Function ensure_float 

pub(crate) fn ensure_float<'i>(
    value: &'i str,
    raw: Raw<'i>,
    error: &mut dyn ErrorSink,
)

float = float-int-part ( exp / frac [ exp ] )

float-int-part = dec-int
frac = decimal-point zero-prefixable-int
decimal-point = %x2E               ; .
zero-prefixable-int = DIGIT *( DIGIT / underscore DIGIT )

exp = "e" float-exp-part
float-exp-part = [ minus / plus ] zero-prefixable-int