Compiling rust-uinput takes almost 20 minutes

[Theemuts]

Hi,

A few minutes ago I opened an issue on the rust-uinput repo, but @meh advised me to open one here. If I compile that library, it takes a significant amount of time which I haven't experienced while compiling before, potentially due to custom_derive. My computer has an i5 2550K, 8GB of ram, is running Ubuntu 16.04 and rustc 1.12.

[jonas-schievink]

It indeed gets stuck inside expansion. Can also reproduce on nightly. I'm on 64-bit Arch Linux with 24GB RAM.

cc @jseyfried and @nrc probably

[durka]

It's probably because of enums (with macros running over them) like this (102 variants) and this (232 variants).

[nrc]

Would be good to narrow this down. It could be the custom derive for a big enum, or it could be the input to the custom_derive macro itself. I'd recommend using macros 1.1 custom derive in any case. But we should try and find out why this takes so long to compile.

cc @rust-lang/compiler

[durka]

Note that custom_derive! (and IterVariants! which it calls here) are token munchers. Is there any way to profile expansion except trace_macros?

[Mark-Simulacrum]

@Theemuts Can you test with a recent nightly and see if the performance improvements were as-expected? We can close this then, since I don't know if there's anything overly helpful to be gained from keeping this open (other than a "expansion can be slow" issue?).

[Mark-Simulacrum]

So there's certainly more room for improvement here, as always, but I think we made some fairly large advances with #37701 and #37569, so I'm going to close. If @Theemuts still sees extremely long compile times then we can reopen, but I think they should be down to something at least a little more manageable.