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
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
# assertr
A fluent assertion library for Rust that enables clear, readable test code with detailed failure
messages that help pinpoint issues quickly.
## Features
- 🔗 **Fluent API**: Chain multiple assertions for improved readability.
Fluent assertions provide better IDE support through method chaining. The IDE can show you exactly what
assertions are available for your specific type, making it hard to write invalid assertions and easier to discover
available checks.
- 🎯 **Type-specific Assertions**: Specialized checks for many Rust types, plus broad generic coverage.
- 📝 **Detailed Error Messages**: Clear, context-rich failure messages. Any assertion can extend the context with
additional descriptive output.
- 🔄 **Capture Mode**: Collect assertion failures for manual inspection instead of immediately panicking.
- 🛠 **Extensible**: Easily add custom assertions for your own types.
- ⚡ **Derive Macros**: Perform partial struct assertions with the help of the `#[derive(AssertrEq)]` macro.
## Installation
### Default setup
```toml
[dependencies]
assertr = "0.5.6"
```
### Cargo features
Available individual features and feature groups:
| feature | description | default feature |
|-----------|-----------------------------------------------------------------------|-----------------|
| std | Assertions for types from the standard library. | yes |
| derive | Enables the `AssertrEq` derive macro. | no |
| fluent | Enables `.must()` / `.verify()` entry points and fluent aliases. | no |
| num | Assertions for numeric types. | yes |
| libm | Use fallback implementations for Rust's float math functions in core. | no |
| serde | Assertions for serializable types (supporting json and toml). | no |
| jiff | Assertions for types from the `jiff` crate. | no |
| http | Assertions for types from the `http` crate. | no |
| tokio | Assertions for types from the `tokio` crate. | no |
| reqwest | Assertions for types from the `reqwest` crate. | no |
| rootcause | Assertions for types from the `rootcause` crate. | no |
| program | Assertions for the provided `Program` type. | no |
| feature-group | description |
|---------------|----------------------------------------------------------------------|
| default | Small set of features, enabling support for `std` types and numbers. |
| full | Enables all features listed above. |
All optional features are additive.
### no_std
Disable the default features in `no_std` environments:
```toml
[dependencies]
assertr = { version = "0.5.6", default-features = false }
```
If you still want numeric assertions in `no_std`, enable `num`. For floating-point classification
helpers such as `is_nan()`, `is_finite()`, or `is_infinite()` without `std`, also enable `libm`.
## Quick start
Always prefer importing the entire prelude:
```rust
use assertr::prelude::*;
#[test]
fn test() {
assert_that!("hello, world!")
.starts_with("hello")
.ends_with("!");
}
```
This gives you full IDE autocomplete for the assertions available on the current subject type.
If the `fluent` feature is enabled, you can also enter assertion contexts directly from values:
```rust
use assertr::prelude::*;
#[test]
fn test() {
"hello, world!"
.must()
.start_with("hello")
.end_with("!");
let failures = 3
.verify()
.be_equal_to(4)
.capture_failures();
assert_that!(failures).have_length(1);
}
```
## Available Assertions
This table is meant as a reference. In day-to-day use, `use assertr::prelude::*;` plus IDE
autocomplete is usually the fastest way to discover what is available.
