1 files changed, 44 insertions, 1 deletions
diff --git a/library/std/src/sys/unix/path.rs b/library/std/src/sys/unix/path.rs
index 717add9ec48..1fdabab4598 100644
--- a/library/std/src/sys/unix/path.rs
+++ b/library/std/src/sys/unix/path.rs
@@ -1,5 +1,8 @@
+use crate::env;
 use crate::ffi::OsStr;
-use crate::path::Prefix;
+use crate::io;
+use crate::os::unix::ffi::OsStrExt;
+use crate::path::{Path, PathBuf, Prefix};
 
 #[inline]
 pub fn is_sep_byte(b: u8) -> bool {
@@ -18,3 +21,43 @@ pub fn parse_prefix(_: &OsStr) -> Option<Prefix<'_>> {
 
 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> {
+    // This is mostly a wrapper around collecting `Path::components`, with
+    // exceptions made where this conflicts with the POSIX specification.
+    // See 4.13 Pathname Resolution, IEEE Std 1003.1-2017
+    // https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_13
+
+    let mut components = path.components();
+    let path_os = path.as_os_str().as_bytes();
+
+    let mut normalized = if path.is_absolute() {
+        // "If a pathname begins with two successive <slash> characters, the
+        // first component following the leading <slash> characters may be
+        // interpreted in an implementation-defined manner, although more than
+        // two leading <slash> characters shall be treated as a single <slash>
+        // character."
+        if path_os.starts_with(b"//") && !path_os.starts_with(b"///") {
+            components.next();
+            PathBuf::from("//")
+        } else {
+            PathBuf::new()
+        }
+    } else {
+        env::current_dir()?
+    };
+    normalized.extend(components);
+
+    // "Interfaces using pathname resolution may specify additional constraints
+    // when a pathname that does not name an existing directory contains at
+    // least one non- <slash> character and contains one or more trailing
+    // <slash> characters".
+    // A trailing <slash> is also meaningful if "a symbolic link is
+    // encountered during pathname resolution".
+    if path_os.ends_with(b"/") {
+        normalized.push("");
+    }
+
+    Ok(normalized)
+}