Some ideas to speed up compilation by not evaluating items that are not used might pan out significantly for big crates in your dep tree (that behavior might never be stable because that would allow items with compile errors in a crate that would still let your application compile, which is against the Rust approach). The same work to do that would also allow overlapping of crate evaluation between different rustc instances called by cargo, as it would require partial evaluation of crates (to do name res only and gather the symbols needed from its deps).
Another thing is that stable rust doesn't treat macros as idempotent (because that wasn't a requirement from the start, there are crates that do dynamic IO to generate types), but if they are then incr comp can be faster by not evaluating them unnecessarily.
I know people are working on a bunch of different strategies to improve both first and incremental compile times, and I'm looking forward to the fruit of their labor.
Compile times are rarely the bottleneck for me, but that doesn't mean I won't welcome any improvements on that front.
Is that still true with modern linkers like `wild`? Seems like it can link Chromium in 1-2s.
> Another thing is that stable rust doesn't treat macros as idempotent
This seems absolutely insane to me given how pervasive macros are in Rust (I wonder how much of incremental compilation time is just repeated serde derives?). Obviously we can't just blindly treat all macros as idempotent, but an opt-in attribute on a macros that pinky-promises that it is seems like it ought to be pretty easy to implement?
Maybe somebody's looked into it and it doesn't help much? But I've heard (unverified) rumours of the opposite.
Wild improves things significantly, but it really depends on the kind of project. For some a third of the time can be linking. And not everyone configures their environment to use a different linker. Another thing that can easily improve performance is changing the global allocator, but I've seen teams that measured 10% perf improvements in their own metrics decide against going with anything other than the "default".
I fully expect that if wild delivers an effective incremental linking architecture, then rustc will be able to produce the patches directly (instead of wild having to produce patches from two versions of the object files), which would mean both that rustc is producing less LLVM bytecode and that wild gets to do what it (will) do best and make linking as fast as mechanically possible.
> This seems absolutely insane to me given how pervasive macros are in Rust
There was some work done on this front, but I haven't kept up to date on the current status of that. There was a PR showing promise https://github.com/rust-lang/rust/pull/129102 (later landed as https://github.com/rust-lang/rust/pull/145354, 10% on a specific serde-heavy crate, reports of 32% improvements in the original PR). The tracking issue doesn't have any updates https://github.com/rust-lang/rust/issues/151364, but you can try out nightly with -Zcache-proc-macros to see what the effect could be on your projects.
Part of the problem I see is that crates will have to opt-in (and we might be able to change the default over an edition boundary) to get the perf benefit, and it might require a granularity lower than "a crate" (which would complicate implementation). I haven't seen additional discussions around these design considerations (needed to stabilize), which will need to happen before people can see progress.
Sadly, Rust is, like many open source projects, a show-up-o-cracy: you need a motivated (group of?) individual(s) to deliver a feature to fruition, and if the person driving a feature is fine with using nightly for their purposes, and only needs a subset of a feature, they will drive the feature to that state (lets say 80% completion), and then the feature will linger (until someone else with enough motivation to see the other 80% through shows up). This is exacerbated because the project is unwilling to have 80% solutions on stable unless the state is very clearly not going to preclude other future work, or the path to completion is 100% visible (but if that were the case, then it would have been completed already). So we end up with situations like the Allocator APIs.
> Another thing that can easily improve performance is changing the global allocator, but I've seen teams that measured 10% perf improvements in their own metrics decide against going with anything other than the "default".
Yeah, although I think there's more of a trade-off there. Alternative allocators can add significant amounts of compile time. Whereas, modulo maturity, I think a faster linker is more of a pure win. I would imagine we will make wild the default linker at some point if development continues on the trajectory it seems to be on.
https://github.com/rust-lang/rustc_codegen_cranelift/issues/...