Auto merge of #23934 - lfairy:write-no-deref, r=alexcrichton

This means passing in e.g. a `Vec<u8>` or `String` will work as
intended, rather than deref-ing to `&mut [u8]` or `&mut str`.

[breaking-change]

Closes #23768

diff --cc src/libcore/macros.rs
Simple merge