48 lines
1.9 KiB
Rust
48 lines
1.9 KiB
Rust
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#![crate_type = "lib"]
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#![feature(transmutability)]
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#![allow(dead_code, incomplete_features, non_camel_case_types)]
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use std::cell::UnsafeCell;
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mod assert {
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use std::mem::{Assume, BikeshedIntrinsicFrom};
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pub fn is_maybe_transmutable<Src, Dst>()
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where
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Dst: BikeshedIntrinsicFrom<Src, { Assume::SAFETY }>
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{}
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}
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fn value_to_value() {
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// We accept value-to-value transmutations of `UnsafeCell`-containing types,
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// because owning a value implies exclusive access.
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assert::is_maybe_transmutable::<UnsafeCell<u8>, u8>();
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assert::is_maybe_transmutable::<u8, UnsafeCell<u8>>();
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assert::is_maybe_transmutable::<UnsafeCell<u8>, UnsafeCell<u8>>();
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}
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fn ref_to_ref() {
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// We forbid `UnsafeCell`-containing ref-to-ref transmutations, because the
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// two types may use different, incompatible synchronization strategies.
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assert::is_maybe_transmutable::<&'static u8, &'static UnsafeCell<u8>>(); //~ ERROR: cannot be safely transmuted
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assert::is_maybe_transmutable::<&'static UnsafeCell<u8>, &'static UnsafeCell<u8>>(); //~ ERROR: cannot be safely transmuted
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}
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fn mut_to_mut() {
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// `UnsafeCell` does't matter for `&mut T` to `&mut U`, since exclusive
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// borrows can't be used for shared access.
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assert::is_maybe_transmutable::<&'static mut u8, &'static mut UnsafeCell<u8>>();
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assert::is_maybe_transmutable::<&'static mut UnsafeCell<u8>, &'static mut u8>();
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assert::is_maybe_transmutable::<&'static mut UnsafeCell<u8>, &'static mut UnsafeCell<u8>>();
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}
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fn mut_to_ref() {
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// We don't care about `UnsafeCell` for transmutations in the form `&mut T
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// -> &U`, because downgrading a `&mut T` to a `&U` deactivates `&mut T` for
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// the lifetime of `&U`.
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assert::is_maybe_transmutable::<&'static mut u8, &'static UnsafeCell<u8>>();
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assert::is_maybe_transmutable::<&'static mut UnsafeCell<u8>, &'static u8>();
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assert::is_maybe_transmutable::<&'static mut UnsafeCell<u8>, &'static UnsafeCell<u8>>();
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}
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