region error "Cannot encode region variables"

[nejucomo]

The following code causes the compiler to fail. Conversation on IRC suggests this is a known issue, but I could not find this specific error by searching tickets for "Cannot encode region variables" nor any of the hits for "region variables".

I apologize if this is a duplicate.

$ cat ./vecpeek.rs
extern mod std; // Only for tests.


fn vec_peek<T>(v: &r/[T]) -> Option< (&r/T, &r/[T]) > {
    if v.len() == 0 {
        None
    } else {
        let head = &v[0];
        let tail = v.view(1, v.len());
        Some( (head, tail) )
    }
}


mod tests {
    /* Testing the different storage locations covers features which
     * should be completely orthogonal (ie: that regions work with any
     * storage area, but the language is young, so we do test just
     * in case.
     */
    #[test]
    fn test_peek_empty_stack() {
        let v : &[int] = &[];
        assert (None == vec_peek(v));
    }

    #[test]
    fn test_peek_empty_unique() {
        let v : ~[int] = ~[];
        assert (None == vec_peek(v));
    }

    #[test]
    fn test_peek_empty_managed() {
        let v : @[int] = @[];
        assert (None == vec_peek(v));
    }

    /*
    #[test]
    fn test_peek_full_stack() {
        let v: &[int] = &[1, 2, 3];
        let head: &int = &v[0];
        let tail: &[int] = v.view(1, v.len());

        match

        assert (None == vec_peek(v));
    }
    */
}

$ rustc --test ./vecpeek.rs
error: internal compiler error: Cannot encode region variables

$ rustc --version
rustc 0.5 (07edf90 2012-10-13 05:57:13 -0700)
host: x86_64-unknown-linux-gnu

[nikomatsakis]

hmm. I wasn't aware of this error. Sounds like a failing in region inference somehow (underconstrained?)

[catamorphism]

As of 89ed595 , this fails with a different error:

------------------------------------------
stderr:
------------------------------------------
/Users/tchevalier/rust/src/test/run-pass/issue-3888.rs:6:19: 6:20 error: illegal borrow: borrowed value does not live long enough
/Users/tchevalier/rust/src/test/run-pass/issue-3888.rs:6         let tail = v.view(1, v.len());
                                                                            ^
/Users/tchevalier/rust/src/test/run-pass/issue-3888.rs:1:54: 9:1 note: borrowed pointer must be valid for the lifetime &r as defined on the block at 1:54...
/Users/tchevalier/rust/src/test/run-pass/issue-3888.rs:1 fn vec_peek<T>(v: &r/[T]) -> Option< (&r/T, &r/[T]) > {
/Users/tchevalier/rust/src/test/run-pass/issue-3888.rs:2     if v.len() == 0 {
/Users/tchevalier/rust/src/test/run-pass/issue-3888.rs:3         None
/Users/tchevalier/rust/src/test/run-pass/issue-3888.rs:4     } else {
/Users/tchevalier/rust/src/test/run-pass/issue-3888.rs:5         let head = &v[0];
/Users/tchevalier/rust/src/test/run-pass/issue-3888.rs:6         let tail = v.view(1, v.len());
                                                         ...
note: ...but borrowed value is only valid for unknown scope: 79.  Please report a bug.
error: aborting due to previous error

Not sure if the illegal borrow part is intended or not, but the "unknown scope" part is at least a bad error message.

[catamorphism]

I fixed the unknown scope error in my branch, but I'm still puzzled. I think the code should be valid.

I figured out that this minimized version still errors out:

fn vec_peek<T>(v: &r/[T]) -> &r/[T] {
    v.view(1, 5)
}

but this one compiles successfully:

fn vec_peek<T>(v: &r/[T]) -> &r/[T] {
    vec::view(v, 1, 5)
}

so I'm guessing this has to do with a bug with autoslicing and methods.

[pnkfelix]

The "cannot encode region variables" error has been fixed.

I verified the results reported by catamorphism. That may be a bug, but the title of this bug is confusing matters. So I will open up a separate bug for the borrow checking issue.

[pnkfelix]

Spawned off new ticket #5541. Closing this one.