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

[huonw]

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.

[huonw]

This feature recently landed in GHC: http://ghc.haskell.org/trac/ghc/ticket/7633. Example of the syntax:

    {-# MINIMAL (.&.), (.|.), xor, complement,
                (shift | (shiftL, shiftR)),
                (rotate | (rotateL, rotateR)),
                bitSize, bitSizeMaybe, isSigned, testBit, bit, popCount #-}

Of particular note is that it supports nested conjunction/disjunction (, == and, | == or).

(taken from http://ghc.haskell.org/trac/ghc/changeset/7ab62490dbc1ea7430fe60d5dad9c3b0fa0bedad/base)

[huonw]

Triage, no change.

[pnkfelix]

cc me

[huonw]

I think this can be entirely implemented as a lint plugin out of the the compiler, although some traits in the standard library (std::rand::Rng is one example I definitely know) would like it.

[SimonSapin]

@huonw would that mean that incorrect impl in a different crate will build without error unless they load the lint themselves?

[huonw]

Yes it would, which is a good point, since it reduces the usefulness of the lint. But, it is just some assistance for avoiding accidental infinite recursion, i.e. not required for safety, or to get a correct impl. (I am personally in favour of it being in the compiler itself.)

It can certainly be prototyped out of tree to avoid the horrendous bootstrap times.