Function to_exact_exp_str 

pub fn to_exact_exp_str<'a, T, F>(
    format_exact: F,
    v: T,
    sign: Sign,
    frac_digits: usize,
    upper: bool,
    buf: &'a mut [MaybeUninit<u8>],
    parts: &'a mut [MaybeUninit<Part<'a>>],
) -> Formatted<'a>
where
    T: DecodableFloat,
    F: FnMut(&Decoded, &'a mut [MaybeUninit<u8>], i16) -> (&'a [u8], i16),

🔬This is a nightly-only experimental API. (flt2dec)

Formats given floating point number into the exponential form with exactly given number of fractional digits. The result is stored to the supplied parts array while utilizing given byte buffer as a scratch. upper is used to determine the case of the exponent prefix (e or E). The first part to be rendered is always a Part::Sign (which can be an empty string if no sign is rendered).

format_exact should be the underlying digit-generation function. It should return the part of the buffer that it initialized. You probably would want strategy::grisu::format_exact for this.

The returned format is [sign][digit][.fraction?][zero padding?][e|E][exponent].

The byte buffer should be at least frac_digits + 1 bytes long unless frac_digits is so large that only the fixed number of digits will be ever written. (The tipping point for f64 is about 800, so 1000 bytes should be enough.) There should be at least 6 parts available, due to the worst case like [+][1][.][2345][e][-][6].