This is a bad and obvious performance issue.
```d
struct S
{
string toString() { return "S"; }
}
void main()
{
import std.conv;
assert(S.init.toString.ptr != S.init.to!string.ptr);
}
```
This assert should not pass -- `to!string` shouldn't do anything other than call `toString` on a struct.
In fact, if `S.toString` *does* allocate, `to!string` is suffering from an *extra allocation* -- one for the call to `S.toString`, and one for the allocated string that `to!string` always builds.
The underlying cause is that the default case for `toImpl!(string, T)` is to create an appender and then use format. There should be a case for when `T.toString` is defined.
See https://github.com/dlang/phobos/blob/f7e523bc3c69506c3e7b4da7e78af93ed8547910/std/conv.d#L1107 and https://github.com/dlang/phobos/blob/f7e523bc3c69506c3e7b4da7e78af93ed8547910/std/conv.d#L136
Comment #1 by robert.schadek — 2024-12-01T16:42:57Z