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.

1 files changed, 1 insertions, 3 deletions

diff --git a/library/std/src/sys/windows/os_str.rs b/library/std/src/sys/windows/os_str.rs
index 11883f15022..4bdd8c505ff 100644
--- a/library/std/src/sys/windows/os_str.rs
+++ b/library/std/src/sys/windows/os_str.rs
@@ -164,9 +164,7 @@ impl Slice {
     }
 
     pub fn to_owned(&self) -> Buf {
-        let mut buf = Wtf8Buf::with_capacity(self.inner.len());
-        buf.push_wtf8(&self.inner);
-        Buf { inner: buf }
+        Buf { inner: self.inner.to_owned() }
     }
 
     pub fn clone_into(&self, buf: &mut Buf) {