Rust 1.81-nightly slower than Rust 1.79 ?

Hi,

I've build my rust project with Rust 1.79 and now test with Rust 1.81 nightly.

I observe slower runtime.

This project is about parsing text files with polars.
Cargo.toml is the same in both context.

With Rust 1.79

# cargo build release with 1.79.0
cargo +1.79.0 build --release      
...

# benchmark on release run 
bench ./target/release/pmsi_rust   
benchmarking ./target/release/pmsi_rust
time                 236.9 ms   (226.6 ms .. 249.0 ms)
                     0.998 R²   (0.991 R² .. 1.000 R²)
mean                 232.1 ms   (228.1 ms .. 236.8 ms)
std dev              5.880 ms   (3.339 ms .. 8.859 ms)
variance introduced by outliers: 14% (moderately inflated)

With Rust 1.81

# With 1.81 version
% cargo --version      
cargo 1.81.0-nightly (bc89bffa5 2024-06-22)
# build release with 1.81
% cargo build --release           
...
# benchmark on release run
% bench ./target/release/pmsi_rust
benchmarking ./target/release/pmsi_rust
time                 614.8 ms   (528.0 ms .. 710.4 ms)
                     0.997 R²   (0.989 R² .. 1.000 R²)
mean                 644.2 ms   (622.9 ms .. 664.8 ms)
std dev              25.50 ms   (11.26 ms .. 31.11 ms)
variance introduced by outliers: 19% (moderately inflated)

It's an issue just to keep trace of this observation, is this feedback something you've encountered before?
Maybe it's just normal as 1.81 is a beta.

If not, I can make a reproducible example I think.

Thanks

Start by rebuilding the same project from its Cargo.toml with cargo +1.79.0 build --release and the reported Rust 1.81 nightly, then rerun the bench command against target/release/pmsi_rust. A useful outcome is a reproducible comparison with the project or a reduced example that identifies whether the slowdown comes from Rust or the polars-based workload.