Added octal literal support.

[mattcarberry]

Associated with Issue #6563.

Useful for Apollo Guidance Computer simulation, Unix file system permissions, and maybe one or two other things.

[brson]

Thanks! Let's discuss this change to the language in a weekly meeting before merging.

[chris-morgan]

I experimented some time back with a couple of ways of doing file modes without needing to resort to comparatively meaningless numbers; for example, defining a macro which could take every possible combination and turn it into a number (that revealed severe performance problems on macros with many thousands of rules... who'd 'a thunked it? If you were doing it seriously, you'd do it as an ultra-fast syntax extension, not as a ridiculously expensive macro that adds three minutes to compile time whether used or not) or constants for every possible combination, like rwxr_S__t. That was a fun experiment in testing unusual conditions in the compiler (it didn't crash anywhere, it merely performed atrociously), but I don't expect it to be taken very seriously.

Still, the principle is right: often defining constants will be sufficient for your purposes. A quick look at the Apollo Guidance Computer suggests it uses octal memory addresses, which would suggest that constants are slightly infeasible and having octal literals would indeed be of value for that case (but you could theoretically define lots of constants or use a macro, like I did!). Still, octal is used by surprisingly few things.

Potentially of more use would be making it possible to have arbitrary base literals. As a demo with no more than twenty-seven seconds' thought put into the syntax, I can imagine something like 8b777 meaning octal 777, 56b14NcYrmxFCN9wAHhxCHkyt4PU9bqnveBUj meaning 14NcYrmxFCN9wAHhxCHkyt4PU9bqnveBUj in base-56—Bitcoin address literals! There are times when writing various algorithms when having base-3 or base-4 would be convenient, for example (alas, base64 probably isn't going to work for tokenising reasons).

[thewonderidiot]

chris-morgan+1

Arbitrary base literals would be really cool. J has them (with the exact same syntax you proposed), and bash and Ada have them with a '#' instead of 'b'.

[mattcarberry]

I like the idea of arbitrary base literals. In thinking about the syntax, it makes a problem for the suffixes. Past a certain base, u32, u16, etc are perfectly valid numbers. Ada wraps the number in #, which is definitely one way to fix it.

[huonw]

This will also need some positive tests, i.e. checking 0o12 == 10 and so on.

[mletterle]

+1 for arbitrary base literals. Though having built in hex, binary, and octal is nice.

[emberian]

I've been ambivalent about this for a while. I lean 👎 but octal is extremely useful in the extremely few cases it's useful in.

[brson]

Let's add octal literals per this pull request and defer the question of arbitrary bases to later.

[alexcrichton]

I'm stopping the build for now, the code looks fine but I agree with @huonw that this requires more tests. Having negative tests is a good thing, but we should also have some positive tests (to make sure they're parsed correctly).

[aaronfranke]

Where did this code go in the latest rust-lang/rust? I am not able to find fn scan_number, the lexer.rs file, or the src/libsyntax folder.