feat: allow const path creation

Author: conradludgate

library/std/src/ffi/os_str.rs

@@ -629,8 +629,15 @@ impl OsStr {
         s.as_ref()
     }
 
+    /// Creates a new `OsStr` from a `str`.
     #[inline]
-    fn from_inner(inner: &Slice) -> &OsStr {
+    #[unstable(feature = "const_path", reason = "TBD", issue = "none")]
+    pub const fn from_str(s: &str) -> &OsStr {
+        Self::from_inner(Slice::from_str(s))
+    }
+
+    #[inline]
+    const fn from_inner(inner: &Slice) -> &OsStr {
         // SAFETY: OsStr is just a wrapper of Slice,
         // therefore converting &Slice to &OsStr is safe.
         unsafe { &*(inner as *const Slice as *const OsStr) }
@@ -1249,7 +1256,7 @@ impl AsRef<OsStr> for OsString {
 impl AsRef<OsStr> for str {
     #[inline]
     fn as_ref(&self) -> &OsStr {
-        OsStr::from_inner(Slice::from_str(self))
+        OsStr::from_str(self)
     }
 }
 

library/std/src/path.rs

@@ -1948,7 +1948,19 @@ impl Path {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn new<S: AsRef<OsStr> + ?Sized>(s: &S) -> &Path {
-        unsafe { &*(s.as_ref() as *const OsStr as *const Path) }
+        Self::from_os_str(s.as_ref())
+    }
+
+    /// Creates a new `Path` from an `OsStr`.
+    ///
+    /// This is a cost-free conversion.
+    ///
+    /// You should probably use the [`Path::new`] method instead,
+    /// however, this method supports const expressions
+    #[inline]
+    #[unstable(feature = "const_path", reason = "TBD", issue = "none")]
+    pub const fn from_os_str(s: &OsStr) -> &Path {
+        unsafe { &*(s as *const OsStr as *const Path) }
     }
 
     /// Yields the underlying [`OsStr`] slice.

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

@@ -186,12 +186,12 @@ impl Buf {
 
 impl Slice {
     #[inline]
-    fn from_u8_slice(s: &[u8]) -> &Slice {
+    const fn from_u8_slice(s: &[u8]) -> &Slice {
         unsafe { mem::transmute(s) }
     }
 
     #[inline]
-    pub fn from_str(s: &str) -> &Slice {
+    pub const fn from_str(s: &str) -> &Slice {
         Slice::from_u8_slice(s.as_bytes())
     }