fn := q.PopLeft().Func

		// Check fn.
		if fn.WBPos.IsKnown() {
			var err strings.Builder
			call := funcs[fn]
			for call.target != nil {
				fmt.Fprintf(&err, "\n\t%v: called by %v", base.FmtPos(call.lineno), call.target.Nname)
				call = funcs[call.target]
			}
			// Seeing this error in a failed CI run? It indicates that
			// a function in the runtime package marked nowritebarrierrec