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Add hexadecimal formatting of integers with fmt::Debug #48978

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SimonSapin commented Mar 13, 2018

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.

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rust-highfive commented Mar 13, 2018

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.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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

```rust
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
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shepmaster commented Mar 18, 2018

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!

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KodrAus commented Mar 18, 2018

@bors r+

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bors commented Mar 18, 2018

📌 Commit 4897935 has been approved by KodrAus

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SimonSapin commented Mar 18, 2018

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.

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bors commented Mar 19, 2018

⌛️ Testing commit 4897935 with merge 152217d...

bors added a commit that referenced this pull request Mar 19, 2018

Auto merge of #48978 - SimonSapin:debug-hex, r=KodrAus
Add hexadecimal formatting of integers with fmt::Debug

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

```rust
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`.
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bors commented Mar 19, 2018

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

@bors bors merged commit 4897935 into rust-lang:master Mar 19, 2018

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add Formatter::debug #49068

@SimonSapin SimonSapin deleted the SimonSapin:debug-hex branch Mar 19, 2018

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SimonSapin commented Mar 23, 2018

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

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