Rust: Middle size, 80 000 lines, project but very slow, 5 hours, compilation

[cheblin]

This code is the MAVLink  protocol handler, and generated by one of the MAVLink specification. It is  C/Rust  mixed project, so to compile it you will need  Rust ,  CC  an any  C  compiler.

To check how slow  rustc  is, check this out , and run  cargo test  in the  GroundControl/demo  dir

Exactly this project is compiled with error, cause the borrow checker complains in one place.

But anyway, the problem is that compilation is unacceptably slow.

Description

/GroundControl/lib   generated API

/GroundControl/demo/src/use_case  is generated sample of using generated API

/GroundControl/demo/src/test  the test

I generate variation of this project, with same functionality, in C, C++, C#.... They are the compiled in seconds !

What wrong with my Rust project? How can I speed up compilation?

My PC

Intel I7-7700K

RAM 32G

SSD Samsung 970 PRO

cargo -V

cargo 1.39.0-nightly (ab6fa8908 2019-09-25)

rustc -V

rustc 1.40.0-nightly (22bc9e1d9 2019-09-30)

[jonas-schievink]

Please provide build instructions for that code. Running cargo build in GroundControl/lib gives:
fatal error: ../config.h: No such file or directory.

[cheblin]

GroundControl/lib gives: gives:.....

But I wrote

and run cargo test in the GroundControl/demo dir

[jonas-schievink]

That gives the same error, since it just has a dependency on the crate in lib...

[cheblin]

Small bug fix in the build.rs file.
Please try again.
Thanks.

[jonas-schievink]

That didn't fix it

[cheblin]

on my debian and windows 10 build script shown:

cargo:warning=Starting search config.h in....
cargo:warning=/root/InRUST/GroundControl/demo/target/debug/build/black_box-sys-57f37c06735e9d12/out
cargo:warning=/root/InRUST/GroundControl/demo/target/debug/build/black_box-sys-57f37c06735e9d12
cargo:warning=/root/InRUST/GroundControl/demo/target/debug/build
cargo:warning=/root/InRUST/GroundControl/demo/target/debug
cargo:warning=/root/InRUST/GroundControl/demo/target
cargo:warning=/root/InRUST/GroundControl/demo
cargo:warning=<<< Find and use generated config.h in root/InRUST/GroundControl/lib/src/ >>>

[cheblin]

made more platform specific fixes.
check now please.

[tanriol]

I've looked into this code and likely found the problem.

The bright side: it compiles in under half a minute if the errors are removed.

The part of the code causing this problem is a test function that tries to test everything at once and has about 8000 lines. The function contains (as of commit ee653b1a0170) 76 borrow checker errors and the part that takes 5 hours is generating the 3-point errors for them. The time is dominated by the repeated recalculation of dominators for the function body.

[tanriol]

A minimized reproduction looks like this

fn get_pair(_a: &mut u32, _b: &mut u32) {
}

macro_rules! x10 {
    ($($t:tt)*) => {
        $($t)* $($t)* $($t)* $($t)* $($t)*
        $($t)* $($t)* $($t)* $($t)* $($t)*
    }
}

#[allow(unused_assignments)]
fn main() {
    let mut x = 1;

    get_pair(&mut x, &mut x);

    x10!{ x10!{ x10!{ if x > 0 { x += 2 } else { x += 1 } } } }
}

On my laptop this code requires about 25 seconds to print the error for the 1000 if statements generated above and scales with the cube of the statement count.

[H2CO3]

On the bright side, waiting 5 hours for borrowck errors is still faster than trying to debug a memory corruption in a language without borrowck. 😜

(edit: lol, why so serious, this was meant to be humorous and not a complaint about Rust…)

[joshtriplett]

Thanks to @cheblin for the report and helping us to work with it, and to @tanriol for the quick analysis and fix!