Macro check for assertion_on_constants lint

[flip1995]

The assertion_on_constants lint currently has following output for this code Playground:

macro_rules! assert_const {
    ($len:expr) => {
        assert!($len > 0);
    }
}

fn main() {
    assert_const!(3);
    assert_const!(-1);
}

warning: assert!(const: true) will be optimized out by the compiler
 --> src/main.rs:3:9
  |
3 |         assert!($len > 0);
  |         ^^^^^^^^^^^^^^^^^^
...
8 |     assert_const!(3);
  |     ---------------- in this macro invocation
  |
  = note: #[warn(clippy::assertions_on_constants)] on by default
  = help: remove it
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#assertions_on_constants

warning: assert!(const: false) should probably be replaced
 --> src/main.rs:3:9
  |
3 |         assert!($len > 0);
  |         ^^^^^^^^^^^^^^^^^^
...
9 |     assert_const!(-1);
  |     ----------------- in this macro invocation
  |
  = help: use panic!() or unreachable!()
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#assertions_on_constants

This is contradictory. This lint should not trigger if the assert! is in a macro itself.

[oli-obk]

@bors r+

[bors]

📌 Commit cb2d987 has been approved by oli-obk

[bors]

⌛ Testing commit cb2d987 with merge af43950...

[bors]

☀️ Test successful - checks-travis, status-appveyor
Approved by: oli-obk
Pushing af43950 to master...