math/big: big.Int `fmt.Formatter` implementation should support `%q`

[azdagron]

I suggest a small quality-of-life improvement to the *big.Int type to support printing as a quoted string. *big.Int implements fmt.Formatter and (contrary to the documentation) supports the s verb, printing the integer in decimal format. This allows you to do:

fmt.Printf("My int: %s\n", big.NewInt(1))

However, it does not support q, which becomes surprising for people accustomed to the special formatting considerations for string formats wherein q is implicitly supported for types implementing fmt.Stringer. Instead you have to explicitly call the String() method:

fmt.Printf("My quoted int: %q\n", big.NewInt(1).String())

And if you forget?
https://play.golang.org/p/5YiBHW5djUM

I don't imagine there would be a big back-compat concern.

I'm happy to provide the code change if folks are on board.

[toothrot]

/cc @griesemer

[AlexanderYastrebov]

(contrary to the documentation) supports the s verb, printing the integer in decimal format.

Undocumented since 10 years https://codereview.appspot.com/4552056 and maybe it should not support it to avoid confusion.

Also it is interesting what output is expected for %q (single/double quotes, number base), e.g. plain int does not support %s at all and %q outputs a single-quoted character literal safely escaped with Go syntax. which might be not intuitive https://play.golang.org/p/o7pBzoVAFdV

I think the correct way for

fmt.Printf("My quoted int: %q\n", big.NewInt(1).String())

is

fmt.Printf("My quoted int: \"%d\"\n", big.NewInt(1))