Rollup merge of rust-lang#141445 - yotamofek:pr/library/from-iter-char-string, r=the8472,joshtriplett

Add `FromIterator` impls for `ascii::Char`s to `String`s

Wanted to `collect` ascii chars into a `String` while working on rust-lang#141369 , and was surprised these impls don't exist. Seems to me to be simply oversight.

BTW, I only added `impl FromIterator<ascii::Char> for Cow<'_, str>`, without a corresponding `FromIterator<&Char>` impl, because there's no existing impl for `FromIterator<&char>`, but that might be oversight too.

cc rust-lang#110998

Author: matthiaskrgr

alloc/src/string.rs

@@ -2378,6 +2378,28 @@ impl<'a> FromIterator<Cow<'a, str>> for String {
     }
 }
 
+#[cfg(not(no_global_oom_handling))]
+#[unstable(feature = "ascii_char", issue = "110998")]
+impl FromIterator<core::ascii::Char> for String {
+    fn from_iter<T: IntoIterator<Item = core::ascii::Char>>(iter: T) -> Self {
+        let buf = iter.into_iter().map(core::ascii::Char::to_u8).collect();
+        // SAFETY: `buf` is guaranteed to be valid UTF-8 because the `core::ascii::Char` type
+        // only contains ASCII values (0x00-0x7F), which are valid UTF-8.
+        unsafe { String::from_utf8_unchecked(buf) }
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[unstable(feature = "ascii_char", issue = "110998")]
+impl<'a> FromIterator<&'a core::ascii::Char> for String {
+    fn from_iter<T: IntoIterator<Item = &'a core::ascii::Char>>(iter: T) -> Self {
+        let buf = iter.into_iter().copied().map(core::ascii::Char::to_u8).collect();
+        // SAFETY: `buf` is guaranteed to be valid UTF-8 because the `core::ascii::Char` type
+        // only contains ASCII values (0x00-0x7F), which are valid UTF-8.
+        unsafe { String::from_utf8_unchecked(buf) }
+    }
+}
+
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Extend<char> for String {
@@ -3184,6 +3206,14 @@ impl<'a> FromIterator<String> for Cow<'a, str> {
     }
 }
 
+#[cfg(not(no_global_oom_handling))]
+#[unstable(feature = "ascii_char", issue = "110998")]
+impl<'a> FromIterator<core::ascii::Char> for Cow<'a, str> {
+    fn from_iter<T: IntoIterator<Item = core::ascii::Char>>(it: T) -> Self {
+        Cow::Owned(FromIterator::from_iter(it))
+    }
+}
+
 #[stable(feature = "from_string_for_vec_u8", since = "1.14.0")]
 impl From<String> for Vec<u8> {
     /// Converts the given [`String`] to a vector [`Vec`] that holds values of type [`u8`].