Auto merge of rust-lang#96881 - est31:join_osstr, r=dtolnay

Implement [OsStr]::join

Implements join for `OsStr` and `OsString` slices:

```Rust
    let strings = [OsStr::new("hello"), OsStr::new("dear"), OsStr::new("world")];
    assert_eq!("hello dear world", strings.join(OsStr::new(" ")));
````

This saves one from converting to strings and back, or from implementing it manually.

This PR has been re-filed after rust-lang#96744 was first accidentally merged and then reverted.

The change is instantly stable and thus:

r? rust-lang/libs-api `@rustbot` label +T-libs-api -T-libs

cc `@thomcc` `@m-ou-se` `@faptc`

library/std/src/ffi/os_str.rs

@@ -1222,6 +1222,23 @@ impl OsStr {
     }
 }
 
+#[unstable(feature = "slice_concat_ext", issue = "27747")]
+impl<S: Borrow<OsStr>> alloc::slice::Join<&OsStr> for [S] {
+    type Output = OsString;
+
+    fn join(slice: &Self, sep: &OsStr) -> OsString {
+        let Some((first, suffix)) = slice.split_first() else {
+            return OsString::new();
+        };
+        let first_owned = first.borrow().to_owned();
+        suffix.iter().fold(first_owned, |mut a, b| {
+            a.push(sep);
+            a.push(b.borrow());
+            a
+        })
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Borrow<OsStr> for OsString {
     #[inline]

library/std/src/ffi/os_str/tests.rs

@@ -84,6 +84,20 @@ fn test_os_string_reserve_exact() {
     assert!(os_string.capacity() >= 33)
 }
 
+#[test]
+fn test_os_string_join() {
+    let strings = [OsStr::new("hello"), OsStr::new("dear"), OsStr::new("world")];
+    assert_eq!("hello", strings[..1].join(OsStr::new(" ")));
+    assert_eq!("hello dear world", strings.join(OsStr::new(" ")));
+    assert_eq!("hellodearworld", strings.join(OsStr::new("")));
+    assert_eq!("hello.\n dear.\n world", strings.join(OsStr::new(".\n ")));
+
+    assert_eq!("dear world", strings[1..].join(&OsString::from(" ")));
+
+    let strings_abc = [OsString::from("a"), OsString::from("b"), OsString::from("c")];
+    assert_eq!("a b c", strings_abc.join(OsStr::new(" ")));
+}
+
 #[test]
 fn test_os_string_default() {
     let os_string: OsString = Default::default();

library/std/src/lib.rs

@@ -241,6 +241,7 @@
 #![feature(intra_doc_pointers)]
 #![feature(lang_items)]
 #![feature(let_chains)]
+#![feature(let_else)]
 #![feature(linkage)]
 #![feature(min_specialization)]
 #![feature(must_not_suspend)]
@@ -301,6 +302,7 @@
 #![feature(toowned_clone_into)]
 #![feature(try_reserve_kind)]
 #![feature(vec_into_raw_parts)]
+#![feature(slice_concat_trait)]
 //
 // Library features (unwind):
 #![feature(panic_unwind)]