error[E0261]: use of undeclared lifetime name `'db`
  --> $DIR/nested-impl-trait-in-tait.rs:3:40
   |
LL | pub type Tait = impl Iterator<Item = (&'db Key, impl Iterator)>;
   |                                        ^^^ undeclared lifetime
   |
   = note: for more information on higher-ranked polymorphism, visit https://doc.rust-lang.org/nomicon/hrtb.html
help: consider making the bound lifetime-generic with a new `'db` lifetime
   |
LL | pub type Tait = impl for<'db> Iterator<Item = (&'db Key, impl Iterator)>;
   |                      ++++++++
help: consider introducing lifetime `'db` here
   |
LL | pub type Tait<'db> = impl Iterator<Item = (&'db Key, impl Iterator)>;
   |              +++++

error[E0412]: cannot find type `Key` in this scope
  --> $DIR/nested-impl-trait-in-tait.rs:3:44
   |
LL | pub type Tait = impl Iterator<Item = (&'db Key, impl Iterator)>;
   |                                            ^^^ not found in this scope
   |
help: consider importing this struct
   |
LL + use std::thread::local_impl::Key;
   |

error: unconstrained opaque type
  --> $DIR/nested-impl-trait-in-tait.rs:3:17
   |
LL | pub type Tait = impl Iterator<Item = (&'db Key, impl Iterator)>;
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `Tait` must be used in combination with a concrete type within the same module

error: unconstrained opaque type
  --> $DIR/nested-impl-trait-in-tait.rs:3:49
   |
LL | pub type Tait = impl Iterator<Item = (&'db Key, impl Iterator)>;
   |                                                 ^^^^^^^^^^^^^
   |
   = note: `Tait` must be used in combination with a concrete type within the same module

error: aborting due to 4 previous errors

Some errors have detailed explanations: E0261, E0412.
For more information about an error, try `rustc --explain E0261`.