Function format_value 

pub fn format_value<V>(
    specifier: &Specifier,
    value: &V,
    f: &mut Formatter<'_>,
) -> Result
where
    V: Display + Debug + Octal + LowerHex + UpperHex + Binary + LowerExp + UpperExp,

Formats the given value using the given formatter and the given format specification.

Since the implementation of format_value employs the write! macro, the value must implement all of the std::fmt formatting traits. Which trait will actually be used is determined at runtime, based on the contents of the specifier.