make path separators available in const context

* consolidate various representations of separators in std::sys::path
  into a single macro_rules invocation per platform to save
  transcription errors
* make `std::path::is_separator()` const
* new constants `std::path::{SEPARATORS, SEPARATORS_STR}`

Author: MikkelPaulson

library/std/src/path.rs

@@ -266,22 +266,33 @@ impl<'a> Prefix<'a> {
 /// ```
 #[must_use]
 #[stable(feature = "rust1", since = "1.0.0")]
-pub fn is_separator(c: char) -> bool {
+#[rustc_const_unstable(feature = "const_path_separators", issue = "153106")]
+pub const fn is_separator(c: char) -> bool {
     c.is_ascii() && is_sep_byte(c as u8)
 }
 
-/// The primary separator of path components for the current platform.
-///
-/// For example, `/` on Unix and `\` on Windows.
+/// All path separators recognized on the current platform, represented as [`char`]s; for example,
+/// this is `&['/'][..]` on Unix and `&['\\', '/'][..]` on Windows. The [primary
+/// separator](MAIN_SEPARATOR) is always element 0 of the slice.
+#[unstable(feature = "const_path_separators", issue = "153106")]
+pub const SEPARATORS: &[char] = crate::sys::path::SEPARATORS;
+
+/// All path separators recognized on the current platform, represented as [`&str`]s; for example,
+/// this is `&["/"][..]` on Unix and `&["\\", "/"][..]` on Windows. The [primary
+/// separator](MAIN_SEPARATOR_STR) is always element 0 of the slice.
+#[unstable(feature = "const_path_separators", issue = "153106")]
+pub const SEPARATORS_STR: &[&str] = crate::sys::path::SEPARATORS_STR;
+
+/// The primary separator of path components for the current platform, represented as a [`char`];
+/// for example, this is `'/'` on Unix and `'\\'` on Windows.
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "path_main_separator")]
-pub const MAIN_SEPARATOR: char = crate::sys::path::MAIN_SEP;
+pub const MAIN_SEPARATOR: char = SEPARATORS[0];
 
-/// The primary separator of path components for the current platform.
-///
-/// For example, `/` on Unix and `\` on Windows.
+/// The primary separator of path components for the current platform, represented as a [`&str`];
+/// for example, this is `"/"` on Unix and `"\\"` on Windows.
 #[stable(feature = "main_separator_str", since = "1.68.0")]
-pub const MAIN_SEPARATOR_STR: &str = crate::sys::path::MAIN_SEP_STR;
+pub const MAIN_SEPARATOR_STR: &str = SEPARATORS_STR[0];
 
 ////////////////////////////////////////////////////////////////////////////////
 // Misc helpers

library/std/src/sys/path/cygwin.rs

@@ -5,25 +5,20 @@ use crate::sys::cvt;
 use crate::sys::helpers::run_path_with_cstr;
 use crate::{io, ptr};
 
-#[inline]
-pub fn is_sep_byte(b: u8) -> bool {
-    b == b'/' || b == b'\\'
-}
+path_separator_bytes!(b'/', b'\\');
 
 /// Cygwin always prefers `/` over `\`, and it always converts all `/` to `\`
 /// internally when calling Win32 APIs. Therefore, the server component of path
 /// `\\?\UNC\localhost/share` is `localhost/share` on Win32, but `localhost`
 /// on Cygwin.
 #[inline]
-pub fn is_verbatim_sep(b: u8) -> bool {
-    b == b'/' || b == b'\\'
+pub const fn is_verbatim_sep(b: u8) -> bool {
+    is_sep_byte(b)
 }
 
 pub use super::windows_prefix::parse_prefix;
 
 pub const HAS_PREFIXES: bool = true;
-pub const MAIN_SEP_STR: &str = "/";
-pub const MAIN_SEP: char = '/';
 
 unsafe extern "C" {
     // Doc: https://cygwin.com/cygwin-api/func-cygwin-conv-path.html

library/std/src/sys/path/mod.rs

@@ -1,3 +1,22 @@
+// There's a lot of necessary redundancy in separator definition. Consolidated into a macro to
+// prevent transcription errors.
+macro_rules! path_separator_bytes {
+    ($($sep:literal),+) => (
+        pub const SEPARATORS: &[char] = &[$($sep as char,)+];
+        pub const SEPARATORS_STR: &[&str] = &[$(
+            match str::from_utf8(&[$sep]) {
+                Ok(s) => s,
+                Err(_) => panic!("path_separator_bytes must be ASCII bytes"),
+            }
+        ),+];
+
+        #[inline]
+        pub const fn is_sep_byte(b: u8) -> bool {
+            $(b == $sep) ||+
+        }
+    )
+}
+
 cfg_select! {
     target_os = "windows" => {
         mod windows;

library/std/src/sys/path/sgx.rs

@@ -3,23 +3,18 @@ use crate::io;
 use crate::path::{Path, PathBuf, Prefix};
 use crate::sys::unsupported;
 
