String: add a FIXME to from_utf16

Author: ljedrz

src/liballoc/string.rs

@@ -618,6 +618,8 @@ impl String {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn from_utf16(v: &[u16]) -> Result<String, FromUtf16Error> {
+        // This isn't done via collect::<Result<_, _>>() for performance reasons.
+        // FIXME: the function can be simplified again when #48994 is closed.
         let mut ret = String::with_capacity(v.len());
         for c in decode_utf16(v.iter().cloned()) {
             if let Ok(c) = c {