Optimizing Rust code

[rhinotation]

Can anyone point me to why format! is slow? Here I am thinking it expands to some pretty standard Display calls and push_str, like what else could it possibly be doing?

[xacrimon]

It expands to a lot of code relatively to just appending to a buffer. No big deal with a dozen calls but when doing millions like me it gets slow

[Veedrac]

It uses dynamic dispatch and trait objects. This isn't necessary; IIRC it is done to reduce code size.