite_values(values.len() as i32, values.as_mut_ptr() as *mut u32); } } pub unsafe fn create_values_transmute() -> [NonZeroU32; 2] { // Reserve some unallocated memory on the stack. let mut values: [Option<NonZeroU32>; 2] = ::std::mem::uninitialized(); // Overwrite the values with unknown new values. overwrite_values(&mut values); // Ensure all values are Some. for value in values.iter() { assert!(value.is_some()); } // Strip away the Option without copying. ::std::mem::transmute::<[Option<NonZeroU32>; 2], [NonZeroU32; 2]>(values) } pub unsafe fn create_values_transmute_unrolled() -> [NonZeroU32; 2] { // Reserve some unallocated memory on the stack. let mut values: [Option<NonZeroU32>; 2] = ::std::mem::uninitialized(); // Overwrite the values with unknown new values. overwrite_values(&mut values); // Ensure all values are Some. values[0].as_ref().unwrap(); values[1].as_ref().unwrap(); // Strip away the Option without copying. ::std::mem::transmute::<[Option<NonZeroU32>; 2], [NonZeroU32; 2]>(values) } // NOTE: Uncommenting create_values_take will cause create_values_replace to change! // pub unsafe fn create_values_take() -> [NonZeroU32; 2] { // // Reserve some unallocated memory on the stack. // let mut values: [Option<NonZeroU32>; 2] = ::std::mem::uninitialized(); // // Overwrite the values with unknown new values. // overwrite_values(&mut values); // // Unwrap the options, hopefully it gets optimized and happens in place. // [ // values[0].take().unwrap(), // values[1].take().unwrap(), // ] // } pub unsafe fn create_values_replace() -> [NonZeroU32; 2] { // Reserve some unallocated memory on the stack. let mut values: [Option<NonZeroU32>; 2] = ::std::mem::uninitialized(); // Overwrite the values with unknown new values. overwrite_values(&mut values); // Unwrap the options, hopefully it gets optimized and happens in place. [ ::std::mem::replace(&mut values[0], ::std::mem::uninitialized()).unwrap(), ::std::mem::replace(&mut values[1], ::std::mem::uninitialized()).unwrap(), ] }
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