https://github.com/fasterthanlime/rustc-superlinear-sadness
Trying to reproduce build time issues with tower
https://github.com/fasterthanlime/rustc-superlinear-sadness
Last synced: 8 months ago
JSON representation
Trying to reproduce build time issues with tower
- Host: GitHub
- URL: https://github.com/fasterthanlime/rustc-superlinear-sadness
- Owner: fasterthanlime
- Created: 2022-07-12T12:39:11.000Z (almost 4 years ago)
- Default Branch: main
- Last Pushed: 2022-07-12T18:24:26.000Z (almost 4 years ago)
- Last Synced: 2025-05-12T20:49:56.319Z (about 1 year ago)
- Language: Rust
- Size: 30.3 KB
- Stars: 6
- Watchers: 3
- Forks: 0
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
Awesome Lists containing this project
README
# rustc-superlinear-sadness
Showcase of how tower-style code can make compile times explode on rustc
nightly as of 2022-07-12.
## Timeline
* 2022-07-12: repro created, profiling work by @fasterthanlime, @eddyb & @BoxyUwU to scope down the problem
* 2022-07-12: [rustc issue 99188](https://github.com/rust-lang/rust/issues/99188) opened by @eddyb
## Repo structure
All the code is in `src/main.rs`, it has no dependencies - it doesn't even
import anything outside the rust 2021 prelude.
The code will only compile if one of those `cfg` is set:
* `assoc_type_0`, `assoc_type_1`, `assoc_type_2`, `assoc_type_3`
* `outlives`
* `clone`
You can "add nesting" to show how compile time evolves, by adding `--cfg more1`,
`--cfg more2`, etc. until `more7`.
## Justfile / commands
`just run --cfg assoc_type_0` will run `rustc`, using the `stage1` toolchain,
enabling self-profiling.
For the rest of the commands to work, you'll need a fork of rustc that records
`SelectionContext::evaluate_predicate_recursively` using the self-profiler.
You can use [this branch of my fork](https://github.com/fasterthanlime/rust/tree/self-profile-evaluate-predicate-recursively). If you don't, you'll still notice
compile times going up but you won't get a nice "leaderboard" like in the README
examples.
`crox` is being run on the most recent `*.mm_profdata` file to generate a
"chrome tracing" file (as `chrome_profiler.json`), and then `jq` is used to
filter only the `evaluate_predicate_recursively` calls, which are then
sorted.
`just seq {name}` will run `just run` multiple times in a row, with a higher
`moreN` value every time, showing compile time growth. This is what is showcased
in the README.
## assoc types
With zero associated types being constrained, even with nested
`MiddleService>` types, the number of checks is constant:
```rust
impl<'a, S> Service<&'a ()> for MiddleService
where
for<'b> S: Service<&'b ()>,
```
```shell
$ just seq assoc_type_0
6 Obligation(predicate=Binder(TraitPredicate(<() as std::marker::Sized>, polarity:Positive), []), depth=?)
6 Obligation(predicate=Binder(TraitPredicate(<() as std::marker::Sized>, polarity:Positive), []), depth=?)
6 Obligation(predicate=Binder(TraitPredicate(<() as std::marker::Sized>, polarity:Positive), []), depth=?)
6 Obligation(predicate=Binder(TraitPredicate(<() as std::marker::Sized>, polarity:Positive), []), depth=?)
6 Obligation(predicate=Binder(TraitPredicate(<() as std::marker::Sized>, polarity:Positive), []), depth=?)
6 Obligation(predicate=Binder(TraitPredicate(<() as std::marker::Sized>, polarity:Positive), []), depth=?)
6 Obligation(predicate=Binder(TraitPredicate(<() as std::marker::Sized>, polarity:Positive), []), depth=?)
```
With one associated type being constrained, it's linear:
```rust
#[cfg(assoc_type_1)]
impl<'a, S> Service<&'a ()> for MiddleService
where
for<'b> S: Service<&'b (), Response = ()>,
```
```shell
$ just seq assoc_type_1
19 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
23 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
27 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
31 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
35 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
39 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
43 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
```
With two, it's exponential ($2^x$ growth):
```rust
#[cfg(assoc_type_2)]
impl<'a, S> Service<&'a ()> for MiddleService
where
for<'b> S: Service<&'b (), Response = (), Error = ()>,
```
```shell
$ just seq assoc_type_2
108 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
220 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
444 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
892 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
1788 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
3580 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
7164 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
```
With three, it's exponential, but worse ($3^x$ growth)
```rust
impl<'a, S> Service<&'a ()> for MiddleService
where
for<'b> S: Service<&'b (), Response = (), Error = (), ThirdType = ()>,
```
```shell
$ just seq assoc_type_3
403 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
1213 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
3643 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
10933 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
32803 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
98413 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
295243 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
```
## outlives constraint
This type of constraint also shows exponential behavior ($2^x$):
```rust
impl<'a, S> Service<&'a ()> for MiddleService
where
for<'b> S: Service<&'b ()>,
for<'b> >::Future: 'b,
```
```shell
$ just seq outlives
26 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
50 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
98 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
194 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
386 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
770 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
1538 Obligation(predicate=Binder(TraitPredicate(, polarity:Positive), []), depth=?)
```
And a `Clone` constraint shows the exact same thing (numbers not shown here):
```rust
impl<'a, S> Service<&'a ()> for MiddleService
where
for<'b> S: Service<&'b ()>,
for<'b> >::Future: Clone,
```
## smaller repro
Here's a smaller repro of the "assoc types" explosion: [Playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=e63793ea5031cb7aa38c64a71e49de23).
Adding another `&` in `main` makes the playground time out (be nice to the playground).
## next steps
Several issues are being filed, let's see where this goes!