-#[inline]
-pub fn is_sep_byte(b: u8) -> bool {
-    b == b'/'
-}
+path_separator_bytes!(b'/');
 
 #[inline]
-pub fn is_verbatim_sep(b: u8) -> bool {
-    b == b'/'
+pub const fn is_verbatim_sep(b: u8) -> bool {
+    is_sep_byte(b)
 }
 
 pub fn parse_prefix(_: &OsStr) -> Option<Prefix<'_>> {
     None
 }
 
 pub const HAS_PREFIXES: bool = false;
-pub const MAIN_SEP_STR: &str = "/";
-pub const MAIN_SEP: char = '/';
 
 pub(crate) fn absolute(_path: &Path) -> io::Result<PathBuf> {
     unsupported()

library/std/src/sys/path/uefi.rs

@@ -5,26 +5,21 @@ use crate::path::{Path, PathBuf, Prefix};
 use crate::sys::pal::helpers;
 use crate::sys::unsupported_err;
 
+path_separator_bytes!(b'\\');
+
 const FORWARD_SLASH: u8 = b'/';
 const COLON: u8 = b':';
 
 #[inline]
-pub fn is_sep_byte(b: u8) -> bool {
-    b == b'\\'
-}
-
-#[inline]
-pub fn is_verbatim_sep(b: u8) -> bool {
-    b == b'\\'
+pub const fn is_verbatim_sep(b: u8) -> bool {
+    is_sep_byte(b)
 }
 
 pub fn parse_prefix(_: &OsStr) -> Option<Prefix<'_>> {
     None
 }
 
 pub const HAS_PREFIXES: bool = true;
-pub const MAIN_SEP_STR: &str = "\\";
-pub const MAIN_SEP: char = '\\';
 
 /// UEFI paths can be of 4 types:
 ///

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

@@ -2,14 +2,11 @@ use crate::ffi::OsStr;
 use crate::path::{Path, PathBuf, Prefix};
 use crate::{env, io};
 
-#[inline]
-pub fn is_sep_byte(b: u8) -> bool {
-    b == b'/'
-}
+path_separator_bytes!(b'/');
 
 #[inline]
-pub fn is_verbatim_sep(b: u8) -> bool {
-    b == b'/'
+pub const fn is_verbatim_sep(b: u8) -> bool {
+    is_sep_byte(b)
 }
 
 #[inline]
@@ -18,8 +15,6 @@ pub fn parse_prefix(_: &OsStr) -> Option<Prefix<'_>> {
 }
 
 pub const HAS_PREFIXES: bool = false;
-pub const MAIN_SEP_STR: &str = "/";
-pub const MAIN_SEP: char = '/';
 
 /// Make a POSIX path absolute without changing its semantics.
 pub(crate) fn absolute(path: &Path) -> io::Result<PathBuf> {

library/std/src/sys/path/unsupported_backslash.rs

@@ -4,23 +4,18 @@ use crate::io;
 use crate::path::{Path, PathBuf, Prefix};
 use crate::sys::unsupported;
 
-#[inline]
-pub fn is_sep_byte(b: u8) -> bool {
-    b == b'\\'
-}
+path_separator_bytes!(b'\\');
 
 #[inline]
-pub fn is_verbatim_sep(b: u8) -> bool {
-    b == b'\\'
+pub const fn is_verbatim_sep(b: u8) -> bool {
+    is_sep_byte(b)
 }
 
 pub fn parse_prefix(_: &OsStr) -> Option<Prefix<'_>> {
     None
 }
 
 pub const HAS_PREFIXES: bool = true;
-pub const MAIN_SEP_STR: &str = "\\";
-pub const MAIN_SEP: char = '\\';
 
 pub(crate) fn absolute(_path: &Path) -> io::Result<PathBuf> {
     unsupported()

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

@@ -9,9 +9,9 @@ mod tests;
 
 pub use super::windows_prefix::parse_prefix;
 
+path_separator_bytes!(b'\\', b'/');
+
 pub const HAS_PREFIXES: bool = true;
-pub const MAIN_SEP_STR: &str = "\\";
-pub const MAIN_SEP: char = '\\';
 
 /// A null terminated wide string.
 #[repr(transparent)]
@@ -48,12 +48,7 @@ pub fn with_native_path<T>(path: &Path, f: &dyn Fn(&WCStr) -> io::Result<T>) ->
 }
 
 #[inline]
-pub fn is_sep_byte(b: u8) -> bool {
-    b == b'/' || b == b'\\'
-}
-
-#[inline]
-pub fn is_verbatim_sep(b: u8) -> bool {
+pub const fn is_verbatim_sep(b: u8) -> bool {
     b == b'\\'
 }