Slow compilation of actix-web routes

[Nutomic]

I noticed that Rust is extremely slow to compile a rather simple file containing route definitions for the actix-web webserver. It has 200 lines of imports, 250 lines of code and no macros, yet takes ~60s to compile. Duration is the same on Rust 1.81, 1.86 and nightly-2025-04-27.

Here is an excerpt from the profiling data, and you can also view the .mm_profdata.

Item	Self time	% of total time	Time	Item count	Incremental result hashing time
layout_of	29.25s	42.066	111.38s	123703	32.73ms
normalize_canonicalized_projection_ty	17.28s	24.851	18.07s	15554	5.31ms
codegen_select_candidate	9.16s	13.173	9.39s	28989	9.75ms
LLVM_module_codegen_emit_obj	5.59s	8.042	5.59s	256	0.00ns
LLVM_passes	2.13s	3.069	2.13s	1	0.00ns
codegen_module	1.40s	2.009	4.43s	256	0.00ns
evaluate_obligation	951.66ms	1.369	1.03s	47932	7.69ms
items_of_instance	301.15ms	0.433	56.05s	57443	34.90ms

Whats strange is that the same file on the main branch takes only ~25s to compile (which is still too long). There it is part of the main binary crate.

To reproduce run:

git clone https://github.com/LemmyNet/lemmy.git --recursive --branch api-routes-crate
cd lemmy
cargo build --timings -p lemmy_api_routes

[lqd]

The above is entirely coming from the layout sanity checks, which is very unexpected.

It looks to be taking a long time to compute uninhabitedness for some of the big types in this crate (big coroutines, a lot of the time), sometimes multiple seconds for a single type. Overall, locally with the latest master commit at the time, the layout sanity checks take 66 seconds with the type-level uninhabitedness check, compared to 220ms without (using try build artifacts of the proposed change below).

@RalfJung would it be acceptable if I moved this uninhabitedness assertion (added in #96872) a bit down, to be gated by cfg(debug_assertions) like the other expensive layout sanity checks in this function?