absent precision parameter for floating point format string is undocumented

[e00E]

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".

[hkBst]

This is parsed here: https://github.com/rust-lang/rust/blob/master/compiler/rustc_parse_format/src/lib.rs#L707