Eagerly collect mono items for non-generic closures

[compiler-errors]

This allows users to use -Zprint-mono-items=eager to eagerly monomorphize closures and coroutine bodies, in case they want to inspect the LLVM or ASM for those items.

-Zprint-mono-items, which used to be called -Zprint-trans-items, was originally added in #30900:

Eager mode is meant to be used in conjunction with incremental compilation
where a stable set of translation items is more important than a minimal
one. Thus, eager mode will instantiate drop-glue for every drop-able type
in the crate, even of no drop call for that type exists (yet). It will
also instantiate default implementations of trait methods, something that
otherwise is only done on demand.

Although it remains an unstable option, its purpose has somewhat expanded since then, and as far as I can tell it's generally useful for cases when you want to monomorphize as many items as possible, even if they're unreachable. Specifically, it's useful for debugging since you can look at the codegen'd body of a function, since we don't emit items that are not reachable in monomorphization.

And even more specifically, it would be very to monomorphize the coroutine body of an async fn, since those you can't easily call those without a runtime. This PR enables this usecase since we now monomorphize DefKind::Closure.

[rustbot]

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[wesleywiser]

Looks like -Clink-dead-code will enable eager collection as well but that seems entirely reasonable to me.

[wesleywiser]

@bors r+

[bors]

📌 Commit 6431504 has been approved by wesleywiser

It is now in the queue for this repository.

[compiler-errors]

Does it trigger eager collection of closures too?

[wesleywiser]

I thought so since it triggers use of Eager mode and the collector doesn't seem to care about -Clink-dead-code specifically, it just changes behavior based on the collection mode

rust/compiler/rustc_monomorphize/src/partitioning.rs

Lines 1118 to 1141 in 336209e

	let collection_strategy = match tcx.sess.opts.unstable_opts.print_mono_items {
	Some(ref s) => {
	let mode = s.to_lowercase();
	let mode = mode.trim();
	if mode == "eager" {
	MonoItemCollectionStrategy::Eager
	} else {
	if mode != "lazy" {
	tcx.dcx().emit_warn(UnknownCguCollectionMode { mode });
	}
	MonoItemCollectionStrategy::Lazy
	}
	}
	None => {
	if tcx.sess.link_dead_code() {
	MonoItemCollectionStrategy::Eager
	} else {
	MonoItemCollectionStrategy::Lazy
	}
	}
	};
	let (items, usage_map) = collector::collect_crate_mono_items(tcx, collection_strategy);