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Rollup merge of #76242 - RalfJung:read-fixme, r=Dylan-DPC

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Read: adjust a FIXME reference

There's already another reference to https://github.com/rust-lang/rust/issues/42788 for basically the same problem, so lets reuse it here:
5e208efaa8/library/std/src/io/mod.rs (L369-L376)

r? @Dylan-DPC
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Dylan DPC 2020-09-03 02:22:11 +02:00 committed by GitHub
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library/std/src/io/util.rs
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@ -52,11 +52,14 @@ where
W: Write,
{
let mut buf = MaybeUninit::<[u8; super::DEFAULT_BUF_SIZE]>::uninit();
// FIXME(#76092): This is calling `get_mut` and `get_ref` on an uninitialized
// `MaybeUninit`. Revisit this once we decided whether that is valid or not.
// This is still technically undefined behavior due to creating a reference
// to uninitialized data, but within libstd we can rely on more guarantees
// than if this code were in an external lib.
// FIXME: #42788
//
// - This creates a (mut) reference to a slice of
// _uninitialized_ integers, which is **undefined behavior**
//
// - Only the standard library gets to soundly "ignore" this,
// based on its privileged knowledge of unstable rustc
// internals;
unsafe {
reader.initializer().initialize(buf.assume_init_mut());
}