Move the implementation of Path::exists to sys_common::fs so platforms can specialize it

Windows implementation of `fs::try_exists`

library/std/src/fs.rs

@@ -2208,3 +2208,29 @@ impl AsInnerMut<fs_imp::DirBuilder> for DirBuilder {
         &mut self.inner
     }
 }
+
+/// Returns `Ok(true)` if the path points at an existing entity.
+///
+/// This function will traverse symbolic links to query information about the
+/// destination file. In case of broken symbolic links this will return `Ok(false)`.
+///
+/// As opposed to the `exists()` method, this one doesn't silently ignore errors
+/// unrelated to the path not existing. (E.g. it will return `Err(_)` in case of permission
+/// denied on some of the parent directories.)
+///
+/// # Examples
+///
+/// ```no_run
+/// #![feature(path_try_exists)]
+/// use std::fs;
+///
+/// assert!(!fs::try_exists("does_not_exist.txt").expect("Can't check existence of file does_not_exist.txt"));
+/// assert!(fs::try_exists("/root/secret_file.txt").is_err());
+/// ```
+// FIXME: stabilization should modify documentation of `exists()` to recommend this method
+// instead.
+#[unstable(feature = "path_try_exists", issue = "83186")]
+#[inline]
+pub fn try_exists<P: AsRef<Path>>(path: P) -> io::Result<bool> {
+    fs_imp::try_exists(path.as_ref())
+}

library/std/src/path.rs

@@ -2507,11 +2507,7 @@ impl Path {
     #[unstable(feature = "path_try_exists", issue = "83186")]
     #[inline]
     pub fn try_exists(&self) -> io::Result<bool> {
-        match fs::metadata(self) {
-            Ok(_) => Ok(true),
-            Err(error) if error.kind() == io::ErrorKind::NotFound => Ok(false),
-            Err(error) => Err(error),
-        }
+        fs::try_exists(self)
     }
 
     /// Returns `true` if the path exists on disk and is pointing at a regular file.

library/std/src/sys/hermit/fs.rs

@@ -12,7 +12,7 @@ use crate::sys::time::SystemTime;
 use crate::sys::unsupported;
 use crate::sys_common::os_str_bytes::OsStrExt;
 
-pub use crate::sys_common::fs::copy;
+pub use crate::sys_common::fs::{copy, try_exists};
 //pub use crate::sys_common::fs::remove_dir_all;
 
 fn cstr(path: &Path) -> io::Result<CString> {

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

@@ -48,7 +48,7 @@ use libc::{
     dirent64, fstat64, ftruncate64, lseek64, lstat64, off64_t, open64, readdir64_r, stat64,
 };
 
-pub use crate::sys_common::fs::remove_dir_all;
+pub use crate::sys_common::fs::{remove_dir_all, try_exists};
 
 pub struct File(FileDesc);
 

library/std/src/sys/unsupported/fs.rs

@@ -275,6 +275,10 @@ pub fn remove_dir_all(_path: &Path) -> io::Result<()> {
     unsupported()
 }
 
+pub fn try_exists(_path: &Path) -> io::Result<bool> {
+    unsupported()
+}
+
 pub fn readlink(_p: &Path) -> io::Result<PathBuf> {
     unsupported()
 }

library/std/src/sys/wasi/fs.rs

@@ -14,7 +14,7 @@ use crate::sys::time::SystemTime;
 use crate::sys::unsupported;
 use crate::sys_common::FromInner;
 
-pub use crate::sys_common::fs::remove_dir_all;
+pub use crate::sys_common::fs::{remove_dir_all, try_exists};
 
 pub struct File {
     fd: WasiFd,

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

@@ -11,6 +11,7 @@ use crate::sync::Arc;
 use crate::sys::handle::Handle;
 use crate::sys::time::SystemTime;
 use crate::sys::{c, cvt};
+pub use crate::sys_common::fs::try_exists;
 use crate::sys_common::FromInner;
 
 use super::to_u16s;

library/std/src/sys_common/fs.rs

@@ -41,3 +41,11 @@ fn remove_dir_all_recursive(path: &Path) -> io::Result<()> {
     }
     fs::remove_dir(path)
 }
+
+pub fn try_exists(path: &Path) -> io::Result<bool> {
+    match fs::metadata(path) {
+        Ok(_) => Ok(true),
+        Err(error) if error.kind() == io::ErrorKind::NotFound => Ok(false),
+        Err(error) => Err(error),
+    }
+}