I'm not sure if changing this is possible, it can break existing Go programs.

cc @griesemer

The %#v is a very simple and effective printing format to log the inner structure of a value at testing. This is not possible without a fix for the current implementation of the printing formats for big.Int and big.Float. If not fixed, the documentation should be updated to clarify the bug. I prefer a fix.

const x = 170141183460469231731687303715884105727
fmt.Printf("const x = %#v\n", new(big.Int).Not(new(big.Int).Lsh(big.NewInt(-1), x&127)))
// prints: const x = 170141183460469231731687303715884105727

I think this is a better go representation of an integer than arbitrary fields and values.
Rat doesn't have any good representations other than String() -> "num/denom", so it doesn't need a Format function that overrides the fmt struct formatting.

Arguably, for ints and floats, 42 and 42.0 are valid "Go-syntax representations of the values" and thus legitimate representations when using the %#v formats. We might even use the fractional representation, say 3/43 for rationals with the %#v format because such fractions are also "valid Go" (at least for small numerators/denominators).