In this case, the interested potential acquirer would have to move heaven and earth to avoid an instant credit downgrade (assuming they have investment grade status now) at current valuations.

Toyota has a lot more debt and a significantly higher debt/equity ratio than Tesla has, a merger shouldn't have a material impact on credit ratings.

Also, I feel quite confident in stating that the short-term market cap of the two companies merged would be significantly higher than the two companies separate. Toyota has a P/E of 10 (!). A merger would make it "cool", giving it a better ratio. Not Tesla's ~50 (based on quadrupling the last quarter earnings; TTM earnings are negative), but something higher than 10. And since Toyota has significant earnings, increasing the multiple of a big number....

Long term I'm not so sure it's a good strategy but a superficially the synergy looks awesome: combine Tesla's innovation and hype with Toyota's ability to deliver high quality cars.