Closures inside `#[pure]` functions trigger "impure function" and "unsupported constant type" errors

[lynae99]

Using a closure inside a function marked with #[pure] causes Prusti to report that std::ops::Fn::call is impure. It also triggers an unsupported constant type error for the closure itself.
Reproduction

use prusti_contracts::*;

#[pure]
fn check() -> bool {
    // A trivial closure inside a pure function
    (|| { true })()
}

fn main() {}

Actual Behavior
Prusti reports the following errors:

1. error: [Prusti: invalid specification] use of impure function "std::ops::Fn::call" in pure code is not allowed
2. error: [Prusti: unsupported feature] unsupported constant type ... Closure(...)

Expected Behavior
Trivially pure closures (like the one above) should be accepted inside #[pure] functions.