Inconsistent format string support for binary and hex representations of arrays

[ChaiTRex]

I tried this code:

fn main() {
    println!("{:02x?}", [10, 11, 12]);
    println!("{:08b?}", [10, 11, 12]);
}

I expected to see this happen:

It prints the given arrays in hexadecimal and in binary.

Instead, this happened:

Hexadecimal works if you comment out the binary line, but binary doesn't work.

Meta

rustc --version --verbose:

rustc 1.74.1 (a28077b28 2023-12-04)
binary: rustc
commit-hash: a28077b28a02b92985b3a3faecf92813155f1ea1
commit-date: 2023-12-04
host: x86_64-unknown-linux-gnu
release: 1.74.1
LLVM version: 17.0.4

rustc 1.75.0-beta.7 (b216e7b0e 2023-12-16)
binary: rustc
commit-hash: b216e7b0e7a2bdf11300a21a614dac6be3e99c5b
commit-date: 2023-12-16
host: x86_64-unknown-linux-gnu
release: 1.75.0-beta.7
LLVM version: 17.0.6

rustc 1.76.0-nightly (3f28fe133 2023-12-18)
binary: rustc
commit-hash: 3f28fe133475ec5faf3413b556bf3cfb0d51336c
commit-date: 2023-12-18
host: x86_64-unknown-linux-gnu
release: 1.76.0-nightly
LLVM version: 17.0.6

   Compiling bug v0.1.0 (/home/chaitrex/dev/bug)
error: invalid format string: expected `'}'`, found `'?'`
 --> src/main.rs:3:20
  |
3 |     println!("{:08b?}", [10, 20, 30]);
  |               -    ^ expected `'}'` in format string
  |               |
  |               because of this opening brace
  |
  = note: if you intended to print `{`, you can escape it using `{{`

error: could not compile `bug` (bin "bug") due to previous error

[asquared31415]

x? and X? are special formatting types. The format docs section about what each of the formatting traits are says:

x? ⇒ Debug with lower-case hexadecimal integers

https://doc.rust-lang.org/beta/std/fmt/index.html#formatting-traits

There is no such formatting for "Debug but with binary integers". It might be a useful feature request though! (I don't know if this has been suggested in the past).

@rustbot label +T-compiler +A-diagnostics +A-fmt +D-papercut -needs-triage