Dereference on single element tags ignores visibility of constructor

[msullivan]

Exporting a tag type but not the constructors for it is a simple way to construct abstract data types. (This is how it is done in Haskell.) This works in rust as well, but only for tags with multiple constructors. If the tag has only a single constructor, * will see through it regardless of the visibility of the constructor.

Example

mod ctr {
    export ctr;
    export new; export inc; export print;

    tag ctr { mkCtr(int); }

    fn new(i: int) -> ctr { mkCtr(i) }
    fn inc(c: ctr) -> ctr { mkCtr(*c + 1) }
    fn print(c: ctr) { log_err *c; }
}


fn main() {
    let c = ctr::new(42);
    ctr::print(c);
    let c2 = ctr::inc(c);
    ctr::print(c2);
    log_err *c2;
}

Part of the problem might be that dereference on single element tags isn't a particularly principled idea to begin with, but it is undeniably convenient.

[msullivan]

This is one of two interrelated complaints in #802.

[brson]

Updated test case:

mod ctr {
    export ctr::{};
    export new; export inc; export print;

    enum ctr { mk_ctr(int) }

    fn new(i: int) -> ctr { mk_ctr(i) }
    fn inc(c: ctr) -> ctr { mk_ctr(*c + 1) }
    fn print(c: ctr) { log(error, *c); }
}


fn main() {
    let c = ctr::new(42);
    ctr::print(c);
    let c2 = ctr::inc(c);
    ctr::print(c2);
    log(error, *c2);
}

[catamorphism]

This seems like it's worth fixing. I will take it on.

[catamorphism]

Blocked on #3993

[catamorphism]

#3993 is fixed so I should be able to fix this now.

[catamorphism]

Done, pending review: #4084