Add OsStr inherent fns to test for and strip str prefixes.

* `OsStr::starts_with()` tests whether an `OsStr` has a prefix matching
  the given `Pattern`.

* `OsStr::strip_prefix()` returns the `OsStr` after removing a prefix
  matching the given `Pattern`.

Author: jmillikin

library/std/src/ffi/os_str.rs

@@ -8,6 +8,7 @@ use crate::fmt;
 use crate::hash::{Hash, Hasher};
 use crate::ops;
 use crate::rc::Rc;
+use crate::str::pattern::Pattern;
 use crate::str::FromStr;
 use crate::sync::Arc;
 
@@ -978,6 +979,69 @@ impl OsStr {
     pub fn eq_ignore_ascii_case<S: AsRef<OsStr>>(&self, other: S) -> bool {
         self.inner.eq_ignore_ascii_case(&other.as_ref().inner)
     }
+
+    /// Returns `true` if the given pattern matches a prefix of this `OsStr`.
+    ///
+    /// Returns `false` if it does not.
+    ///
+    /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
+    /// function or closure that determines if a character matches.
+    ///
+    /// [`char`]: prim@char
+    /// [pattern]: crate::str::pattern
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(osstr_str_prefix_fns)]
+    ///
+    /// use std::ffi::OsString;
+    ///
+    /// let bananas = OsString::from("bananas");
+    ///
+    /// assert!(bananas.starts_with("bana"));
+    /// assert!(!bananas.starts_with("nana"));
+    /// ```
+    #[unstable(feature = "osstr_str_prefix_fns", issue = "none")]
+    #[must_use]
+    #[inline]
+    pub fn starts_with<'a, P: Pattern<'a>>(&'a self, pattern: P) -> bool {
+        self.inner.starts_with(pattern)
+    }
+
+    /// Returns this `OsStr` with the given prefix removed.
+    ///
+    /// If the `OsStr` starts with the pattern `prefix`, returns the substring
+    /// after the prefix, wrapped in `Some`.
+    ///
+    /// If the `OsStr` does not start with `prefix`, returns `None`.
+    ///
+    /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
+    /// function or closure that determines if a character matches.
+    ///
+    /// [`char`]: prim@char
+    /// [pattern]: crate::str::pattern
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(osstr_str_prefix_fns)]
+    ///
+    /// use std::ffi::{OsStr, OsString};
+    ///
+    /// let foobar = OsString::from("foo:bar");
+    ///
+    /// assert_eq!(foobar.strip_prefix("foo:"), Some(OsStr::new("bar")));
+    /// assert_eq!(foobar.strip_prefix("bar"), None);
+    /// ```
+    #[unstable(feature = "osstr_str_prefix_fns", issue = "none")]
+    #[must_use]
+    #[inline]
+    pub fn strip_prefix<'a, P: Pattern<'a>>(&'a self, prefix: P) -> Option<&'a OsStr> {
+        Some(OsStr::from_inner(self.inner.strip_prefix(prefix)?))
+    }
 }
 
 #[stable(feature = "box_from_os_str", since = "1.17.0")]

library/std/src/lib.rs

@@ -264,6 +264,7 @@
 #![feature(needs_panic_runtime)]
 #![feature(negative_impls)]
 #![feature(never_type)]
+#![feature(pattern)]
 #![feature(platform_intrinsics)]
 #![feature(prelude_import)]
 #![feature(rustc_attrs)]

library/std/src/sys/unix/os_str.rs

@@ -8,6 +8,7 @@ use crate::fmt::Write;
 use crate::mem;
 use crate::rc::Rc;
 use crate::str;
+use crate::str::pattern::{Pattern, SearchStep, Searcher};
 use crate::sync::Arc;
 use crate::sys_common::{AsInner, IntoInner};
 
