1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82

//! Rust wrapper function which checks that the given closure does not modify input data.
//!
//! This is helpful when checking that a function which returns Err(...) does not have
//! side-effects.
//!
//! # Example
//!
//! ```
//! use assert_not_modified::assert_not_modified;
//!
//! // This bugged function wil return Err but still modify the data.
//! fn misleading_err(x: &mut i32) -> Result<(), String> {
//!     *x = *x + 1;
//!     // Throws an error but x is modified anyway. This is misleading.
//!     Err("Something wrong happened !".to_owned())
//! }
//!
//! // This test will expose the lying function :
//! assert!(std::panic::catch_unwind(|| {
//!     let mut x = 3;
//!     assert_not_modified(&mut x, |x| misleading_err(x)); // Panics
//! })
//! .is_err());
//! ```

/// Helper function which panics if the given closure mutates the input data.
///
/// # Panics
///
/// Panics if data is modified with message "Data was modified where it should not have been".
///
/// # Example
///
/// ```
/// use assert_not_modified::assert_not_modified;
///
/// // This bugged function wil return Err but still modify the data.
/// fn misleading_err(x: &mut i32) -> Result<(), String> {
///     *x = *x + 1;
///     // Throws an error but x is modified anyway. This is misleading.
///     Err("Something wrong happened !".to_owned())
/// }
///
/// // This test will expose the lying function :
/// assert!(std::panic::catch_unwind(|| {
///     let mut x = 3;
///     assert_not_modified(&mut x, |x| misleading_err(x)); // Panics
/// })
/// .is_err());
/// ```
pub fn assert_not_modified<F, Data, Any>(data: &mut Data, test: F)
where
    F: FnOnce(&mut Data) -> Any,
    Data: Clone + Eq + std::fmt::Debug,
{
    let old_data = data.clone();
    test(data);
    assert_eq!(
        &old_data, data,
        "Data was modified where it should not have been"
    );
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    #[should_panic(expected = "Data was modified where it should not have been")]
    fn test_should_not_modify_data_panics() {
        let mut x = 4;

        assert_not_modified(&mut x, |x| *x = 1);
    }

    #[test]
    fn test_should_not_modify_data_does_not_panic() {
        let mut x = 4;

        assert_not_modified(&mut x, |x| *x + 1);
    }
}