Roughly, the table is grouped like this:
- Core assertions and collection/string helpers first
- `std` assertions next
- Optional integrations (`http`, `tokio`, `reqwest`, `program`, `rootcause`, `jiff`) last
| type / required bounds | assertion | note | required features |
|-------------------------------------------|---------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------|-------------------|
| `T: PartialEq` | `is_equal_to(expected)` | | |
| `T: PartialEq` | `is_not_equal_to(expected)` | | |
| `T: PartialOrd<E>` | `is_less_than(expected)` | | |
| `T: PartialOrd<E>` | `is_greater_than(expected)` | | |
| `T: PartialOrd<E>` | `is_less_or_equal_to(expected)` | | |
| `T: PartialOrd<E>` | `is_greater_or_equal_to(expected)` | | |
| `bool` | `is_true()` | | |
| `bool` | `is_false()` | | |
| `char` | `is_equal_to_ignoring_ascii_case(expected)` | | |
| `char` | `is_lowercase()` | | |
| `char` | `is_uppercase()` | | |
| `char` | `is_ascii_lowercase()` | | |
| `char` | `is_ascii_uppercase()` | | |
| `&str` | `is_blank()` | | |
| `&str` | `is_not_blank()` | | |
| `&str` | `is_blank_ascii()` | | |
| `&str` | `is_equal_to_ignoring_ascii_case(expected)` | | |
| `&str` | `contains(expected)` | | |
| `&str` | `does_not_contain(unexpected)` | | |
| `&str` | `starts_with(expected)` | | |
| `&str` | `does_not_start_with(unexpected)` | | |
| `&str` | `ends_with(expected)` | | |
| `&str` | `does_not_end_with(unexpected)` | | |
| `String` | `is_not_blank()` | | |
| `String` | `is_equal_to_ignoring_ascii_case(expected)` | | |
| `String` | `contains(expected)` | | |
| `String` | `does_not_contain(unexpected)` | | |
| `String` | `starts_with(expected)` | | |
| `String` | `does_not_start_with(unexpected)` | | |
| `String` | `ends_with(expected)` | | |
| `String` | `does_not_end_with(unexpected)` | | |
| `&[T]` | `contains(expected)` | | |
| `&[T]` | `does_not_contain(not_expected)` | | |
| `&[T]` | `contains_exactly(expected)` | | |
| `&[T]` | `contains_exactly_in_any_order(expected)` | | |
| `&[T]` | `contains_exactly_matching_in_any_order(expected)` | | |
| `[T; N]` | `contains(expected)` | | |
| `[T; N]` | `does_not_contain(not_expected)` | | |
| `[T; N]` | `contains_exactly(expected)` | | |
| `[T; N]` | `contains_exactly_in_any_order(expected)` | | |
| `[T; N]` | `contains_exactly_matching_in_any_order(expected)` | | |
| `Vec<T>` | `contains(expected)` | | |
| `Vec<T>` | `does_not_contain(not_expected)` | | |
| `Vec<T>` | `contains_exactly(expected)` | | |
| `Vec<T>` | `contains_exactly_in_any_order(expected)` | | |
| `Vec<T>` | `contains_exactly_matching_in_any_order(expected)` | | |
| `T: Debug` | `has_debug_string(expected)` | | |
| `T: Debug` | `has_debug_value(expected)` | | |
| `T: Display` | `has_display_value(expected)` | | |
| `F: FnOnce() -> R` | `panics()` | Panic mode only | std |
| `F: FnOnce() -> R` | `does_not_panic()` | Panic mode only | std |
| `F: FnOnce() -> impl Future<Output = R>` | `panics_async()` | Panic mode only | std |
| `F: FnOnce() -> impl Future<Output = R>` | `does_not_panic_async()` | Panic mode only | std |
| `I: Iterator<Item = T>` | `contains(expected)` | Terminal assertion | |
| `I: Iterator<Item = T>` | `does_not_contain(not_expected)` | Terminal assertion | |
| `I: Iterator<Item = T>` | `contains_exactly(expected)` | Terminal assertion | |
