range syntax doesn't work with non-literal endpoints in for statements

[blaenk]

Range syntax doesn't work with non-literal endpoints. It seems that the parser tries to parse a structure definition or instantiation.

I don't know if this is known and just not completely implemented yet, but I figured I'd give the heads up.

fn main() {
  let x = 10;

  for z in 0u..x {
    println!("{}", z);
  }
}

:5:12: 5:13 error: expected :, found !
:5 println!("{}", z);

playpen

[huonw]

It should work with explicit disambiguation via parens for i in (0..n) { ....

This seems like it may be something that crops up often so it's probably worth at least improving the diagnostics.

[bstrie]

@huonw I don't understand why the parens are necessary, surely that's not our plan for 1.0. Why can't the parser unambiguously detect that .. denotes a range in that context?

[blaenk]

Especially since we've already gone out of our way to prevent other ambiguities.

[huonw]

@bstrie it is detecting that .. denotes a range. This isn't an ambiguity with .. specifically, it's a problem with the <ident> { token sequence at the end of the .. expression. It looks it just needs to have the same special case in for position as exists for for i in x { ... and for i in x+y { ....

[huonw]

#20830 (and #20858) has some more analysis.