Document that assert! format arguments are evaluated lazily

It can be useful to do some computation in `assert!` format arguments, in order to get better error messages. For example:

```rust
assert!(
    some_condition,
    "The state is invalid. Details: {}",
    expensive_call_to_get_debugging_info(),
);
```

It seems like `assert!` only evaluates the format arguments if the assertion fails, which is useful but doesn't appear to be documented anywhere. This PR documents the behavior and adds some tests.

Author: not-an-aardvark

library/core/src/macros/mod.rs

@@ -1208,7 +1208,8 @@ pub(crate) mod builtin {
     ///
     /// This macro has a second form, where a custom panic message can
     /// be provided with or without arguments for formatting. See [`std::fmt`]
-    /// for syntax for this form.
+    /// for syntax for this form. Expressions used as format arguments will only
+    /// be evaluated if the assertion fails.
     ///
     /// [`std::fmt`]: ../std/fmt/index.html
     ///

src/test/ui/macros/assert-format-lazy.rs

@@ -0,0 +1,11 @@
+// run-pass
+
+#[allow(unreachable_code)]
+fn main() {
+    assert!(true, "Failed: {:?}", panic!("assert! evaluated format expressions"));
+    debug_assert!(true, "Failed: {:?}", panic!("debug_assert! evaluated format expressions"));
+    assert_eq!(1, 1, "Failed: {:?}", panic!("assert_eq! evaluated format expressions"));
+    debug_assert_eq!(1, 1, "Failed: {:?}", panic!("debug_assert_eq! evaluated format expressions"));
+    assert_ne!(1, 2, "Failed: {:?}", panic!("assert_ne! evaluated format expressions"));
+    debug_assert_ne!(1, 2, "Failed: {:?}", panic!("debug_assert_ne! evaluated format expressions"));
+}