| `I where &I: IntoIterator<Item = &T>` | `into_iter_contains(expected)` | Prefixed to avoid overlap with more specific collection assertions | |
| `I where &I: IntoIterator<Item = &T>` | `into_iter_does_not_contain(not_expected)` | Prefixed to avoid overlap with more specific collection assertions | |
| `I where &I: IntoIterator<Item = &T>` | `into_iter_contains_exactly(expected)` | Prefixed to avoid overlap with more specific collection assertions | |
| `I where &I: IntoIterator<Item = &T>` | `into_iter_iterator_is_empty()` | Prefixed to avoid overlap with more specific collection assertions | |
| `T: HasLength` | `is_empty()` | Implemented for strings, slices, arrays, `Vec`, `HashMap`/`HashSet`, numeric ranges, and feature-gated rootcause collections/attachments | |
| `T: HasLength` | `is_not_empty()` | Implemented for strings, slices, arrays, `Vec`, `HashMap`/`HashSet`, numeric ranges, and feature-gated rootcause collections/attachments | |
| `T: HasLength` | `has_length(expected)` | Implemented for strings, slices, arrays, `Vec`, `HashMap`/`HashSet`, numeric ranges, and feature-gated rootcause collections/attachments | |
| `T: Num` | `is_zero()` | | num |
| `T: Num` | `is_additive_identity()` | Synonym for `is_zero` | num |
| `T: Num` | `is_one()` | | num |
| `T: Num` | `is_multiplicative_identity()` | Synonym for `is_one` | num |
| `T: Num + Signed` | `is_negative()` | | num |
| `T: Num + Signed` | `is_positive()` | | num |
| `T: Num + PartialOrd + Clone` | `is_close_to(expected, allowed_deviation)` | | num |
| `T: Num + Float` | `is_nan()` | Requires either `std` or `libm` in addition to `num` | num |
| `T: Num + Float` | `is_finite()` | Requires either `std` or `libm` in addition to `num` | num |
| `T: Num + Float` | `is_infinite()` | Requires either `std` or `libm` in addition to `num` | num |
| `Option<T>` | `is_some()` | Panic mode only | |
| `Option<T>` | `is_some_satisfying(assertions)` | | |
| `Option<T>` | `is_none()` | | |
| `Poll<T>` | `is_pending()` | | |
| `Poll<T>` | `is_ready()` | Panic mode only | |
| `Poll<T>` | `is_ready_satisfying(assertions)` | | |
| `R: RangeBounds<B>, B: PartialOrd` | `contains_element(expected)` | | |
| `R: RangeBounds<B>, B: PartialOrd` | `does_not_contain_element(expected)` | | |
| `B: PartialOrd` | `is_in_range(expected)` | | |
| `B: PartialOrd` | `is_not_in_range(expected)` | | |
| `B: PartialOrd` | `is_outside_of_range(expected)` | Synonym for `is_not_in_range` | |
| `RefCell<T>` | `is_borrowed()` | | |
| `RefCell<T>` | `is_mutably_borrowed()` | | |
| `RefCell<T>` | `is_not_mutably_borrowed()` | | |
| `Mutex<T>` | `is_locked()` | | std |
| `Mutex<T>` | `is_not_locked()` | | std |
| `Mutex<T>` | `is_free()` | Synonym for `is_not_locked` | std |
| `Result<T, E>` | `is_ok()` | Panic mode only | |
| `Result<T, E>` | `is_err()` | Panic mode only | |
| `Result<T, E>` | `is_ok_satisfying(assertions)` | | |
| `Result<T, E>` | `is_err_satisfying(assertions)` | | |
| `PathBuf` | `exists()` | | std |
| `PathBuf` | `does_not_exist()` | | std |
| `PathBuf` | `is_a_file()` | | std |
| `PathBuf` | `is_a_directory()` | | std |
| `PathBuf` | `is_a_symlink()` | | std |
| `PathBuf` | `has_a_root()` | | std |
| `PathBuf` | `is_relative()` | | std |
| `PathBuf` | `has_file_name(expected)` | | std |
| `PathBuf` | `has_file_stem(expected)` | | std |
| `PathBuf` | `has_extension(expected)` | | std |
| `PathBuf` | `starts_with(expected)` | | std |
| `PathBuf` | `ends_with(expected)` | | std |
