Reading into uninitialized buffers

[sunfishcode]

Currently, read takes a &mut [u8]. That's fine for many use cases, but it does require the buffer to be initialized before calling read, so it's currently not possible to read directly into uninitialized memory.

For Mustang, the C read API takes a raw pointer which may be pointing to an uninitialized buffer. Currently mustang does slice::from_raw_parts_mut, however this has undefined behavior if the slice is uninitialized.

[sunfishcode]

This issue affects Rust more broadly; here are some links:

• https://rust-lang.github.io/rfcs/2930-read-buf.html
• https://internals.rust-lang.org/t/reading-into-uninitialized-buffers-yet-again/13282/4
• https://yosh.is/writing/uninit-read-write
• std::io: migrate ReadBuf to BorrowedBuf/BorrowedCursor rust-lang/rust#97015

[sunfishcode]

Mustang is now using temporary buffers to avoid having UB, but it could avoid this overhead if rustix had a way to read into uninitialized buffers directly.