Implement and forward more formatting traits for slices and arrays

[SimonSapin]

For example:

impl<T: Octal> Octal for [T; 5] { /* … */ }

This is mostly useful for showing binary (non-text) data in hexadecimal.

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[SimonSapin]

CC @rust-lang/libs

I’ve added #[unstable] to avoid "error: This node does not have a stability attribute", but I don’t remember if stability attributes actually work on impls or if they’re insta-stable regardless.

[dtolnay]

I believe the libs team discussed this in a PR or RFC a couple months ago and decided against adding these impls. I will try to find the discussion.

[dtolnay]

"Added additional formatting options for slices" #44751

[SimonSapin]

#44751 (comment)

an entire library dedicated to providing high-quality formatting-related utilities for slices and other datatypes, handling information like formatting flags, customization, etc.

That feels very “perfect is the enemy of good” :(

[dtolnay]

Later in the same comment,

This we felt was best prototyped outside the standard library before moving in.

Which is to say experience is the enemy of blind speculation. The design space here is wide enough that we would benefit from lots of experience with this in library form, rather than guessing what people want based on a few people using this while it is only usable on nightly.

[SimonSapin]

At least as far as I’m concerned, I’ve wanted this in two kinds of cases: for “printf-debugging” with temporary println!(…); statements that are removed after running few times before committing anything to version control, and for assertion failure messages in tests. In both cases, the audience is developers, not end users, so the exact formatting doesn’t matter too much.

So what is (in my opinion) lacking here is the opposite of an entire library that supports all kinds of combinations, it is “Debug, but in hex” for quick one-liners.

Perhaps a better solution would be a (pair of) flags for Debug to opt into (upper-case/lower-case) hexadecimal for numbers? Something like format!("{:02X?}", slice_of_u8).

(By the way, {:02?} seems to have no effect different from {:?}, is that intentional?)

I hadn’t really managed to find how to express it before, but I’ve always felt like the Octal/UpperHex/LowerHex traits were out of place: the base used for numbers should be orthogonal to the Debug vs Display split.

[SimonSapin]

(By the way, {:02?} seems to have no effect different from {:?}, is that intentional?)

It does work on integers, not on slices. I think this is due to not using the original Formatter in fmt::DebugList /~https://github.com/rust-lang/rust/blob/1.22.1/src/libcore/fmt/builders.rs#L256.

Edit: #46233 should fix this.

[SimonSapin]

This is PR is unlikely to succeed as-is since pretty much the same was rejected before. Closing in favor of rust-lang/rfcs#2226.