Bad parse error message for missing alt close brace

[catamorphism]

In this code:

fn foo() {
  match Some(x) {
      Some(y) { fail!(); }
      None    { fail!(); }
}

fn bar() {
    let mut i = 0;
    while (i < 1000) {}
}

fn main() {}

foo is missing the close brace for the alt, but the resulting parse error message is very unhelpful:

src/test/compile-fail/bad.rs:12:13: 12:13 error: unexpected token: '<eof>'
src/test/compile-fail/bad.rs:12 fn main() {}
                                                                   ^

It would be much more helpful to report the error at the location where the un-closed brace was (in this case, after the alt in foo). I don't know how easy it would be to do with the current parser, though.

[nikomatsakis]

I have often wanted this too, though I imagine it is quite hard to correctly decide which brace is the unclosed one. After all, if you put a } at the end of the file, this would be perfectly valid Rust. The only way I can see to decide that the alt is at fault is through a heuristic based on the indentation---which would be quite useful! Something like missing close brace, most likely here.

[kud1ing]

Maybe duplicate of #643

[nikomatsakis]

The example in #643 is considerably easier than this one.

[kud1ing]

@nikomatsakis: Do you mean it does not cover the same issue? Isn't the underlying problem "we don't know where the missing brace is, until we hit EOF" the same?

[emberian]

$ rustc foo.rs
foo.rs:12:11: 12:12 error: file ended with unbalanced delimiters
foo.rs:12 fn main() {}
                     ^

Is the current error.

[graydon]

Updated code during triage. I get the same error @cmr is seeing now, which is not bad but we could possibly do better if we pointed to, say, the current still-open delimiter. It might not be the unbalanced one, but it's a place to point the user at least.

[catamorphism]

(bug triage) Milestone looks right; carry on.

[catamorphism]

I'm getting annoyed with this enough to want to fix it. For me, the issue comes up pretty often when I do a merge and accidentally delete a brace, and then it can be quite a while before I see the compile error and I've forgotten what part of the code I edited.

It doesn't seem like it would be impossible to keep a stack of open delimiters and refer to their spans in the error message. In any case, I'm going to try to fix this in my next spare moment.