There was an interesting talk on type system tips at RustConf 2017. Sean Griffin at one point walks through some macro expansions and compile-time optimizations that are performed for a typical Diesel query generator.
He ends by talking about the last step which the compiler couldn’t do (at least at the time of the talk):
a.push_str("h") a.push_str("e") a.push_str("y")
is equivalent to:
but was not an available transformation.
Still the video is really fun to watch.