Bevy should become less of a rust-analyzer stress test

[workingjubilee]

Bevy version

I first observed this working from 1a410ef but I believe it remains relevant today, 3 days later.

What you did

I tried to edit a file in Bevy. Specifically, crates/bevy_reflect/src/lib.rs. I was trying to remove some of the glob imports, so I used rust-analyzer's expand-glob-imports assist.

What went wrong

Honestly I was expecting things to be slow. But I encountered outright pathological behavior from rust-analyzer, as it hung after every "unglob". See rust-lang/rust-analyzer#20173 for more on why, but the details here are that while the glob assist worked fine, it happened to perturb the state of the file enough that rust-analyzer recomputed the diagnostics for the entire file, including every glob import.

This happens to be pretty slow, and I believe it scales with the number of globs, possibly nonlinearly? I don't know what else can trigger it except this assist, but I believe it is not unique to that assist, i.e. other things can cause diagnostics to be recomputed. It probably just depends on how much the file changes.

Additional information

My understanding of the rust-analyzer architectural issues is that, while we can expect this to speed up ~soon, rust-analyzer still handles globs extremely poorly and will have other correctness and performance issues.

I believe rust-analyzer's team will be able to extract a number of very useful benchmarks and tests from the historical information available in Bevy's repo. Meanwhile, Bevy should, on future commits, reduce the amount of patterns it uses that are outright problematic for performance in That Other Rust Compiler People Use.

[workingjubilee]

@davidbarsky Am I correct in my belief that other things can cause this performance bug, not just the assist, it just happens to be that the assist manages to do enough to trigger the bug?

[Veykril]

The assist is irrelevant here I believe. Just changing those imports in general will likely surface this, I wrote down some notes on the rust-analyzer issue in here rust-lang/rust-analyzer#20173.

tl;dr (I think) part of the issue is the vast amount of traits the globs might be bringing into scope (guessing), so changing imports and the like will invalidate the entire crate causing us to re-typecheck things (which thanks to the traits in scope) is fairly expensive -> noticable slow down.