Auto merge of #71447 - cuviper:unsized_cow, r=dtolnay

impl From<Cow> for Box, Rc, and Arc

These forward `Borrowed`/`Owned` values to existing `From` impls.

- `Box<T>` is a fundamental type, so it would be a breaking change to add a blanket impl. Therefore, `From<Cow>` is only implemented for `[T]`, `str`, `CStr`, `OsStr`, and `Path`.
- For `Rc<T>` and `Arc<T>`, `From<Cow>` is implemented for everything that implements `From` the borrowed and owned types separately.

src/libstd/path.rs

@@ -1433,6 +1433,17 @@ impl From<&Path> for Box<Path> {
     }
 }
 
+#[stable(feature = "box_from_cow", since = "1.45.0")]
+impl From<Cow<'_, Path>> for Box<Path> {
+    #[inline]
+    fn from(cow: Cow<'_, Path>) -> Box<Path> {
+        match cow {
+            Cow::Borrowed(path) => Box::from(path),
+            Cow::Owned(path) => Box::from(path),
+        }
+    }
+}
+
 #[stable(feature = "path_buf_from_box", since = "1.18.0")]
 impl From<Box<Path>> for PathBuf {
     /// Converts a `Box<Path>` into a `PathBuf`