I am talking about format strings of the form format!"{:.}", 1.234). The corresponding documentation for the precision argument is here. The documentation writes that the format is .N where N is an integer specifying the precision.

The bug is that it is not documented that N can also be absent. Either this should not be accepted by the compiler or it should be documented.

I believe the intention is that an absent N means the same thing as if there was no precision specified at all (no :.). Note that this is not the same as "as much precision as needed".