`lemmy_db_views_modlog_combined` has a weird performance regression

https://github.com/rust-lang/rust/issues/132064#issuecomment-2862157673

We recently split up our code into multiple crates for better parallel compilation. This also helped to narrow down where exactly the slowdown discussed here is happening, namely in this file. I already removed most irrelevant code, and it seems clear that the slowdown is somehow related to diesel. You can reproduce with the following commands:

git clone https://github.com/LemmyNet/lemmy.git --recursive --branch modlog-combined-remove-derives
cd lemmy
cargo build --timings -p lemmy_db_views_modlog_combined --all-features

Build timings only for lemmy_db_views_modlog_combined crate: 1.81.0: 23s 1.86.0: 188s 1.88.0-nightly (7918c7eb5 2025-04-27): 188s

I hope this will help to fix the problem, because we are stuck on 1.81 until it is resolved.

cc @weiznich: Are you aware of problems with slow compilation of diesel code starting from Rust 1.82?

In case its helpful here is profiling data for lemmy_db_views_modlog_combined using cargo 1.88.0-nightly (7918c7eb5 2025-04-27):

Item	Self time	% of total time	Time	Item count	Incremental result hashingtime
check_liveness	82.00s	42.409	82.04s	43	4.60µs
mir_built	76.86s	39.752	169.03s	43	562.14µs
layout_of	16.74s	8.656	84.04s	5691	1.26ms

Originally posted by @Nutomic in #132064