internal compiler error: no enclosing scope with id XY

[Blub]

When a method returns an &self/mut Foo, and the caller doesn't store the returned value somewhere, there's an internal compiler error. See the following code. Note how when storing the result of x.stuff() in a variable makes it work.

struct Foo { x: int }

impl Foo {
    fn stuff(&mut self) -> &self/mut Foo {
        return self;
    }
}

fn main() {
    let mut x = @mut Foo { x: 3 };
    x.stuff(); // error: internal compiler error: no enclosing scope with id 49
    // storing the result removes the error, so replacing the above
    // with the following, works:
    // let _y = x.stuff()

    // also making 'stuff()' not return anything fixes it
    // I guess the "dangling &ptr" cuases issues?
}

[timford]

I was asked to add the following, which exhibits the same or a similar problem, encountered using:
rustc 0.5 (07edf90 2012-10-13 05:57:13 -0700)
host: x86_64-unknown-linux-gnu

use std::net_url;
use core::result;

fn main() {
        let urlstr = "test";

        // this works without error or warning
        let r: &result::Result<net_url::Url,~str> = &net_url::from_str(urlstr);
        result::get_ref( r );

        // this gives me:
        // error: illegal borrow: borrowed value does not live long enough
        // note: borrowed pointer must be valid for unknown scope: 59.  Please report a bug....
        result::get_ref( &net_url::from_str(urlstr) );
}

[catamorphism]

Reproduced as of d2ad028

[catamorphism]

I have a fix; running tests.

[erickt]

I just ran into a similar error with this code:

struct Parser {
    mut otag: @~str,
}

fn add_tag(self: &Parser) {
    *self.otag + *self.otag;
}

fn main() {}

That resulted in this error:

error: internal compiler error: no enclosing scope with id 21

I haven't been able to reduce it down beyond this.

[catamorphism]

I know what the bug is here, and had a fix in #4192, but the problem was that it degraded performance. I can probably come up with a fix to borrowck instead of trans that won't degrade performance. Just letting you know, @erickt , that the bug is still being worked on (I've been on leave so I didn't get to it sooner).

[catamorphism]

Fixed in 9d67267 -- but see the comments in borrowck::preserve from that commit for a warning.