Convert T::Foo resolution to use where clauses, not bounds
#20300
Comments
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Nominating and placing on 1.0 alpha milestone. Ultimately PROBABLY a nice to have, but it's really surprising that this doesn't work. |
nikomatsakis
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T::Foo resolution to use where clauses, not bounds`T::Foo resolution to use where clauses, not bounds
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cc me |
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Until this is implemented, you can also use a qualified type path trait Trait {
type Item;
}
struct Foo;
impl Foo {
fn foo1<T: Trait>(_x: T::Item) {}
fn foo2<T>(_x: <T as Trait>::Item) where T: Trait {}
}
struct Bar;
impl Trait for Bar {
type Item = i32;
}
fn main() {
Foo::foo1::<Bar>(5i32);
Foo::foo2::<Bar>(5i32);
} |
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cc me |
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Would anyone mind mentoring this? |
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@ahmedcharles a fix is in progress already, actually, but it turns out to be quite a tricky problem. Much trickier than it appeared at first. |
This was referenced Feb 21, 2015
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Feb 25, 2015
…ds, r=nikomatsakis This is a fix for #20300 though as a side-sweep it fixes a number of stack overflows because it integrates cycle detection into the conversion process. I didn't go through and retest everything. The tricky part here is that in some cases we have to go find the information we need from the AST -- we can't use the converted form of the where-clauses because we often have to handle something like `T::Item` *while converting the where-clauses themselves*. Since this is also not a fixed-point process we can't just try and keep trying to find the best order. So instead I modified the `AstConv` interface to allow you to request the bounds for a type parameter; we'll then do a secondary scan of the where-clauses to figure out what we need. This may create a cycle in some cases, but we have code to catch that. Another approach that is NOT taken by this PR would be to "convert" `T::Item` into a form that does not specify what trait it's using. This then kind of defers the problem of picking the trait till later. That might be a good idea, but it would make normalization and everything else much harder, so I'm holding off on that (and hoping to find a better way for handling things like `i32::T`). This PR also removes "most of" the `bounds` struct from `TypeParameterDef`. Still a little ways to go before `ParamBounds` can be removed entirely -- it's used for supertraits, for example (though those really ought to be converted, I think, to a call to `get_type_parameter_bounds` on `Self` from within the trait definition). cc @jroesch Fixes #20300
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Currently referencing an associated type like
T::Foosearches the bounds onTrather than the full set of where clauses. This is no good.The text was updated successfully, but these errors were encountered: