shadow-core: drop the single-use strerror wrapper

Per review on #171, inline the one caller: permission_denied() now calls
uucore::error::strip_errno directly. Also disambiguate the module doc now
that the local strerror fn is gone (it refers to libc's strerror).

src/shadow-core/src/os_error.rs

@@ -5,30 +5,23 @@
 
 //! Error text sourced from the operating system instead of hardcoded.
 //!
-//! Wording for conditions that map to a libc `errno` is taken from
-//! `strerror` (via [`std::io::Error`]) rather than carried as a string
-//! literal in our tree. This keeps the text matching the host OS and lets
-//! glibc translate it on localized systems — the same way GNU coreutils
-//! renders system errors (e.g. `cat: /tmp: Is a directory`). See issue #159.
-
-/// The operating system's message for a raw `errno`.
-///
-/// For example `EACCES` renders as "Permission denied" on English locales
-/// and the translated equivalent elsewhere. On targets whose libc does not
-/// translate (musl), this is the untranslated English text.
-#[must_use]
-pub fn strerror(errno: i32) -> String {
-    // `io::Error::from_raw_os_error` carries libc's `strerror` text, but its
-    // `Display` appends Rust's own " (os error N)" suffix. `strip_errno` (the
-    // same helper uucore uses for its own I/O errors) removes it, leaving the
-    // bare OS message — matching GNU/coreutils output.
-    uucore::error::strip_errno(&std::io::Error::from_raw_os_error(errno))
-}
+//! Wording for conditions that map to a libc `errno` is taken from the OS
+//! (libc's `strerror`, surfaced through [`std::io::Error`]) rather than
+//! carried as a string literal in our tree. This keeps the text matching the
+//! host OS and lets glibc translate it on localized systems — the same way
+//! GNU coreutils renders system errors (e.g. `cat: /tmp: Is a directory`).
+//! See issue #159.
 
 /// The OS message for `EACCES` ("Permission denied"), sourced from libc.
+///
+/// Rendered as "Permission denied" on English locales and the translated
+/// equivalent elsewhere; on a libc that does not translate (musl) it is the
+/// untranslated English text. `strip_errno` (the helper uucore uses for its
+/// own I/O errors) drops the " (os error N)" suffix that `io::Error`'s
+/// `Display` appends, leaving the bare OS message — matching coreutils output.
 #[must_use]
 pub fn permission_denied() -> String {
-    strerror(libc::EACCES)
+    uucore::error::strip_errno(&std::io::Error::from_raw_os_error(libc::EACCES))
 }
 
 #[cfg(test)]
@@ -36,22 +29,12 @@ mod tests {
     use super::*;
 
     #[test]
-    fn permission_denied_is_nonempty_and_os_sourced() {
-        // We assert the shape, not the exact text: the wording comes from the
-        // host libc and may be localized, so hardcoding it would defeat the
-        // purpose of this module.
+    fn permission_denied_is_bare_os_message() {
         let msg = permission_denied();
+        // Non-empty, and the bare OS text — not Rust's "... (os error 13)"
+        // rendering (the regression that suffix-stripping prevents). We assert
+        // the shape, not the exact wording, since libc may localize it.
         assert!(!msg.is_empty());
-        // Same value libc would give for EACCES directly.
-        assert_eq!(msg, strerror(libc::EACCES));
-    }
-
-    #[test]
-    fn strerror_drops_the_rust_os_error_suffix() {
-        // Regression guard: the bare OS message must not carry Rust's
-        // " (os error N)" rendering (the bug that made permission_denied()
-        // return "Permission denied (os error 13)").
-        let msg = strerror(libc::EACCES);
         assert!(!msg.contains("(os error"), "got: {msg:?}");
     }
 }