expected expression, found ..

[CAD97]

Given the following code:

let _ = &..;

The current output is:

error: expected expression, found `..`
 --> src/main.rs:2:14
  |
2 |     let _ = &..;
  |              ^^ expected expression

.. is a valid expression (a RangeFull literal), so this error is misleading. It should at a minimum have a suggestion to replace with the valid version &(..), and probably should just work. It also imho ideally shouldn't be a hard parser error blocking semantic passes; the parse of &(..) is fairly unambiguously intended, even if we decide we should still require the parenthesis.

[agluszak]

I'm interested! Where should I look first? (I have no experience in rustc, some general experience in compilers tho)

[CAD97]

The rustc dev guide is a good place to start. If you have some compiler development experience already, that should get you going quickly enough.

One specifically relevant section is -Ztreat-err-as-bug, which will allow you to get a backtrace for what code is emitting this error. Then it should just be a matter of threading the proper information through. The rustdoc for the compiler crates is also available online; rustc_parse is likely to be where you'll need to change.

Also, the rustbot docs; you can @rustbot claim to assign yourself to the issue to track that you're working on it.

[CAD97]

For extra context, .. is accepted in expression position at least for both let _ = ..;, macro_rules! m {($x:expr) => {}} m!(..), and obviously &slice[..], so I have absolutely no idea what makes &.. different that it causes a parser error in this case.

[RedDocMD]

@rustbot claim