Compilation taking unexpected long after 1.84

[mrain]

I'm building jf-pcs library from https://github.com/EspressoSystems/jellyfish

On rust version 1.83

rust version meta

[$] rustc --version --verbose                                                                                             
rustc 1.83.0 (90b35a623 2024-11-26)
binary: rustc
commit-hash: 90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf
commit-date: 2024-11-26
host: aarch64-apple-darwin
release: 1.83.0
LLVM version: 19.1.1

It only took ~11s to build the library

[$] cargo build -p jf-pcs                                                                                                        
   Compiling proc-macro2 v1.0.86
   Compiling unicode-ident v1.0.12
   ... [excess outputs omitted]
   Compiling jf-utils v0.4.4 (https://github.com/EspressoSystems/jellyfish?tag=0.4.5#7d71dbef)
   Compiling jf-pcs v0.1.0 (/Users/chengyu/espressosys/jellyfish/pcs)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 11.43s

Now on rust 1.84 the compilation runs almost forever:

rust version meta

rustc 1.84.0 (9fc6b4312 2025-01-07)
binary: rustc
commit-hash: 9fc6b43126469e3858e2fe86cafb4f0fd5068869
commit-date: 2025-01-07
host: aarch64-apple-darwin
release: 1.84.0
LLVM version: 19.1.5

[$] cargo build -p jf-pcs                                                                                                  
   Compiling proc-macro2 v1.0.86
   Compiling unicode-ident v1.0.12
   ... [excess outputs omitted]
   Compiling jf-utils v0.4.4 (https://github.com/EspressoSystems/jellyfish?tag=0.4.5#7d71dbef)
   Compiling jf-pcs v0.1.0 (/Users/chengyu/espressosys/jellyfish/pcs)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 26m 43s

Suspect that this is due to the new trait solver.

[lqd]

Indeed, this bisects to #130654 cc @rust-lang/initiative-trait-system-refactor. I'll look into minimizing this.

@rustbot label -E-needs-bisection

[lqd]

2 crates:

• src/lib.rs

pub trait Pairing {}

pub trait AffineRepr:
    // without the following line: super fast.
    Mul<Self::ScalarField, Output = Self::Group> +
    // without the following line: super fast.
    for<'a> Mul<&'a Self::ScalarField, Output = Self::Group>
    // note: combinations of the `Output`s above can be changed to `ScalarField` for 2x faster
{
    type ScalarField;
    type Group;
}

pub trait Mul<T> { type Output; }

• src/main.rs:

use repro::{Pairing, AffineRepr};

struct S<E>(E);
impl<T, E> From<T> for S<E>
where
    T: AffineRepr,
    E: Pairing,
{
    fn from(_value: T) -> Self {
        unimplemented!()
    }
}

fn main() {}

$ cargo clean -q && time CARGO_INCREMENTAL=0 cargo rustc -q --bin=repro -- -Znext-solver=no

real    0m0.206s
user    0m0.083s
sys     0m0.159s

$ cargo clean -q && time CARGO_INCREMENTAL=0 cargo rustc -q --bin=repro -- -Znext-solver=coherence

real    0m5.636s
user    0m5.519s
sys     0m0.152s

There may be others in there, I'm not sure.

[lcnr]

@rustbot claim

Thank you @mrain for reporting this and an enormous thank you to @lqd for the very nice minimization ❤️

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-high

[lcnr]

reopening to track stable-backport (if it happens)

[cuviper]

Should be fixed now in 1.84.1.