path: Move is_absolute check to sys::path

I am working on fs support for UEFI [0], which similar to windows has prefix
components, but is not quite same as Windows. It also seems that Prefix
is tied closely to Windows and cannot really be extended [1].

This PR just tries to remove coupling between Prefix and absolute path
checking to allow platforms to provide there own implementation to check
if a path is absolute or not.

I am not sure if any platform other than windows currently uses Prefix,
so I have kept the path.prefix().is_some() check in most cases.

[0]: rust-lang#135368
[1]: rust-lang#52331 (comment)

Signed-off-by: Ayush Singh <ayush@beagleboard.org>

std/src/path.rs

@@ -298,7 +298,7 @@ where
 }
 
 // Detect scheme on Redox
-fn has_redox_scheme(s: &[u8]) -> bool {
+pub(crate) fn has_redox_scheme(s: &[u8]) -> bool {
     cfg!(target_os = "redox") && s.contains(&b':')
 }
 
@@ -2155,7 +2155,7 @@ impl Path {
         unsafe { Path::new(OsStr::from_encoded_bytes_unchecked(s)) }
     }
     // The following (private!) function reveals the byte encoding used for OsStr.
-    fn as_u8_slice(&self) -> &[u8] {
+    pub(crate) fn as_u8_slice(&self) -> &[u8] {
         self.inner.as_encoded_bytes()
     }
 
@@ -2323,14 +2323,7 @@ impl Path {
     #[must_use]
     #[allow(deprecated)]
     pub fn is_absolute(&self) -> bool {
-        if cfg!(target_os = "redox") {
-            // FIXME: Allow Redox prefixes
-            self.has_root() || has_redox_scheme(self.as_u8_slice())
-        } else {
-            self.has_root()
-                && (cfg!(any(unix, target_os = "hermit", target_os = "wasi"))
-                    || self.prefix().is_some())
-        }
+        sys::path::is_absolute(self)
     }
 
     /// Returns `true` if the `Path` is relative, i.e., not absolute.
@@ -2353,7 +2346,7 @@ impl Path {
         !self.is_absolute()
     }
 
-    fn prefix(&self) -> Option<Prefix<'_>> {
+    pub(crate) fn prefix(&self) -> Option<Prefix<'_>> {
         self.components().prefix
     }
 

std/src/sys/path/sgx.rs

@@ -23,3 +23,7 @@ pub const MAIN_SEP: char = '/';
 pub(crate) fn absolute(_path: &Path) -> io::Result<PathBuf> {
     unsupported()
 }
+
+pub(crate) fn is_absolute(path: &Path) -> bool {
+    path.has_root() && path.prefix().is_some()
+}

std/src/sys/path/unix.rs

@@ -60,3 +60,14 @@ pub(crate) fn absolute(path: &Path) -> io::Result<PathBuf> {
 
     Ok(normalized)
 }
+
+pub(crate) fn is_absolute(path: &Path) -> bool {
+    if cfg!(target_os = "redox") {
+        // FIXME: Allow Redox prefixes
+        path.has_root() || crate::path::has_redox_scheme(path.as_u8_slice())
+    } else if cfg!(any(unix, target_os = "hermit", target_os = "wasi")) {
+        path.has_root()
+    } else {
+        path.has_root() && path.prefix().is_some()
+    }
+}

std/src/sys/path/unsupported_backslash.rs

@@ -24,3 +24,7 @@ pub const MAIN_SEP: char = '\\';
 pub(crate) fn absolute(_path: &Path) -> io::Result<PathBuf> {
     unsupported()
 }
+
+pub(crate) fn is_absolute(path: &Path) -> bool {
+    path.has_root() && path.prefix().is_some()
+}

std/src/sys/path/windows.rs

@@ -346,3 +346,7 @@ pub(crate) fn absolute(path: &Path) -> io::Result<PathBuf> {
         os2path,
     )
 }
+
+pub(crate) fn is_absolute(path: &Path) -> bool {
+    path.has_root() && path.prefix().is_some()
+}