Add hexadecimal formatting of integers with fmt::Debug

[SimonSapin]

This can be used for integers within a larger types which implements Debug (possibly through derive) but not fmt::UpperHex or fmt::LowerHex.

assert!(format!("{:02x?}", b"Foo\0") == "[46, 6f, 6f, 00]");
assert!(format!("{:02X?}", b"Foo\0") == "[46, 6F, 6F, 00]");

RFC: rust-lang/rfcs#2226

The new formatting string syntax (x? and X?) is insta-stable in this PR because I don’t know how to change a built-in proc macro’s behavior based of a feature gate. I can look into adding that, but I also strongly suspect that keeping this feature unstable for a time period would not be useful as possibly no-one would use it during that time.

This PR does not add the new (public) fmt::Formatter proposed in the API because:

• There was some skepticism on response to this part of the RFC
• It is not possible to implement as-is without larger changes to fmt, because Formatter at the moment has no easy way to tell apart for example Octal from Binary: it only has a function pointer for the relevant fmt() method.

If some integer-like type outside of std want to implement this behavior, another RFC will likely need to propose a different public API for Formatter.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @KodrAus (or someone else) soon.

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[shepmaster]

Ping from triage, @KodrAus !

[KodrAus]

Thanks @SimonSapin! So, as I understand it, we've got special DebugLowerHex, and DebugUpperHex variants because hex is a nice representation for bit-packed data, so we don't need to combine other formatting flags with ?.

At first I thought it was a little unfortunate that we can't treat ? like a flag that could be combined with others arbitrarily, but looking at the changes that need to be made to the Debug impls on numbers this doesn't seem like such a problem.

So all that's to say this looks good to me!

[KodrAus]

@bors r+

[bors]

📌 Commit 4897935 has been approved by KodrAus

[SimonSapin]

DebugLowerHex and DebugUpperHex are bits inside a semi-private flags: u32 field. (It has pub so that the expansion format_args! can construct it, but it is an #[doc(hidden)] module marked as "internal".)

And yes, in the current implementation the fact that ? vs other "formatter types" was used is not easy to determine form Formatter. We can still change that internal representation, though.

[bors]

⌛️ Testing commit 4897935 with merge 152217d...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: KodrAus
Pushing 152217d to master...

[SimonSapin]

I’m unsure if this is release-notes-worthy, tagging to let someone else decide.