Auto merge of #96869 - sunfishcode:main, r=joshtriplett

Optimize `Wtf8Buf::into_string` for the case where it contains UTF-8.

Add a `is_known_utf8` flag to `Wtf8Buf`, which tracks whether the
string is known to contain UTF-8. This is efficiently computed in many
common situations, such as when a `Wtf8Buf` is constructed from a `String`
or `&str`, or with `Wtf8Buf::from_wide` which is already doing UTF-16
decoding and already checking for surrogates.

This makes `OsString::into_string` O(1) rather than O(N) on Windows in
common cases.

And, it eliminates the need to scan through the string for surrogates in
`Args::next` and `Vars::next`, because the strings are already being
translated with `Wtf8Buf::from_wide`.

Many things on Windows construct `OsString`s with `Wtf8Buf::from_wide`,
such as `DirEntry::file_name` and `fs::read_link`, so with this patch,
users of those functions can subsequently call `.into_string()` without
paying for an extra scan through the string for surrogates.

r? `@ghost`

0 files changed, 0 insertions, 0 deletions