Initial support for empty named constraints

[danakj]

Type check named constraint decls and definitions. We don't correctly error if you put a fn inside them. There is no support for require or alias yet, so there's nothing useful you can do with them yet.

We have attempted to share code between interface and constraint as they are quite similar. First by splitting out some of handle_interface.cpp to a separate file. Second by sharing some code paths when you want a facet type from either one, as they both turn into a facet type.

[danakj]

Going to toss this toward @jonmeow for review. A lot of it is just boilerplate. I think the interesting question is what is the best way to share code between interface and constraint.

[jonmeow]

I did a quick pass, but I'm going to be off next week so re-adding @geoffromer who got the initial autoassign.

[danakj]

I did a quick pass, but I'm going to be off next week so re-adding @geoffromer who got the initial autoassign.

Oops, thanks, I removed him as I thought something went wrong. Re-added back. But @geoffromer you can hold off reviewing until I address this feedback. I will ping you when it's ready.

[geoffromer]

But @geoffromer you can hold off reviewing until I address this feedback. I will ping you when it's ready.

Sounds good. (Replying here so the bot stops pinging me :-) )

[danakj]

PTAL

[danakj]

PTAL

[geoffromer]

Optional: I think you can make these into a conventional overload set with something like this:

Suggested change

	template <typename EntityT>
	requires std::same_as<EntityT, SemIR::Interface>
	static auto TryGetEntity(Context& context, SemIR::Inst inst)
	-> const SemIR::EntityWithParamsBase* {
	if (auto decl = inst.TryAs<SemIR::InterfaceDecl>()) {
	return &context.interfaces().Get(decl->interface_id);
	} else {
	return nullptr;
	}
	}
	template <typename EntityT>
	requires std::same_as<EntityT, SemIR::NamedConstraint>
	static auto TryGetEntity(Context& context, SemIR::Inst inst)
	-> const SemIR::EntityWithParamsBase* {
	if (auto decl = inst.TryAs<SemIR::NamedConstraintDecl>()) {
	return &context.named_constraints().Get(decl->named_constraint_id);
	} else {
	return nullptr;
	}
	}
	template <typename T>
	struct Tag {};
	static auto TryGetEntity(Tag<SemIR::Interface>, Context& context, SemIR::Inst inst)
	-> const SemIR::EntityWithParamsBase* {
	if (auto decl = inst.TryAs<SemIR::InterfaceDecl>()) {
	return &context.interfaces().Get(decl->interface_id);
	} else {
	return nullptr;
	}
	}
	static auto TryGetEntity(Tag<SemIR::NamedConstraint>, Context& context, SemIR::Inst inst)
	-> const SemIR::EntityWithParamsBase* {
	if (auto decl = inst.TryAs<SemIR::NamedConstraintDecl>()) {
	return &context.named_constraints().Get(decl->named_constraint_id);
	} else {
	return nullptr;
	}
	}

Then the callsite looks like TryGetEntity(Tag<EntityT>{}, context, existing_decl_inst).

This technique trades off a slightly more awkward interface for a more straightforward implementation, so I'd be more hesitant to do it with a public function, but it can be a good fit for implementation details within a .cpp file.

[danakj]

My reply here isn't showing easily on the convo tab so surfacing it for you.