perf: cost-free conversion from paths to &str

.github/workflows/ci.yml

@@ -51,6 +51,7 @@ jobs:
           - 1.63
           - nightly-2022-12-14
           - 1.68
+          - 1.74
           - stable
         exclude:
           # These versions started failing with "archive member 'lib.rmeta' with length 26456 is not

build.rs

@@ -18,6 +18,7 @@ fn main() {
     println!("cargo:rustc-check-cfg=cfg(path_buf_capacity)");
     println!("cargo:rustc-check-cfg=cfg(shrink_to)");
     println!("cargo:rustc-check-cfg=cfg(try_reserve_2)");
+    println!("cargo:rustc-check-cfg=cfg(os_str_bytes)");
 
     let compiler = match rustc_version() {
         Some(compiler) => compiler,
@@ -49,6 +50,11 @@ fn main() {
     {
         println!("cargo:rustc-cfg=path_buf_deref_mut");
     }
+    // os_str_bytes was added in a 1.74 stable.
+    if (compiler.minor >= 74 && compiler.channel == ReleaseChannel::Stable) || compiler.minor >= 75
+    {
+        println!("cargo:rustc-cfg=os_str_bytes");
+    }
 
     // Catch situations where the actual features aren't enabled. Currently, they're only shown with
     // `-vv` output, but maybe that will be noticed.

src/lib.rs

@@ -701,10 +701,10 @@ impl Utf8Path {
     /// the current directory.
     ///
     /// * On Unix, a path is absolute if it starts with the root, so
-    /// `is_absolute` and [`has_root`] are equivalent.
+    ///   `is_absolute` and [`has_root`] are equivalent.
     ///
     /// * On Windows, a path is absolute if it has a prefix and starts with the
-    /// root: `c:\windows` is absolute, while `c:temp` and `\temp` are not.
+    ///   root: `c:\windows` is absolute, while `c:temp` and `\temp` are not.
     ///
     /// # Examples
     ///
@@ -3067,9 +3067,22 @@ impl_cmp_os_str!(&'a Utf8Path, OsString);
 // invariant: OsStr must be guaranteed to be utf8 data
 #[inline]
 unsafe fn str_assume_utf8(string: &OsStr) -> &str {
-    // Adapted from the source code for Option::unwrap_unchecked.
-    match string.to_str() {
-        Some(val) => val,
-        None => std::hint::unreachable_unchecked(),
+    #[cfg(os_str_bytes)]
+    {
+        // SAFETY: OsStr is guaranteed to be utf8 data from the invariant
+        unsafe {
+            std::str::from_utf8_unchecked(
+                #[allow(clippy::incompatible_msrv)]
+                string.as_encoded_bytes(),
+            )
+        }
+    }
+    #[cfg(not(os_str_bytes))]
+    {
+        // Adapted from the source code for Option::unwrap_unchecked.
+        match string.to_str() {
+            Some(val) => val,
+            None => std::hint::unreachable_unchecked(),
+        }
     }
 }