Parallel frontend tests failures due to different post-monomorphization errors

[zetanumbers]

Part of #154314.

PR #153801 did introduce an option to run rustc tests with parallel compiler enabled using option --parallel-frontend-threads. There a collection of tests was marked ignore-parallel-frontend due to subtle difference in output diagnostics. For example:

---- [ui] tests/ui/structs/default-field-values/post-mono.rs#indirect stdout ----
Saved the actual stderr to `build/x86_64-unknown-linux-gnu/test/ui/structs/default-field-values/post-mono.indirect/post-mono.indirect.stderr`
diff of stderr:
24      LL |     indirect::<1>();
25         |     ^^^^^^^^^^^^^^^
26
+       note: the above error was encountered while instantiating `fn std::rt::lang_start::<()>`
+
27      error: aborting due to 1 previous error
28
29      For more information about this error, try `rustc --explain E0080`.

The addition of message "the above error was encountered while instantiating fn std::rt::lang_start::<()>" in output diagnostics was the sole cause of failures in these tests with parallel frontend enabled:

Failed tests

• tests/ui/abi/simd-abi-checks-avx.rs
• tests/ui/consts/const-eval/issue-50814-2.rs
• tests/ui/consts/const-eval/issue-50814.rs
• tests/ui/consts/mono-reachable-invalid-const.rs
• tests/ui/consts/required-consts/collect-in-called-fn.rs
• tests/ui/consts/required-consts/collect-in-dead-closure.rs
• tests/ui/consts/required-consts/collect-in-dead-drop.rs
• tests/ui/consts/required-consts/collect-in-dead-fn-behind-assoc-type.rs
• tests/ui/consts/required-consts/collect-in-dead-fn-behind-generic.rs
• tests/ui/consts/required-consts/collect-in-dead-fn-behind-opaque-type.rs
• tests/ui/consts/required-consts/collect-in-dead-fn.rs
• tests/ui/consts/required-consts/collect-in-dead-fnptr-in-const.rs
• tests/ui/consts/required-consts/collect-in-dead-fnptr.rs
• tests/ui/consts/required-consts/collect-in-dead-move.rs
• tests/ui/consts/required-consts/collect-in-promoted-const.rs
• tests/ui/inline-const/required-const.rs
• tests/ui/lint/large_assignments/copy_into_box_rc_arc.rs
• tests/ui/lint/large_assignments/copy_into_fn.rs
• tests/ui/lint/large_assignments/inline_mir.rs
• tests/ui/lint/large_assignments/large_future.rs
• tests/ui/lint/large_assignments/move_into_fn.rs
• tests/ui/simd/const-err-trumps-simd-err.rs
• tests/ui/structs/default-field-values/post-mono.rs

Meanwhile there were similar tests with //@ ignore-parallel-frontend post-monomorphization errors I was unable to reproduce a failure for with --parallel-frontend-threads set:

Other tests

• tests/ui/consts/const-eval/index-out-of-bounds-never-type.rs
• tests/ui/consts/required-consts/collect-in-dead-forget.rs
• tests/ui/consts/required-consts/collect-in-dead-vtable.rs
• tests/ui/consts/required-consts/interpret-in-const-called-fn.rs
• tests/ui/consts/required-consts/interpret-in-promoted.rs
• tests/ui/consts/required-consts/interpret-in-static.rs

collect_items_rec uses tcx.dcx().err_count() global error counter to count errors before and after checking items, difference of which is assumed to be number of errors for checking said items:

rust/compiler/rustc_monomorphize/src/collector.rs

Lines 417 to 418 in bbe8536

	// FIXME: don't rely on global state, instead bubble up errors. Note: this is very hard to do.
	let error_count = tcx.dcx().err_count();

and later

rust/compiler/rustc_monomorphize/src/collector.rs

Lines 541 to 564 in bbe8536

	// Check for PMEs and emit a diagnostic if one happened. To try to show relevant edges of the
	// mono item graph.
	if tcx.dcx().err_count() > error_count
	&& starting_item.node.is_generic_fn()
	&& starting_item.node.is_user_defined()
	{
	match starting_item.node {
	MonoItem::Fn(instance) => tcx.dcx().emit_note(EncounteredErrorWhileInstantiating {
	span: starting_item.span,
	kind: "fn",
	instance,
	}),
	MonoItem::Static(def_id) => tcx.dcx().emit_note(EncounteredErrorWhileInstantiating {
	span: starting_item.span,
	kind: "static",
	instance: Instance::new_raw(def_id, GenericArgs::empty()),
	}),
	MonoItem::GlobalAsm(_) => {
	tcx.dcx().emit_note(EncounteredErrorWhileInstantiatingGlobalAsm {
	span: starting_item.span,
	})
	}
	}
	}

However we call collect_items_rec in parallel in which leads to this assumption to fail:

rust/compiler/rustc_monomorphize/src/collector.rs

Lines 1820 to 1822 in bbe8536

	par_for_each_in(roots, |root| {
	collect_items_root(tcx, dummy_spanned(*root), &state, recursion_limit);
	});

Thus it creates unintended inconsistency in tests' outputs.

[zetanumbers]

PR #153194 seems to modify this section of the compiler, although it doesn't fix this issue.