std: add pattern matching to OsStr

Implement Haystack for &OsStr and Pattern<&OsStr> for &str, char and
Predicate.  Furthermore, add prefix/suffix matching/stripping and
splitting methods to OsStr type to make use of those patterns.

Using OsStr as a pattern is *not* implemented.

With #118485 - I added
find and rfind

Author: pacak

library/core/src/str_bytes.rs

@@ -67,7 +67,7 @@ impl<F: Flavour> Bytes<F> {
     ///
     /// It may be more convenient to use [`Bytes::from_str`] for `&str` and
     /// [`Bytes::from_bytes`] for `&[u8]`.
-    unsafe fn new(bytes: &[u8]) -> &Self {
+    pub unsafe fn new(bytes: &[u8]) -> &Self {
         // SAFETY: `Bytes` is a `repr(transparent)` wrapper around `[u8]`.
         unsafe { &*(bytes as *const [u8] as *const Bytes<F>) }
     }

library/core/src/wtf8.rs

@@ -503,6 +503,14 @@ pub enum Utf8BoundaryError {
 }
 
 /// Copied from core::str::raw::slice_unchecked
+impl Wtf8 {
+    #[inline]
+    pub unsafe fn get_unchecked(&self, range: core::ops::Range<usize>) -> &Self {
+        // SAFETY: Caller promises `range` is valid.
+        unsafe { slice_unchecked(self, range.start, range.end) }
+    }
+}
+
 #[inline]
 unsafe fn slice_unchecked(s: &Wtf8, begin: usize, end: usize) -> &Wtf8 {
     // SAFETY: memory layout of a &[u8] and &Wtf8 are the same
@@ -662,3 +670,20 @@ unsafe impl CloneToUninit for Wtf8 {
         unsafe { self.bytes.clone_to_uninit(dst) }
     }
 }
+
+#[unstable(feature = "pattern", issue = "27721")]
+impl<'a> From<&'a Wtf8> for &'a core::str_bytes::Bytes<core::str_bytes::Wtf8> {
+    fn from(wtf8: &'a Wtf8) -> Self {
+        // SAFETY: As name implies, `Wtf8`’s bytes are guaranteed to be WTF-8
+        // so `Wtf8` flavour is correct.
+        unsafe { core::str_bytes::Bytes::new(&wtf8.bytes) }
+    }
+}
+
+#[unstable(feature = "pattern", issue = "27721")]
+impl<'a> From<&'a core::str_bytes::Bytes<core::str_bytes::Wtf8>> for &'a Wtf8 {
+    fn from(bytes: &'a core::str_bytes::Bytes<core::str_bytes::Wtf8>) -> Self {
+        // SAFETY: `Bytes<Wtf8>` is guaranteed to be well-formed WTF-8.
+        unsafe { Wtf8::from_bytes_unchecked(bytes.as_bytes()) }
+    }
+}