Merge pull request #4642 from JackStouffer/utf2

Added fast paths in std.utf.byUTF

Author: andralex

std/utf.d

@@ -3599,8 +3599,27 @@ template byUTF(C) if (isSomeChar!C)
                     if (pos == fill)
                     {
                         pos = 0;
-                        fill = cast(ushort)encode!(UseReplacementDchar.yes)(
-                            buf, decodeFront!(UseReplacementDchar.yes)(r));
+                        auto c = r.front;
+
+                        if (c <= 0x7F)
+                        {
+                            fill = 1;
+                            r.popFront;
+                            buf[pos] = cast(C) c;
+                        }
+                        else
+                        {
+                            static if (is(RC == dchar))
+                            {
+                                fill = cast(ushort) encode!(UseReplacementDchar.yes)(buf, c);
+                                r.popFront;
+                            }
+                            else
+                            {
+                                fill = cast(ushort) encode!(UseReplacementDchar.yes)(
+                                    buf, decodeFront!(UseReplacementDchar.yes)(r));
+                            }
+                        }
                     }
                     return buf[pos];
                 }