Implement `impl Trait` in argument position (RFC1951, Universal quantification) #45918

With the check for impl trait moving from type checking to HIR lowering the error needs to move too.

Replace hir::TyImplTrait with TyImplTraitUniversal and TyImplTraitExistential. Add an ImplTraitContext enum to rustc::hir::lowering to track the kind and allowedness of an impl Trait. Significantly alter lowering to thread ImplTraitContext and one other boolean parameter described below throughought much of lowering. The other parameter is for tracking if lowering a function is in a trait impl, as there is not enough information to otherwise know this information during lowering otherwise. This change also removes the checks from ast_ty_to_ty for impl trait allowedness as they are now all taking place in HIR lowering.

This is for tracking if an ImplItem is part of a trait impl. Add a with_trait_impl_ref method to ItemLowerer to appropriately save the state to allow appropriate nesting of trait and non-trait impls.

In ast_generics extraction in generics_of and explicit_predicates_of, also collect inputs if there are any. Then use a Visitor to extract the necessary information from the TyImplTraitUniversal types before extending generics and predicates with the new information.

First some background: To the compiler, the following two signatures in the trait vs the impl are the same. ```rust trait Foo { fn foo(&self, &impl Debug); } impl Foo for () { fn foo<U: Debug>(&self, x: &U) { ... } } ``` We do not want to allow this, and so we add a new error and check. The check just tests that all paramters 'syntheticness' match up. As during collection, the impl Trait parameters are transformed into anonymous synthetic generics. Furthermore, causes a check for unused type parameters to be skipped in check_bounds_are_used if there is already a TyError. Thus, an unused input will not trigger `type parameter unused` errors. Update the one test that checked for this error in the case of a TyError.

Move feature gate check to inside HIR lowering. Change error messages and update tests.

also merge disallowed and disallowed-2 into that set

It'd be good to have a positive test for each case where it is allowed, I should think.

Uses Symbol::intern and hir.node_to_pretty_string to create a name for the impl Trait parameter that is just impl and then a ' + ' separated list of bounds that the user typed.

Add requested comments, restructure some small bits of code. Fix extern declarations allowing impl Trait.

We already disallowed them to be in the arg list, such as Fn(impl Debug), but now we disallow Fn() -> impl Debug. Also remove the ImplTraitContext argument from the function lower_parenthesized_parameter_data as it is now unused. Comment out part of test run-pass/impl-trait/xcrate.rs that now fails.

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Implement `impl Trait` in argument position (RFC1951, Universal quantification) #45918

Implement `impl Trait` in argument position (RFC1951, Universal quantification) #45918

Commits on Nov 15, 2017

Implement impl Trait in argument position (RFC1951, Universal quantification) #45918

Implement impl Trait in argument position (RFC1951, Universal quantification) #45918

Commits on Nov 15, 2017

Implement `impl Trait` in argument position (RFC1951, Universal quantification) #45918

Implement `impl Trait` in argument position (RFC1951, Universal quantification) #45918