# Roadmap 2020 Compiler Discussion

# 2019 roadmap (for reference)

The compiler team has five main themes of work for this year:

- **Improving "core strength"** by lowering raw compilation times and also generating better code (which in turn can help with compilation times)
- **Improved IDE integration**, including support for typed completions
- Extracting parts of rustc into **libraries**, which are easier to understand and maintain but also help in developing a Rust specification
- Supporting lang team in **key feature development**, such as around `async fn` support, or improvements to the trait system
- Improving our **documentation** and **organization**, helping to make it clearer what is going on at any given time, and how people can get involved.

# Big challenges for rustc and compiler team

[Poll to vote](https://poll.ly/#/P76nbdyb) -- feel free to add your own items to the poll; you can also add notes below with a few lines of elaboration.

* compilation times remain stubbornly high
* many companies have huge codebases and compilation time can be an existential crisis
* we feel this ourselves with bootstrapping
* band-aids like "factor librustc into finer grain crates" might work; cannot be "official answer"
* incremental has helped but it's not like a 1-line change is 'instantaneous'
* this bullet is talking about the time it takes to "run tests" and do full builds; it does NOT refer to the "instantaneous feedback" for things like completion, which falls under IDE
* tech debt around the trait system
* blocking a lot of lang team progress (async fn in traits, GATs, specialization)
* "lazy normalization"
* also maybe blocking const generics work
* also support for higher-ranked lifetimes etc remains a common place where you can hit ICEs that have no real fix
* possibly performance impact
* maintenance and triage work
* we do a decent job of keeping up with work but work burden is unevenly distributed
* non-urgent / non-regression bugs can "pile up" -- are we making progress on them?
* (note absence of data here; maybe we *are* keeping up, relative to bug-report rate and/or bugs that actually matter...)
* (simulacrum): we are not really keeping up
* long-term trend is up; most current issues are fairly recently triaged,
count has been going up: ~3000 last summer, up to 4900 now. Not all compiler team, but "most".
* IDE story is confusing, we're not focusing our efforts
* we need a strong IDE story
* new features draw in new volunteers more easily than maintenance/paying-down-debt
* can we make such work more exciting? E.g. via gamification?
* documentation and design
* many parts of the compiler remain undocumented
* writing mentoring instructions often requires recounting same details over and over
* we have a lot of great design ideas but there are often ill-documented
* ambitious rewrites can be hard to review without knowing 'the plan'
* also, often it's not clear how much consensus there is *on* the plan
* paying for labor
* we rely on volunteers for vast majority of coding effort
* some of this is coding done while "on the clock" for some boss, with that boss's blessing
* (there are also contract laborers, financed by companies with specific needs)
* is this model of compensation going to suffice going forward?
* team growth
* Find more people to help out
* Improve the onboarding process
* Mentoring/help

# Discussion summaries

## compilation times

* benchmark vs end-user programs
* maybe set a strict goal of improving bootstrap times ([link](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/design.20meeting.202019-10-04/near/177342591))
* rough conclusions
* bootstrap is worth focusing on
* codegen time is a big deal
* [measurements suggest](https://mark.rousskov.org/parallel-compiler-data/master/rustc-compiler-opt-timing.html) librustc is long pole in the tent, also
* incremental could be improved, e.g. measure/improve the "edit comment" use case
* tracked by rust-lang/rust#47389
* eddyb writes:
* don't do any work past the HIR when only a comment changed (targeted Span improvements may help)
* move incremental back so you don't need to spend entire seconds getting to HIR

## trait system

* made progress this year but not as much as hoped
* [conclusion](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/design.20meeting.202019-10-04/near/177344654):
* we need most of all a clear roadmap here, and that should be an immediate focus
* good to try and separate out lazy normalization from other goals

##

# See also

* [2019 roadmap](https://raw.githubusercontent.com/rust-lang/rfcs/master/text/2657-roadmap-2019.md)
* [random brainstorming by pnkfelix + nikomatsakis](/iHCbm2sWQ-y2KuWFjEpUog)