reference lexer and grammar incorrectly handle tuple field expressions

[tromey]

The reference grammar in rust/src/grammar/lexer.l returns LIT_INTEGER for any sort of integer, including hex, octal, etc.

Then parser-lalr.y implements tuple field expressions as:

| expr '.' LIT_INTEGER                                { $$ = mk_node("ExprTupleIndex", 1, $1); }

However, rustc rejects hexadecimal here:

b7.rs:4:31: 4:34 error: invalid tuple or tuple struct index
b7.rs:4     println!("{}, {}", x.0, x.0x1);

My guess is that the reference lexer is incorrect here.

[tromey]

rustc also seems to reject decimal literals including underscores, so "x.1_" is invalid.

[alexcrichton]

cc #15880

[steveklabnik]

Triage: all of this is vestigial at this point, and the grammar WG is handling these kinds of issues. Closing.