alloc: implement FromIterator for Box<str>

Box<[T]> implements FromIterator<T> using Vec<T> + into_boxed_slice().
Add analogous FromIterator implementations for Box<str>
matching the current implementations for String.
Remove the Global allocator requirement for FromIterator<Box<str>> too.

library/alloc/src/boxed.rs

@@ -2080,6 +2080,54 @@ impl<I> FromIterator<I> for Box<[I]> {
     }
 }
 
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "boxed_str_from_iter", since = "CURRENT_RUSTC_VERSION")]
+impl FromIterator<char> for Box<str> {
+    fn from_iter<T: IntoIterator<Item = char>>(iter: T) -> Self {
+        String::from_iter(iter).into_boxed_str()
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "boxed_str_from_iter", since = "CURRENT_RUSTC_VERSION")]
+impl<'a> FromIterator<&'a char> for Box<str> {
+    fn from_iter<T: IntoIterator<Item = &'a char>>(iter: T) -> Self {
+        String::from_iter(iter).into_boxed_str()
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "boxed_str_from_iter", since = "CURRENT_RUSTC_VERSION")]
+impl<'a> FromIterator<&'a str> for Box<str> {
+    fn from_iter<T: IntoIterator<Item = &'a str>>(iter: T) -> Self {
+        String::from_iter(iter).into_boxed_str()
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "boxed_str_from_iter", since = "CURRENT_RUSTC_VERSION")]
+impl FromIterator<String> for Box<str> {
+    fn from_iter<T: IntoIterator<Item = String>>(iter: T) -> Self {
+        String::from_iter(iter).into_boxed_str()
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "boxed_str_from_iter", since = "CURRENT_RUSTC_VERSION")]
+impl<A: Allocator> FromIterator<Box<str, A>> for Box<str> {
+    fn from_iter<T: IntoIterator<Item = Box<str, A>>>(iter: T) -> Self {
+        String::from_iter(iter).into_boxed_str()
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "boxed_str_from_iter", since = "CURRENT_RUSTC_VERSION")]
+impl<'a> FromIterator<Cow<'a, str>> for Box<str> {
+    fn from_iter<T: IntoIterator<Item = Cow<'a, str>>>(iter: T) -> Self {
+        String::from_iter(iter).into_boxed_str()
+    }
+}
+
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "box_slice_clone", since = "1.3.0")]
 impl<T: Clone, A: Allocator + Clone> Clone for Box<[T], A> {

library/alloc/src/string.rs

@@ -59,6 +59,8 @@ use core::ptr;
 use core::slice;
 use core::str::pattern::Pattern;
 
+#[cfg(not(no_global_oom_handling))]
+use crate::alloc::Allocator;
 #[cfg(not(no_global_oom_handling))]
 use crate::borrow::{Cow, ToOwned};
 use crate::boxed::Box;
@@ -2155,8 +2157,8 @@ impl FromIterator<String> for String {
 
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "box_str2", since = "1.45.0")]
-impl FromIterator<Box<str>> for String {
-    fn from_iter<I: IntoIterator<Item = Box<str>>>(iter: I) -> String {
+impl<A: Allocator> FromIterator<Box<str, A>> for String {
+    fn from_iter<I: IntoIterator<Item = Box<str, A>>>(iter: I) -> String {
         let mut buf = String::new();
         buf.extend(iter);
         buf
@@ -2237,8 +2239,8 @@ impl<'a> Extend<&'a str> for String {
 
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "box_str2", since = "1.45.0")]
-impl Extend<Box<str>> for String {
-    fn extend<I: IntoIterator<Item = Box<str>>>(&mut self, iter: I) {
+impl<A: Allocator> Extend<Box<str, A>> for String {
+    fn extend<I: IntoIterator<Item = Box<str, A>>>(&mut self, iter: I) {
         iter.into_iter().for_each(move |s| self.push_str(&s));
     }
 }