Fix Bugzilla Issue 24739 - to!string creates unnecessary allocation for structs with toString

Author: deveshidwivedi

std/conv.d

@@ -1013,7 +1013,13 @@ if (!(is(S : T) &&
     !isEnumStrToStr!(S, T) && !isNullToStr!(S, T)) &&
     !isInfinite!S && isExactSomeString!T)
 {
-    static if (isExactSomeString!S && value[0].sizeof == ElementEncodingType!T.sizeof)
+    static if (is(typeof(S.init.toString())) &&
+               is(typeof(S.init.toString()) == string) &&
+               !is(S == class))
+    {
+            return value.toString();
+    }
+    else static if (isExactSomeString!S && value[0].sizeof == ElementEncodingType!T.sizeof)
     {
         // string-to-string with incompatible qualifier conversion
         static if (is(ElementEncodingType!T == immutable))
@@ -1121,6 +1127,19 @@ if (!(is(S : T) &&
         return toStr!T(value);
     }
 }
+// https://issues.dlang.org/show_bug.cgi?id=24739
+@system unittest
+{
+    import std.conv : to;
+    import std.exception : enforce;
+
+    struct S
+    {
+        string toString() { return "S"; }
+    }
+
+    enforce(S.init.toString().ptr == S.init.to!string.ptr);
+}
 
 // https://issues.dlang.org/show_bug.cgi?id=14042
 @system unittest