Make decoding non-optional LazyArray panic if not set

[compiler-errors]

Tables may be defined as optional: or defaulted:. If optional, if we try to read a value from a key that was never encoded, we should panic. This has high value in ensuring correctness over a defaulted table, so the tradeoff is worth considering, since it signals the compiler has a buggy encode impl, rather than just defaulting to a value.

HOWEVER, optional: arrays were side-stepping this. So this PR fixes that, and makes optional: tables of LazyArray act like LazyValue, and panic if it's not assigned a value during encoding.

During this PR, I found that deduced_param_attrs has buggy (?? i think??) implementation where it will refuse to encode cross-crate deduced_param_attrs unless we're codegening, we're optimizing the library, and incremental is disabled. This seems incredibly wrong, but I don't want to fix it in this PR.

rust/compiler/rustc_metadata/src/rmeta/encoder.rs

Lines 1733 to 1747 in 9649706

	// Encode all the deduced parameter attributes for everything that has MIR, even for items
	// that can't be inlined. But don't if we aren't optimizing in non-incremental mode, to
	// save the query traffic.
	if tcx.sess.opts.output_types.should_codegen()
	&& tcx.sess.opts.optimize != OptLevel::No
	&& tcx.sess.opts.incremental.is_none()
	{
	for &local_def_id in tcx.mir_keys(()) {
	if let DefKind::AssocFn | DefKind::Fn = tcx.def_kind(local_def_id) {
	record_array!(self.tables.deduced_param_attrs[local_def_id.to_def_id()] <-
	self.tcx.deduced_param_attrs(local_def_id.to_def_id()));
	}
	}
	}
	}

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[lcnr]

@bors r+ rollup

[bors]

📌 Commit 175238b has been approved by lcnr

It is now in the queue for this repository.