New API: `char::is_ascii_octdigit`?

internals
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Is there a reason why char::is_ascii_digit and char::is_ascii_hexdigit exist, whereas there is no shorthand to check if a character is a valid octal? While it is as simple as matches!(c, '0'..='7'), I would prefer to have the proposed API because:

  • it is a lot more ergonomic in point-free form, i.e.: my_string.chars().filter(char::is_ascii_octdigit) as compared to my_string.chars().filter(|c| matches!(c, '0'..='7')) or my_string.chars().filter(|c| '0'..='7'.contains(c))
  • the intent is a lot more obvious
  • completeness, I just think it fills a gap(?) in the list of char functions
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There's always c.is_digit(8) whose intent is perhaps a little more obvious than using explicit ranges.

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And if the libs API is to be expanded, it should probably now use const generics to be more general eg fn is_digit_const<const BASE: u32>(self) -> bool which could then be invoked in "point-free" form as eg my_string.chars().filter(char::is_digit_const::<8>).

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I’m curious what you’re using octal for in 2022!

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Can't speak for the OP, but chmod for example still takes octal here in the year 03746.

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I've used it when parsing git ls-tree output.

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I myself happened to use it earlier today to represent bit lengths, because the second position onward represent the number of bytes and the units represent offset into the final byte.

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Yeah octal is perfect for thinking about memory.

On x86-64 linux, in octal:

1 byte         = 10 bits
1 machine word = 100 bits
1 cache line   = 1000 bits
1 page         = 100000 bits
1 huge page    = 100000000 bits
1 huger page   = 100000000000 bits
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as @quinedot mentioned, i realized this method was not present when i was validating a string containing file permissions!

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I would be happy to merge a PR adding this method (as unstable, with a tracking issue). As several people in this thread observed, it does still come up in the context of file modes.

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thanks, i'd love to give that a shot!

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It would be great to have both a const generic version and char::is_ascii_octdigit as a special case since it's so useful.

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