Rollup merge of rust-lang#55530 - ljedrz:speed_up_String_from_utf16, r=SimonSapin

Speed up String::from_utf16

Collecting into a `Result` is idiomatic, but not necessarily fast due to rustc not being able to preallocate for the resulting collection. This is fine in case of an error, but IMO we should optimize for the common case, i.e. a successful conversion.

This changes the behavior of `String::from_utf16` from collecting into a `Result` to pushing to a preallocated `String` in a loop.

According to [my simple benchmark](https://gist.github.com/ljedrz/953a3fb74058806519bd4d640d6f65ae) this change makes `String::from_utf16` around **twice** as fast.

Author: pietroalbini

Diff for: src/liballoc/string.rs

@@ -618,7 +618,15 @@ impl String {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn from_utf16(v: &[u16]) -> Result<String, FromUtf16Error> {
-        decode_utf16(v.iter().cloned()).collect::<Result<_, _>>().map_err(|_| FromUtf16Error(()))
+        let mut ret = String::with_capacity(v.len());
+        for c in decode_utf16(v.iter().cloned()) {
+            if let Ok(c) = c {
+                ret.push(c);
+            } else {
+                return Err(FromUtf16Error(()));
+            }
+        }
+        Ok(ret)
     }
 
     /// Decode a UTF-16 encoded slice `v` into a `String`, replacing