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.

5 files changed, 103 insertions, 0 deletions

diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index b3a771a721d..8ef6090c743 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -146,6 +146,7 @@ use core::ptr::{self, NonNull, Unique};
 use core::task::{Context, Poll};
 
 use crate::alloc::{self, AllocInit, AllocRef, Global};
+use crate::borrow::Cow;
 use crate::raw_vec::RawVec;
 use crate::str::from_boxed_utf8_unchecked;
 use crate::vec::Vec;
@@ -800,6 +801,17 @@ impl<T: Copy> From<&[T]> for Box<[T]> {
     }
 }
 
+#[stable(feature = "box_from_cow", since = "1.45.0")]
+impl<T: Copy> From<Cow<'_, [T]>> for Box<[T]> {
+    #[inline]
+    fn from(cow: Cow<'_, [T]>) -> Box<[T]> {
+        match cow {
+            Cow::Borrowed(slice) => Box::from(slice),
+            Cow::Owned(slice) => Box::from(slice),
+        }
+    }
+}
+
 #[stable(feature = "box_from_slice", since = "1.17.0")]
 impl From<&str> for Box<str> {
     /// Converts a `&str` into a `Box<str>`
@@ -818,6 +830,17 @@ impl From<&str> for Box<str> {
     }
 }
 
+#[stable(feature = "box_from_cow", since = "1.45.0")]
+impl From<Cow<'_, str>> for Box<str> {
+    #[inline]
+    fn from(cow: Cow<'_, str>) -> Box<str> {
+        match cow {
+            Cow::Borrowed(s) => Box::from(s),
+            Cow::Owned(s) => Box::from(s),
+        }
+    }
+}
+
 #[stable(feature = "boxed_str_conv", since = "1.19.0")]
 impl From<Box<str>> for Box<[u8]> {
     /// Converts a `Box<str>>` into a `Box<[u8]>`
diff --git a/src/liballoc/rc.rs b/src/liballoc/rc.rs
index 76213e65bab..ad96a138d7e 100644
--- a/src/liballoc/rc.rs
+++ b/src/liballoc/rc.rs
@@ -252,6 +252,7 @@ use core::ptr::{self, NonNull};
 use core::slice::from_raw_parts_mut;
 
 use crate::alloc::{box_free, handle_alloc_error, AllocInit, AllocRef, Global, Layout};
+use crate::borrow::{Cow, ToOwned};
 use crate::string::String;
 use crate::vec::Vec;
 
@@ -1497,6 +1498,21 @@ impl<T> From<Vec<T>> for Rc<[T]> {
     }
 }
 
+#[stable(feature = "shared_from_cow", since = "1.45.0")]
+impl<'a, B> From<Cow<'a, B>> for Rc<B>
+where
+    B: ToOwned + ?Sized,
+    Rc<B>: From<&'a B> + From<B::Owned>,
+{
+    #[inline]
+    fn from(cow: Cow<'a, B>) -> Rc<B> {
+        match cow {
+            Cow::Borrowed(s) => Rc::from(s),
+            Cow::Owned(s) => Rc::from(s),
+        }
+    }
+}
+
 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
 impl<T, const N: usize> TryFrom<Rc<[T]>> for Rc<[T; N]>
 where
diff --git a/src/liballoc/sync.rs b/src/liballoc/sync.rs
index 2c6d130826b..b7be8042ea4 100644
--- a/src/liballoc/sync.rs
+++ b/src/liballoc/sync.rs
@@ -25,6 +25,7 @@ use core::sync::atomic;
 use core::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst};
 
 use crate::alloc::{box_free, handle_alloc_error, AllocInit, AllocRef, Global, Layout};
+use crate::borrow::{Cow, ToOwned};
 use crate::boxed::Box;
 use crate::rc::is_dangling;
 use crate::string::String;
@@ -2128,6 +2129,21 @@ impl<T> From<Vec<T>> for Arc<[T]> {
     }
 }
 
+#[stable(feature = "shared_from_cow", since = "1.45.0")]
+impl<'a, B> From<Cow<'a, B>> for Arc<B>
+where
+    B: ToOwned + ?Sized,
+    Arc<B>: From<&'a B> + From<B::Owned>,
+{
+    #[inline]
+    fn from(cow: Cow<'a, B>) -> Arc<B> {
+        match cow {
+            Cow::Borrowed(s) => Arc::from(s),
+            Cow::Owned(s) => Arc::from(s),
+        }
+    }
+}
+
 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
 impl<T, const N: usize> TryFrom<Arc<[T]>> for Arc<[T; N]>
 where
diff --git a/src/liballoc/tests/borrow.rs b/src/liballoc/tests/borrow.rs
new file mode 100644
index 00000000000..8bfcf323f67
--- /dev/null
+++ b/src/liballoc/tests/borrow.rs
@@ -0,0 +1,47 @@
+use std::borrow::{Cow, ToOwned};
+use std::ffi::{CStr, OsStr};
+use std::path::Path;
+use std::rc::Rc;
+use std::sync::Arc;
+
+macro_rules! test_from_cow {
+    ($value:ident => $($ty:ty),+) => {$(
+        let borrowed = <$ty>::from(Cow::Borrowed($value));
+        let owned = <$ty>::from(Cow::Owned($value.to_owned()));
+        assert_eq!($value, &*borrowed);
+        assert_eq!($value, &*owned);
+    )+};
+    ($value:ident : & $ty:ty) => {
+        test_from_cow!($value => Box<$ty>, Rc<$ty>, Arc<$ty>);
+    }
+}
+
+#[test]
+fn test_from_cow_slice() {
+    let slice: &[i32] = &[1, 2, 3];
+    test_from_cow!(slice: &[i32]);
+}
+
+#[test]
+fn test_from_cow_str() {
+    let string = "hello";
+    test_from_cow!(string: &str);
+}
+
+#[test]
+fn test_from_cow_c_str() {
+    let string = CStr::from_bytes_with_nul(b"hello\0").unwrap();
+    test_from_cow!(string: &CStr);
+}
+
+#[test]
+fn test_from_cow_os_str() {
+    let string = OsStr::new("hello");
+    test_from_cow!(string: &OsStr);
+}
+
+#[test]
+fn test_from_cow_path() {
+    let path = Path::new("hello");
+    test_from_cow!(path: &Path);
+}
diff --git a/src/liballoc/tests/lib.rs b/src/liballoc/tests/lib.rs
index 78d49558262..f3da46bd0cc 100644
--- a/src/liballoc/tests/lib.rs
+++ b/src/liballoc/tests/lib.rs
@@ -21,6 +21,7 @@ use std::hash::{Hash, Hasher};
 
 mod arc;
 mod binary_heap;
+mod borrow;
 mod boxed;
 mod btree;
 mod cow_str;