impl From<Cow> for boxed slices and strings

These forward `Borrowed`/`Owned` values to existing `Box::from` impls.

- `From<Cow<'_, [T]>> for Box<[T]>`
- `From<Cow<'_, str>> for Box<str>`
- `From<Cow<'_, CStr>> for Box<CStr>`
- `From<Cow<'_, OsStr>> for Box<OsStr>`
- `From<Cow<'_, Path>> for Box<Path>`

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;
@@ -774,6 +775,17 @@ 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>`
@@ -792,6 +804,17 @@ 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]>`

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.

src/libstd/ffi/os_str.rs

@@ -849,6 +849,17 @@ 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

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`