error: const `impl` for trait `A` which is not `const`
  --> $DIR/const-impl-requires-const-trait.rs:6:12
   |
LL | impl const A for () {}
   |            ^ this trait is not `const`
   |
   = note: marking a trait with `const` ensures all default method bodies are `const`
   = note: adding a non-const method body in the future would be a breaking change
help: mark `A` as `const` to allow it to have `const` implementations
   |
LL | #[const_trait] pub trait A {}
   | ++++++++++++++

error: aborting due to 1 previous error