Overriding default ToStr impl for @A where A: ToStr

[pkgw]

I'm running into an issue with an interaction between ToStr and pointers that I'm not sure how to handle. I have a struct with a ToStr, and I want to call that method on a @-pointer to the struct. However, to_str.rs provides a generic implementation:

impl<A: ToStr> @A: ToStr { ... }

that returns the to_str prefixed with "@". I do not want this behavior, but I can't figure out how to override it. My naive implementation is:

#[crate_type = "bin"];

pub struct Foo {
    v: uint,
}

impl Foo: to_str::ToStr {
    pure fn to_str (&self) -> ~str { ~"stacky" }
}

impl @Foo: to_str::ToStr {
    pure fn to_str (&self) -> ~str { ~"at-pointer" }
}

fn main () {
    let q = @Foo { v: 0 };
    io::print (q.to_str () + "\n");
}

But this gives a compile error about multiple implementations. Is there a way around this?

[nikomatsakis]

No, you can't do this.

[nikomatsakis]

One possible workaround is to make a "newtype" for your @Foo type, like this:

struct AtFoo(@Foo)

then use AtFoo everywhere.

Closing since this check is working as designed.

[pkgw]

If there's no simple workaround on that end, I'd argue that the default ToStr implementations for pointers-to-ToStr should be removed. If I have some non-small data structure and I care about the to_str(), I'm probably going to also be passing around pointers to the structure. The way I see it, it's far more likely that I'll want

(@foo).to_str() = foo.to_str()

rather than

(@foo).to_str() = "@" + foo.to_str()

but more broadly, I will want control over the to_str result without having to resort to awkward hacks such as AtFoo. As evidence for the superfluity of the current @(ToStr) and ~(ToStr) implementations, if I comment them out, the whole system recompiles just fine.

[pkgw]

Oh, and here's a variation that seems more like a genuine error in the impl-finder to me:

If I comment out both impls in my first example, unsurprisingly I get an error that there are no implementations for ToStr for <V7> which presumably corresponds to @Foo.

If I provide only the implementation for @Foo, but comment out the impl for plain Foo, I still get an error that there are two impls available: __extensions__::to_str and core::to_str::__extensions__::to_str. Looking at the set of impls in core.to_str; I think that the impl for @(ToStr) must be kicking in even though in this case only @Foo is ToStr, not Foo.

[catamorphism]

Removing the default implementations here is a change worth considering -- @pkgw , please file a new issue with the RFC tag if you want to propose that. Thanks!