diff options
3 files changed, 44 insertions, 1 deletions
diff --git a/compiler/rustc_trait_selection/src/traits/coherence.rs b/compiler/rustc_trait_selection/src/traits/coherence.rs index 8ab1aa65d3a..a54b36262b2 100644 --- a/compiler/rustc_trait_selection/src/traits/coherence.rs +++ b/compiler/rustc_trait_selection/src/traits/coherence.rs @@ -734,7 +734,21 @@ impl<'tcx> TypeVisitor<'tcx> for OrphanChecker<'tcx> { result } - // FIXME: Constants should participate in orphan checking. + /// All possible values for a constant parameter already exist + /// in the crate defining the trait, so they are always non-local[^1]. + /// + /// Because there's no way to have an impl where the first local + /// generic argument is a constant, we also don't have to fail + /// the orphan check when encountering a parameter or a generic constant. + /// + /// This means that we can completely ignore constants during the orphan check. + /// + /// See `src/test/ui/coherence/const-generics-orphan-check-ok.rs` for examples. + /// + /// [^1]: This might not hold for function pointers or trait objects in the future. + /// As these should be quite rare as const arguments and especially rare as impl + /// parameters, allowing uncovered const parameters in impls seems more useful + /// than allowing `impl<T> Trait<local_fn_ptr, T> for i32` to compile. fn visit_const(&mut self, _c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> { ControlFlow::CONTINUE } diff --git a/src/test/ui/coherence/auxiliary/trait-with-const-param.rs b/src/test/ui/coherence/auxiliary/trait-with-const-param.rs new file mode 100644 index 00000000000..a44eb14f8e4 --- /dev/null +++ b/src/test/ui/coherence/auxiliary/trait-with-const-param.rs @@ -0,0 +1 @@ +pub trait Trait<const N: usize, T> {} diff --git a/src/test/ui/coherence/const-generics-orphan-check-ok.rs b/src/test/ui/coherence/const-generics-orphan-check-ok.rs new file mode 100644 index 00000000000..217e8aed234 --- /dev/null +++ b/src/test/ui/coherence/const-generics-orphan-check-ok.rs @@ -0,0 +1,28 @@ +// check-pass +// aux-build:trait-with-const-param.rs +extern crate trait_with_const_param; +use trait_with_const_param::*; + +// Trivial case, const param after local type. +struct Local1; +impl<const N: usize, T> Trait<N, T> for Local1 {} + +// Concrete consts behave the same as foreign types, +// so this also trivially works. +impl Trait<3, Local1> for i32 {} + +// This case isn't as trivial as we would forbid type +// parameters here, we do allow const parameters though. +// +// The reason that type parameters are forbidden for +// `impl<T> Trait<T, LocalInA> for i32 {}` is that another +// downstream crate can add `impl<T> Trait<LocalInB, T> for i32`. +// As these two impls would overlap we forbid any impls which +// have a type parameter in front of a local type. +// +// With const parameters this issue does not exist as there are no +// constants local to another downstream crate. +struct Local2; +impl<const N: usize> Trait<N, Local2> for i32 {} + +fn main() {} |