Support RangeFrom ([x..]) and RangeFromTo ([x..y]) in the parser

Parsing the .. (DOT_DOT) operator to get a range had two
issues. Trying to compile:

  let block = [1,2,3,4,5];
  let _rf = &block[1..];
  let _rt = &block[..3];
  let _rft = &block[2..4];

range.rs:4:23: error: found unexpected token ‘]’ in null denotation
    4 |   let _rf = &block[1..];
      |                       ^
range.rs:4:24: error: expecting ‘]’ but ‘;’ found
    4 |   let _rf = &block[1..];
      |                        ^

Since .. can represent either a range from or a range from-to it can
be followed by an expression or not. We do have a hack in our
pratt-parser so that it is allowed to return a nullptr. But even in
that case it will have swallowed the next token. Add another hack to
the pratt-parser so that if the next token is one that cannot start an
expression and the caller allows a nullptr return then don't skip the
token and return immediately.

After this patch we can parse the above range expressions, but we
still don't handle them fully:

range.rs:4:20: fatal error: Failed to lower expr: [1..]
    4 |   let _rf = &block[1..];
      |                    ^

Ranges are actually syntactic sugar for std::ops::Range[From|To].

0 files changed, 0 insertions, 0 deletions