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Fix the rule introduced by #176 and accord doc with rustc-guide #184

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merged 1 commit into from Dec 17, 2018

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scalexm commented Oct 17, 2018

The rule introduced in #176 was too restrictive, as can be seen with this example:

trait Bar { }

trait Baz { }
// Implemented(Self: Baz) :- FromEnv(Self: Baz).

trait Foo {
    type Item<T>: Baz where T: Bar;
}

// (*) FromEnv(<Self as Foo>::Item<T>: Baz) :- FromEnv(Self: Foo), FromEnv(T: Bar).

struct AlwaysBar<T> { }
impl<T> Bar for AlwaysBar<T> { }

fn baz<X: Baz>() { }

fn foo<X: Foo, T>() // where AlwaysBar<T>: Bar
{
    // We must prove: `Implemented(<X as Foo>::Item<AlwaysBar<T>>: Baz)`.
    // The only way to prove it is to use the rule (*).
    // This requires `FromEnv(Always<T>: Bar)`, which we don't have in our env.
    baz::<X::Item<AlwaysBar<T>>>()
}

So we change it to be:

FromEnv(<Self as Foo>::Item<T>: Baz) :- FromEnv(Self: Foo), Implemented(T: Bar).

(I also accorded the doc with rust-lang/rustc-guide#210)

@nikomatsakis nikomatsakis self-assigned this Dec 17, 2018
@nikomatsakis nikomatsakis merged commit cd35595 into rust-lang:master Dec 17, 2018
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@scalexm scalexm deleted the scalexm:fix-176-and-doc branch Jan 8, 2019
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