@@ -270,4 +271,38 @@ impl Slice {
     pub fn eq_ignore_ascii_case(&self, other: &Self) -> bool {
         self.inner.eq_ignore_ascii_case(&other.inner)
     }
+
+    fn to_str_prefix(&self) -> &str {
+        let utf8_err = match str::from_utf8(&self.inner) {
+            Ok(prefix) => return prefix,
+            Err(err) => err,
+        };
+        let utf8_len = utf8_err.valid_up_to();
+        if utf8_len == 0 {
+            return "";
+        }
+        // SAFETY: `Utf8Error::valid_up_to()` returns an index up to which
+        // valid UTF-8 has been verified.
+        unsafe { str::from_utf8_unchecked(&self.inner[..utf8_len]) }
+    }
+
+    #[inline]
+    pub fn starts_with<'a, P: Pattern<'a>>(&'a self, pattern: P) -> bool {
+        self.to_str_prefix().starts_with(pattern)
+    }
+
+    pub fn strip_prefix<'a, P: Pattern<'a>>(&'a self, prefix: P) -> Option<&'a Slice> {
+        let p = self.to_str_prefix();
+        let prefix_len = match prefix.into_searcher(p).next() {
+            SearchStep::Match(0, prefix_len) => prefix_len,
+            _ => return None,
+        };
+
+        // SAFETY: `p` is guaranteed to be a prefix of `self.inner`,
+        // and `Searcher` is known to return valid indices.
+        unsafe {
+            let suffix = self.inner.get_unchecked(prefix_len..);
+            Some(Slice::from_u8_slice(suffix))
+        }
+    }
 }

library/std/src/sys/unix/os_str/tests.rs

@@ -16,3 +16,37 @@ fn display() {
         Slice::from_u8_slice(b"Hello\xC0\x80 There\xE6\x83 Goodbye").to_string(),
     );
 }
+
+#[test]
+fn slice_starts_with() {
+    let mut string = Buf::from_string(String::from("héllô="));
+    string.push_slice(Slice::from_u8_slice(b"\xFF"));
+    string.push_slice(Slice::from_str("wørld"));
+    let slice = string.as_slice();
+
+    assert!(slice.starts_with('h'));
+    assert!(slice.starts_with("héllô"));
+    assert!(!slice.starts_with("héllô=wørld"));
+}
+
+#[test]
+fn slice_strip_prefix() {
+    let mut string = Buf::from_string(String::from("héllô="));
+    string.push_slice(Slice::from_u8_slice(b"\xFF"));
+    string.push_slice(Slice::from_str("wørld"));
+    let slice = string.as_slice();
+
+    assert!(slice.strip_prefix("héllô=wørld").is_none());
+
+    {
+        let suffix = slice.strip_prefix('h');
+        assert!(suffix.is_some());
+        assert_eq!(&suffix.unwrap().inner, b"\xC3\xA9ll\xC3\xB4=\xFFw\xC3\xB8rld",);
+    }
+
+    {
+        let suffix = slice.strip_prefix("héllô");
+        assert!(suffix.is_some());
+        assert_eq!(&suffix.unwrap().inner, b"=\xFFw\xC3\xB8rld");
+    }
+}

library/std/src/sys/windows/os_str.rs

@@ -5,6 +5,7 @@ use crate::collections::TryReserveError;
 use crate::fmt;
 use crate::mem;
 use crate::rc::Rc;
+use crate::str::pattern::Pattern;
 use crate::sync::Arc;
 use crate::sys_common::wtf8::{Wtf8, Wtf8Buf};
 use crate::sys_common::{AsInner, FromInner, IntoInner};
@@ -156,6 +157,13 @@ impl Slice {
         unsafe { mem::transmute(Wtf8::from_str(s)) }
     }
 
+    #[inline]
+    fn from_inner(inner: &Wtf8) -> &Slice {
+        // SAFETY: Slice is just a wrapper of Wtf8,
+        // therefore converting &Wtf8 to &Slice is safe.
+        unsafe { &*(inner as *const Wtf8 as *const Slice) }
+    }
+
     pub fn to_str(&self) -> Option<&str> {
         self.inner.as_str()
     }
@@ -222,4 +230,14 @@ impl Slice {
     pub fn eq_ignore_ascii_case(&self, other: &Self) -> bool {
         self.inner.eq_ignore_ascii_case(&other.inner)
     }
+
+    #[inline]
+    pub fn starts_with<'a, P: Pattern<'a>>(&'a self, pattern: P) -> bool {
+        self.inner.starts_with(pattern)
+    }
+
+    #[inline]
+    pub fn strip_prefix<'a, P: Pattern<'a>>(&'a self, prefix: P) -> Option<&'a Slice> {
+        Some(Slice::from_inner(self.inner.strip_prefix(prefix)?))
+    }
 }

