`dcx.try_steal_modify_and_emit_err` race

[zetanumbers]

Part of #154314

There is a possible race for this dcx.try_steal_modify_and_emit_err call:

rust/compiler/rustc_hir_analysis/src/collect/type_of.rs

Lines 437 to 464 in a962594

	let guar = cx
	.dcx()
	.try_steal_modify_and_emit_err(ty_span, StashKey::ItemNoType, |err| {
	if !ty.references_error() {
	// Only suggest adding `:` if it was missing (and suggested by parsing diagnostic).
	let colon = if ty_span == item_ident.span.shrink_to_hi() { ":" } else { "" };
	// The parser provided a sub-optimal `HasPlaceholders` suggestion for the type.
	// We are typeck and have the real type, so remove that and suggest the actual type.
	if let Suggestions::Enabled(suggestions) = &mut err.suggestions {
	suggestions.clear();
	}
	if let Some(ty) = ty.make_suggestable(tcx, false, None) {
	err.span_suggestion(
	ty_span,
	format!("provide a type for the {kind}"),
	with_types_for_suggestion!(format!("{colon} {ty}")),
	Applicability::MachineApplicable,
	);
	} else {
	with_forced_trimmed_paths!(err.span_note(
	body_span,
	format!("however, the inferred type `{ty}` cannot be named"),
	));
	}
	}
	})

Such that inferred type for a diagnostic suggestion can vary between different compilations on the parallel front-end.

[Dnreikronos]

I'm investigating this issue and I've been able to reproduce it locally, so I'll try to write a PR solving the problem itself.