Rustdoc takes 8.5 GB of memory to document stm32h7

[jyn514]

Similar to but not the same as #78761: rustdoc uses an inordinate amount of memory to document large crates.

heaptrack_gui shows almost a third of the memory being allocated in get_blanket_impls:

However, the peak usage is actually in check_mod_item_types, which rustdoc can't do much about:

Rustdoc should work improving on both the total memory usage and peak memory usage.

This is meant as a tracking issue, since there are issues other than get_blanket_impls causing high memory use, such as #76382. There is a Zulip stream for technical discussion.

cc @adamgreig

[jyn514]

This is causing docs.rs to have to periodically bump the sandbox limits when documenting stm32 crates: rust-lang/docs.rs#1179.

[camelid]

P-medium: it's annoying that it takes so much memory, but it's not like you can't document it. And it's being worked on.

[jyn514]

I ran Rustdoc under DHAT and found that a good tenth of the allocations come from get_blanket_impls alone:

rust/src/librustdoc/clean/blanket_impl.rs

Line 130 in 2225ee1

items: self

. I'm not sure how to upload the full 81 MB file to GitHub, but feel free to ping me here or on Zulip, I'll keep a copy around Here's a link to Google Drive with the full file. Unfortunately DHAT takes a full 50 minutes to run rustdoc on stm32 so it's a pain to collect on your own.

[jyn514]

Another 7% just storing the structs themselves:

rust/src/librustdoc/passes/collect_trait_impls.rs

Line 128 in 2225ee1

self.impls.extend(get_auto_trait_and_blanket_impls(

. Maybe I should be asking just how many impls there are?