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0879-small-base-lexing.md

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Summary

Lex binary and octal literals as if they were decimal.

Motivation

Lexing all digits (even ones not valid in the given base) allows for improved error messages & future proofing (this is more conservative than the current approach) and less confusion, with little downside.

Currently, the lexer stops lexing binary and octal literals (0b10 and 0o12345670) as soon as it sees an invalid digit (2-9 or 8-9 respectively), and immediately starts lexing a new token, e.g. 0b0123 is two tokens, 0b01 and 23. Writing such a thing in normal code gives a strange error message:

<anon>:2:9: 2:11 error: expected one of `.`, `;`, `}`, or an operator, found `23`
<anon>:2     0b0123
                 ^~

However, it is valid to write such a thing in a macro (e.g. using the tt non-terminal), and thus lexing the adjacent digits as two tokens can lead to unexpected behaviour.

macro_rules! expr { ($e: expr) => { $e } }

macro_rules! add {
    ($($token: tt)*) => {
        0 $(+ expr!($token))*
    }
}
fn main() {
    println!("{}", add!(0b0123));
}

prints 24 (add expands to 0 + 0b01 + 23).

It would be nicer for both cases to print an error like:

error: found invalid digit `2` in binary literal
0b0123
    ^

(The non-macro case could be handled by detecting this pattern in the lexer and special casing the message, but this doesn't not handle the macro case.)

Code that wants two tokens can opt in to it by 0b01 23, for example. This is easy to write, and expresses the intent more clearly anyway.

Detailed design

The grammar that the lexer uses becomes

(0b[0-9]+ | 0o[0-9]+ | [0-9]+ | 0x[0-9a-fA-F]+) suffix

instead of just [01] and [0-7] for the first two, respectively.

However, it is always an error (in the lexer) to have invalid digits in a numeric literal beginning with 0b or 0o. In particular, even a macro invocation like

macro_rules! ignore { ($($_t: tt)*) => { {} } }

ignore!(0b0123)

is an error even though it doesn't use the tokens.

Drawbacks

This adds a slightly peculiar special case, that is somewhat unique to Rust. On the other hand, most languages do not expose the lexical grammar so directly, and so have more freedom in this respect. That is, in many languages it is indistinguishable if 0b1234 is one or two tokens: it is always an error either way.

Alternatives

Don't do it, obviously.

Consider 0b123 to just be 0b1 with a suffix of 23, and this is an error or not depending if a suffix of 23 is valid. Handling this uniformly would require "foo"123 and 'a'123 also being lexed as a single token. (Which may be a good idea anyway.)

Unresolved questions

None.