error[E0635]: unknown feature `const_default_impls`
  --> $DIR/derive-const-use.rs:3:41
   |
LL | #![feature(const_trait_impl, const_cmp, const_default_impls, derive_const, effects)]
   |                                         ^^^^^^^^^^^^^^^^^^^

error: const `impl` for trait `Default` which is not marked with `#[const_trait]`
  --> $DIR/derive-const-use.rs:7:12
   |
LL | impl const Default for A {
   |            ^^^^^^^
   |
   = note: marking a trait with `#[const_trait]` ensures all default method bodies are `const`
   = note: adding a non-const method body in the future would be a breaking change

error: const `impl` for trait `Default` which is not marked with `#[const_trait]`
  --> $DIR/derive-const-use.rs:15:16
   |
LL | #[derive_const(Default, PartialEq)]
   |                ^^^^^^^
   |
   = note: marking a trait with `#[const_trait]` ensures all default method bodies are `const`
   = note: adding a non-const method body in the future would be a breaking change
   = note: this error originates in the derive macro `Default` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0207]: the const parameter `host` is not constrained by the impl trait, self type, or predicates
  --> $DIR/derive-const-use.rs:7:6
   |
LL | impl const Default for A {
   |      ^^^^^ unconstrained const parameter
   |
   = note: expressions using a const parameter must map each value to a distinct output value
   = note: proving the result of expressions other than the parameter are unique is not supported

error[E0207]: the const parameter `host` is not constrained by the impl trait, self type, or predicates
   |
   = note: expressions using a const parameter must map each value to a distinct output value
   = note: proving the result of expressions other than the parameter are unique is not supported

error: aborting due to 5 previous errors

Some errors have detailed explanations: E0207, E0635.
For more information about an error, try `rustc --explain E0207`.