Auto merge of #31386 - tbu-:pr_cow_from_vec, r=alexcrichton

Fixes #31354.

src/libcollections/vec.rs

@@ -1505,6 +1505,20 @@ impl<'a> From<&'a str> for Vec<u8> {
 // Clone-on-write
 ////////////////////////////////////////////////////////////////////////////////
 
+#[stable(feature = "cow_from_vec", since = "1.7.0")]
+impl<'a, T: Clone> From<&'a [T]> for Cow<'a, [T]> {
+    fn from(s: &'a [T]) -> Cow<'a, [T]> {
+        Cow::Borrowed(s)
+    }
+}
+
+#[stable(feature = "cow_from_vec", since = "1.7.0")]
+impl<'a, T: Clone> From<Vec<T>> for Cow<'a, [T]> {
+    fn from(v: Vec<T>) -> Cow<'a, [T]> {
+        Cow::Owned(v)
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, T> FromIterator<T> for Cow<'a, [T]> where T: Clone {
     fn from_iter<I: IntoIterator<Item = T>>(it: I) -> Cow<'a, [T]> {

src/libcollectionstest/str.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use std::borrow::Cow;
 use std::cmp::Ordering::{Equal, Greater, Less};
 use std::str::from_utf8;
 
@@ -1267,6 +1268,16 @@ fn test_box_slice_clone() {
     assert_eq!(data, data2);
 }
 
+#[test]
+fn test_cow_from() {
+    let borrowed = "borrowed";
+    let owned = String::from("owned");
+    match (Cow::from(owned.clone()), Cow::from(borrowed)) {
+        (Cow::Owned(o), Cow::Borrowed(b)) => assert!(o == owned && b == borrowed),
+        _ => panic!("invalid `Cow::from`"),
+    }
+}
+
 mod pattern {
     use std::str::pattern::Pattern;
     use std::str::pattern::{Searcher, ReverseSearcher};

src/libcollectionstest/vec.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use std::borrow::Cow;
 use std::iter::{FromIterator, repeat};
 use std::mem::size_of;
 
@@ -466,6 +467,16 @@ fn test_into_iter_count() {
     assert_eq!(vec![1, 2, 3].into_iter().count(), 3);
 }
 
+#[test]
+fn test_cow_from() {
+    let borrowed: &[_] = &["borrowed", "(slice)"];
+    let owned = vec!["owned", "(vec)"];
+    match (Cow::from(owned.clone()), Cow::from(borrowed)) {
+        (Cow::Owned(o), Cow::Borrowed(b)) => assert!(o == owned && b == borrowed),
+        _ => panic!("invalid `Cow::from`"),
+    }
+}
+
 #[bench]
 fn bench_new(b: &mut Bencher) {
     b.iter(|| {