error[E0015]: cannot call non-const closure in constant functions
  --> $DIR/issue-56164.rs:1:18
   |
LL | const fn foo() { (||{})() }
   |                  ^^^^^^^^
   |
   = note: closures need an RFC before allowed to be called in constant functions
   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants

error: function pointer calls are not allowed in constant functions
  --> $DIR/issue-56164.rs:5:5
   |
LL |     input()
   |     ^^^^^^^

note: erroneous constant used
  --> $DIR/issue-56164.rs:1:18
   |
LL | const fn foo() { (||{})() }
   |                  ^^^^^^

note: erroneous constant used
  --> $DIR/issue-56164.rs:1:18
   |
LL | const fn foo() { (||{})() }
   |                  ^^^^^^

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0015`.