Allow a minimum complete implementation to be specified for mutually recursive default methods

Issue by huonw
Saturday Jul 13, 2013 at 08:41 GMT

For earlier discussion, see https://github.com/rust-lang/rust/issues/7771

This issue was labelled with: A-attributes, A-diagnostics, A-lint, A-traits, B-RFC, I-wishlist in the Rust repository

It should be possible to tell the compiler that at least one of a given set of default methods is required to be implemented, e.g.

#[requires(one_of(foo, bar), one_of(baz, qux))]
trait A {
   fn foo(&self) { self.bar() }
   fn bar(&self) { self.foo() }

   fn baz(&self) { self.qux() }
   fn qux(&self) { self.baz() }
}

impl A for int { // ok
   fn foo(&self) {}
   fn bar(&self) {}

   fn baz(&self) {} 
}

impl A for uint { // "requires at least one of foo or bar"
   fn baz(&self) {}
}

impl A for float { // "requires at least one of baz or qux"
   fn foo(&self) {}
}

This allows e.g. Eq to write eq and ne in terms of each other, without allowing the infinitely recursive impl Eq for Foo {}.

It could even allow saying "either foo or both bar and baz", e.g. #[requires(one_of(foo, all_of(bar, baz)))], and would hopefully warn if a non-default method was listed.

No files, tests, or compiler entry points are named. Start by reading the earlier discussion in issue 7771 and the mutually recursive trait examples here; clarify the semantics for one_of, all_of, and non-default methods. Done means the compiler can enforce the stated minimum implementations and diagnose an empty implementation.