library/std/src/sys_common/wtf8.rs

@@ -31,6 +31,7 @@ use crate::ops;
 use crate::rc::Rc;
 use crate::slice;
 use crate::str;
+use crate::str::pattern::{Pattern, SearchStep, Searcher};
 use crate::sync::Arc;
 use crate::sys_common::AsInner;
 
@@ -781,6 +782,42 @@ impl Wtf8 {
     pub fn eq_ignore_ascii_case(&self, other: &Self) -> bool {
         self.bytes.eq_ignore_ascii_case(&other.bytes)
     }
+
+    fn to_str_prefix(&self) -> &str {
+        let utf8_bytes = match self.next_surrogate(0) {
+            None => &self.bytes,
+            Some((0, _)) => b"",
+            Some((surrogate_pos, _)) => {
+                let (utf8_bytes, _) = self.bytes.split_at(surrogate_pos);
+                utf8_bytes
+            }
+        };
+
+        // SAFETY: `utf8_bytes` is a prefix of a WTF-8 value that contains no
+        // surrogates, and well-formed WTF-8 that contains no surrogates is
+        // also well-formed UTF-8.
+        unsafe { str::from_utf8_unchecked(utf8_bytes) }
+    }
+
+    #[inline]
+    pub fn starts_with<'a, P: Pattern<'a>>(&'a self, pattern: P) -> bool {
+        self.to_str_prefix().starts_with(pattern)
+    }
+
+    pub fn strip_prefix<'a, P: Pattern<'a>>(&'a self, prefix: P) -> Option<&'a Wtf8> {
+        let p = self.to_str_prefix();
+        let prefix_len = match prefix.into_searcher(p).next() {
+            SearchStep::Match(0, prefix_len) => prefix_len,
+            _ => return None,
+        };
+
+        // SAFETY: `p` is guaranteed to be a prefix of `self.bytes`,
+        // and `Searcher` is known to return valid indices.
+        unsafe {
+            let suffix = self.bytes.get_unchecked(prefix_len..);
+            Some(Wtf8::from_bytes_unchecked(suffix))
+        }
+    }
 }
 
 /// Returns a slice of the given string for the byte range \[`begin`..`end`).

library/std/src/sys_common/wtf8/tests.rs

@@ -664,3 +664,37 @@ fn wtf8_to_owned() {
     assert_eq!(string.bytes, b"\xED\xA0\x80");
     assert!(!string.is_known_utf8);
 }
+
+#[test]
+fn wtf8_starts_with() {
+    let mut string = Wtf8Buf::from_str("héllô=");
+    string.push(CodePoint::from_u32(0xD800).unwrap());
+    string.push_str("wørld");
+    let slice = string.as_slice();
+
+    assert!(slice.starts_with('h'));
+    assert!(slice.starts_with("héllô"));
+    assert!(!slice.starts_with("héllô=wørld"));
+}
+
+#[test]
+fn wtf8_strip_prefix() {
+    let mut string = Wtf8Buf::from_str("héllô=");
+    string.push(CodePoint::from_u32(0xD800).unwrap());
+    string.push_str("wørld");
+    let slice = string.as_slice();
+
+    assert!(slice.strip_prefix("héllô=wørld").is_none());
+
+    {
+        let suffix = slice.strip_prefix('h');
+        assert!(suffix.is_some());
+        assert_eq!(&suffix.unwrap().bytes, b"\xC3\xA9ll\xC3\xB4=\xED\xA0\x80w\xC3\xB8rld",);
+    }
+
+    {
+        let suffix = slice.strip_prefix("héllô");
+        assert!(suffix.is_some());
+        assert_eq!(&suffix.unwrap().bytes, b"=\xED\xA0\x80w\xC3\xB8rld");
+    }
+}