This is more nuanced, as there are three separate components: * time to compile the syn itself * time to run syn on the inputs * time to compile whatever

This is more nuanced, as there are three separate components: * time to compile the syn itself * time to run syn on the inputs * time to compile whatever syn generated I didn’t do a super thorough studies of things, but my impression is that 2, performance of syn itself, is rarely an issue. Most of the time it is 1) (and the associated problem of decreased build parallelism because half of the crates wait for syn to compile) and 3). To get a feeling how costly a simple proc macro is, run this benchmark: https://github.com/matklad/xshell/blob/4e5090e9f79baeed1037bc0925a6a84174566eb1/tests/it/main.rs#L441 https://github.com/matklad/xshell/blob/4e5090e9f79baeed1037b....