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Document that assert! format arguments are evaluated lazily

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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.
This commit is contained in:
Teddy Katz 2021-02-15 21:06:00 -05:00
parent d1206f950f
commit cb653b100c
2 changed files with 13 additions and 1 deletions
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11
src/test/ui/macros/assert-format-lazy.rs Normal file
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// 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"));
}