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.

3 files changed, 33 insertions, 0 deletions

diff --git a/src/libstd/ffi/c_str.rs b/src/libstd/ffi/c_str.rs
index 0a4802fb2c8..4bac9a4917d 100644
--- a/src/libstd/ffi/c_str.rs
+++ b/src/libstd/ffi/c_str.rs
@@ -730,6 +730,17 @@ impl From<&CStr> for Box<CStr> {
     }
 }
 
+#[stable(feature = "box_from_cow", since = "1.45.0")]
+impl From<Cow<'_, CStr>> for Box<CStr> {
+    #[inline]
+    fn from(cow: Cow<'_, CStr>) -> Box<CStr> {
+        match cow {
+            Cow::Borrowed(s) => Box::from(s),
+            Cow::Owned(s) => Box::from(s),
+        }
+    }
+}
+
 #[stable(feature = "c_string_from_box", since = "1.18.0")]
 impl From<Box<CStr>> for CString {
     /// Converts a [`Box`]`<CStr>` into a [`CString`] without copying or allocating.
diff --git a/src/libstd/ffi/os_str.rs b/src/libstd/ffi/os_str.rs
index 7a05aaf71f2..d1eaf3c583f 100644
--- a/src/libstd/ffi/os_str.rs
+++ b/src/libstd/ffi/os_str.rs
@@ -850,6 +850,17 @@ impl From<&OsStr> for Box<OsStr> {
     }
 }
 
+#[stable(feature = "box_from_cow", since = "1.45.0")]
+impl From<Cow<'_, OsStr>> for Box<OsStr> {
+    #[inline]
+    fn from(cow: Cow<'_, OsStr>) -> Box<OsStr> {
+        match cow {
+            Cow::Borrowed(s) => Box::from(s),
+            Cow::Owned(s) => Box::from(s),
+        }
+    }
+}
+
 #[stable(feature = "os_string_from_box", since = "1.18.0")]
 impl From<Box<OsStr>> for OsString {
     /// Converts a [`Box`]`<`[`OsStr`]`>` into a `OsString` without copying or
diff --git a/src/libstd/path.rs b/src/libstd/path.rs
index 8516e80f3b8..0fe5451bb95 100644
--- a/src/libstd/path.rs
+++ b/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`