add uitest for issue 12816

Author: tesuji

tests/ui/assertions_on_constants.rs

@@ -53,3 +53,9 @@ fn main() {
     const N: usize = 1024;
     const _: () = assert!(N.is_power_of_two());
 }
+
+#[allow(clippy::eq_op)]
+const _: () = {
+    assert!(true);
+    assert!(8 == (7 + 1));
+};

tests/ui/assertions_on_constants.stderr

@@ -80,5 +80,21 @@ LL |     const _: () = assert!(true);
    |
    = help: remove it
 
-error: aborting due to 10 previous errors
+error: `assert!(true)` will be optimized out by the compiler
+  --> tests/ui/assertions_on_constants.rs:59:5
+   |
+LL |     assert!(true);
+   |     ^^^^^^^^^^^^^
+   |
+   = help: remove it
+
+error: `assert!(true)` will be optimized out by the compiler
+  --> tests/ui/assertions_on_constants.rs:60:5
+   |
+LL |     assert!(8 == (7 + 1));
+   |     ^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: remove it
+
+error: aborting due to 12 previous errors