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transmute: improve Vec example

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This commit is contained in:
Ralf Jung 2020-02-29 14:03:25 +01:00
parent 04e7f96dd8
commit b961a03060
1 changed files with 10 additions and 6 deletions
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16
src/libcore/intrinsics.rs
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@ -881,7 +881,8 @@ extern "rust-intrinsic" {
/// // clone the vector as we will reuse them later
/// let v_clone = v_orig.clone();
///
/// // Using transmute: this is Undefined Behavior, and a bad idea.
/// // Using transmute: this relies on the unspecified data layout of `Vec`, which is a
/// // bad idea and could cause Undefined Behavior
/// // However, it is no-copy.
/// let v_transmuted = unsafe {
/// std::mem::transmute::<Vec<&i32>, Vec<Option<&i32>>>(v_clone)
@ -897,13 +898,14 @@ extern "rust-intrinsic" {
///
/// let v_clone = v_orig.clone();
///
/// // The no-copy, unsafe way, still using transmute, but not UB.
/// // This is equivalent to the original, but safer, and reuses the
/// // same `Vec` internals. Therefore, the new inner type must have the
/// // exact same size, and the same alignment, as the old type.
/// // The no-copy, unsafe way, still using transmute, but not relying on the data layout.
/// // Like the first approach, this reuses the `Vec` internals.
/// // Therefore, the new inner type must have the
/// // exact same size, *and the same alignment*, as the old type.
/// // The same caveats exist for this method as transmute, for
/// // the original inner type (`&i32`) to the converted inner type
/// // (`Option<&i32>`), so read the nomicon pages linked above.
/// // (`Option<&i32>`), so read the nomicon pages linked above and also
/// // consult the [`from_raw_parts`] documentation.
/// let v_from_raw = unsafe {
// FIXME Update this when vec_into_raw_parts is stabilized
/// // Ensure the original vector is not dropped.
@ -914,6 +916,8 @@ extern "rust-intrinsic" {
/// };
/// ```
///
/// [`from_raw_parts`]: ../../std/vec/struct.Vec.html#method.from_raw_parts
///
/// Implementing `split_at_mut`:
///
/// ```