Move platform-specific implementation of libstd/path.rs

This removes one exception in the tidy lint 'pal'.

src/libstd/path.rs

@@ -127,7 +127,7 @@ use ops::{self, Deref};
 
 use ffi::{OsStr, OsString};
 
-use sys::path::{is_sep_byte, is_verbatim_sep, MAIN_SEP_STR, parse_prefix};
+use sys::path::{ABSOLUTE_NEEDS_PREFIX, is_sep_byte, is_verbatim_sep, MAIN_SEP_STR, parse_prefix};
 
 ////////////////////////////////////////////////////////////////////////////////
 // GENERAL NOTES
@@ -1509,7 +1509,7 @@ impl Path {
     #[allow(deprecated)]
     pub fn is_absolute(&self) -> bool {
         // FIXME: Remove target_os = "redox" and allow Redox prefixes
-        self.has_root() && (cfg!(unix) || cfg!(target_os = "redox") || self.prefix().is_some())
+        self.has_root() && (!ABSOLUTE_NEEDS_PREFIX || self.prefix().is_some())
     }
 
     /// A path is *relative* if it is not absolute.

src/libstd/sys/redox/path.rs

@@ -37,3 +37,5 @@ pub fn parse_prefix(path: &OsStr) -> Option<Prefix> {
 
 pub const MAIN_SEP_STR: &'static str = "/";
 pub const MAIN_SEP: char = '/';
+// FIXME: Change to 'true' and allow Redox prefixes
+pub const ABSOLUTE_NEEDS_PREFIX: bool = false;

src/libstd/sys/unix/path.rs

@@ -27,3 +27,4 @@ pub fn parse_prefix(_: &OsStr) -> Option<Prefix> {
 
 pub const MAIN_SEP_STR: &'static str = "/";
 pub const MAIN_SEP: char = '/';
+pub const ABSOLUTE_NEEDS_PREFIX: bool = false;

src/libstd/sys/windows/path.rs

@@ -106,3 +106,4 @@ pub fn parse_prefix<'a>(path: &'a OsStr) -> Option<Prefix> {
 
 pub const MAIN_SEP_STR: &'static str = "\\";
 pub const MAIN_SEP: char = '\\';
+pub const ABSOLUTE_NEEDS_PREFIX: bool = true;

src/tools/tidy/src/pal.rs

@@ -66,7 +66,6 @@ const EXCEPTION_PATHS: &'static [&'static str] = &[
 
     // temporary exceptions
     "src/libstd/rtdeps.rs", // Until rustbuild replaces make
-    "src/libstd/path.rs",
     "src/libstd/f32.rs",
     "src/libstd/f64.rs",
     "src/libstd/sys_common/mod.rs",