Add FromIterator implementations for Box<str>

These are analogous to impl FromIterator<A> for Box<[A]> (rust-lang#55843).

Fixes rust-lang#65163.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

src/liballoc/boxed.rs

@@ -147,8 +147,10 @@ use core::slice;
 use core::task::{Context, Poll};
 
 use crate::alloc::{self, AllocRef, Global};
+use crate::borrow::Cow;
 use crate::raw_vec::RawVec;
 use crate::str::from_boxed_utf8_unchecked;
+use crate::string::String;
 use crate::vec::Vec;
 
 /// A pointer type for heap allocation.
@@ -1045,6 +1047,41 @@ impl<A> FromIterator<A> for Box<[A]> {
     }
 }
 
+#[stable(feature = "boxed_str_from_iter", since = "1.44.0")]
+impl<'a> FromIterator<&'a char> for Box<str> {
+    fn from_iter<T: IntoIterator<Item = &'a char>>(iter: T) -> Self {
+        iter.into_iter().collect::<String>().into_boxed_str()
+    }
+}
+
+#[stable(feature = "boxed_str_from_iter", since = "1.44.0")]
+impl<'a> FromIterator<&'a str> for Box<str> {
+    fn from_iter<T: IntoIterator<Item = &'a str>>(iter: T) -> Self {
+        iter.into_iter().collect::<String>().into_boxed_str()
+    }
+}
+
+#[stable(feature = "boxed_str_from_iter", since = "1.44.0")]
+impl<'a> FromIterator<Cow<'a, str>> for Box<str> {
+    fn from_iter<T: IntoIterator<Item = Cow<'a, str>>>(iter: T) -> Self {
+        iter.into_iter().collect::<String>().into_boxed_str()
+    }
+}
+
+#[stable(feature = "boxed_str_from_iter", since = "1.44.0")]
+impl FromIterator<String> for Box<str> {
+    fn from_iter<T: IntoIterator<Item = String>>(iter: T) -> Self {
+        iter.into_iter().collect::<String>().into_boxed_str()
+    }
+}
+
+#[stable(feature = "boxed_str_from_iter", since = "1.44.0")]
+impl FromIterator<char> for Box<str> {
+    fn from_iter<T: IntoIterator<Item = char>>(iter: T) -> Self {
+        iter.into_iter().collect::<String>().into_boxed_str()
+    }
+}
+
 #[stable(feature = "box_slice_clone", since = "1.3.0")]
 impl<T: Clone> Clone for Box<[T]> {
     fn clone(&self) -> Self {

src/liballoc/tests.rs

@@ -8,7 +8,9 @@ use core::i64;
 use core::ops::Deref;
 use core::result::Result::{Err, Ok};
 
+use std::borrow::Cow;
 use std::boxed::Box;
+use std::string::String;
 
 #[test]
 fn test_owned_clone() {
@@ -140,6 +142,22 @@ fn boxed_slice_from_iter() {
     assert_eq!(boxed[7], 7);
 }
 
+#[test]
+fn boxed_str_from_iter() {
+    let s = "Hello, world!";
+    let chars: Box<[char]> = s.chars().collect();
+    let boxed: Box<str> = chars.iter().collect();
+    assert_eq!(&*boxed, s);
+    let boxed: Box<str> = [s].iter().cloned().collect();
+    assert_eq!(&*boxed, s);
+    let boxed: Box<str> = [Cow::from(s)].iter().cloned().collect();
+    assert_eq!(&*boxed, s);
+    let boxed: Box<str> = [String::from(s)].iter().cloned().collect();
+    assert_eq!(&*boxed, s);
+    let boxed: Box<str> = s.chars().collect();
+    assert_eq!(&*boxed, s);
+}
+
 #[test]
 fn test_array_from_slice() {
     let v = vec![1, 2, 3];