The current rule is that all commits must pass tests. I suggest to make exception for rustfmt and only require that the final commit after merging passes rustfmt. There are two reasons:

• Sometimes review can be made easier by intentionally "mis-formatting" - see Support weight prediction in const context #1710 for an example
• If it ever happens that merge fails because of formatting only it's a bunch of wasted time. (Not sure if this can happen.)

IIUC the rule is to make bisecting reliable and I don't think anyone will ever need to bisect formatting. Bisecting is generally used to find bugs.