| `&Path` | `exists()` | | std |
| `&Path` | `does_not_exist()` | | std |
| `&Path` | `is_a_file()` | | std |
| `&Path` | `is_a_directory()` | | std |
| `&Path` | `is_a_symlink()` | | std |
| `&Path` | `has_a_root()` | | std |
| `&Path` | `is_relative()` | | std |
| `&Path` | `has_file_name(expected)` | | std |
| `&Path` | `has_file_stem(expected)` | | std |
| `&Path` | `has_extension(expected)` | | std |
| `&Path` | `starts_with(expected)` | | std |
| `&Path` | `ends_with(expected)` | | std |
| `HashMap<K, V>` | `contains_key(expected)` | | std |
| `HashMap<K, V>` | `does_not_contain_key(not_expected)` | | std |
| `HashMap<K, V>` | `contains_value(expected)` | | std |
| `HashMap<K, V>` | `does_not_contain_value(not_expected)` | | std |
| `HashMap<K, V>` | `contains_entry(expected_key, expected_value)` | | std |
| `HashMap<K, V>` | `does_not_contain_entry(unexpected_key, unexpected_value)` | | std |
| `HashMap<K, V>` | `contains_keys(expected)` | | std |
| `HashMap<K, V>` | `contains_exactly_entries(expected)` | | std |
| `HashSet<T>` | `contains(expected)` | | std |
| `HashSet<T>` | `does_not_contain(not_expected)` | | std |
| `HashSet<T>` | `contains_all(expected)` | | std |
| `HashSet<T>` | `is_subset_of(expected_superset)` | | std |
| `HashSet<T>` | `is_superset_of(expected_subset)` | | std |
| `HashSet<T>` | `is_disjoint_from(other)` | | std |
| `Command` | `has_arg(expected)` | | std |
| `Type<T>` | `needs_drop()` | | std |
| `Type<T>` | `need_drop()` | Synonym for `needs_drop` | std |
| `Box<dyn Any>` | `has_type::<Expected>()` | Panic mode only | |
| `Box<dyn Any>` | `has_type_ref::<Expected>()` | Panic mode only | |
| `PanicValue` | `has_type::<Expected>()` | Panic mode only | |
| `PanicValue` | `has_type_ref::<Expected>()` | Panic mode only | |
| `http::HeaderValue` | `is_empty()` | | http |
| `http::HeaderValue` | `is_not_empty()` | | http |
| `http::HeaderValue` | `is_sensitive()` | | http |
| `http::HeaderValue` | `is_insensitive()` | | http |
| `http::HeaderValue` | `is_ascii_satisfying(assertions)` | | http |
| `http::HeaderValue` | `is_ascii()` | Panic mode only | http |
| `tokio::sync::Mutex<T>` | `is_locked()` | | tokio |
| `tokio::sync::Mutex<T>` | `is_not_locked()` | | tokio |
| `tokio::sync::Mutex<T>` | `is_free()` | Synonym for `is_not_locked` | tokio |
| `tokio::sync::RwLock<T>` | `is_not_locked()` | | tokio |
| `tokio::sync::RwLock<T>` | `is_free()` | Synonym for `is_not_locked` | tokio |
| `tokio::sync::RwLock<T>` | `is_read_locked()` | | tokio |
| `tokio::sync::RwLock<T>` | `is_write_locked()` | | tokio |
| `tokio::sync::watch::Receiver<T>` | `has_current_value(expected)` | | tokio |
| `tokio::sync::watch::Receiver<T>` | `has_changed()` | Panic mode only | tokio |
| `tokio::sync::watch::Receiver<T>` | `has_not_changed()` | Panic mode only | tokio |
| `reqwest::Response` | `has_status_code(expected)` | | reqwest |
| `Program<'a>` | `exists()` | | program |
| `Program<'a>` | `exists_and()` | Panic mode only | program |
| `rootcause::ReportCollection<C, T>` | `is_empty()` | via `HasLength` | rootcause |
| `rootcause::ReportCollection<C, T>` | `is_not_empty()` | via `HasLength` | rootcause |
| `rootcause::ReportCollection<C, T>` | `has_length(expected)` | via `HasLength` | rootcause |
| `rootcause::ReportAttachments<T>` | `is_empty()` | via `HasLength` | rootcause |
| `rootcause::ReportAttachments<T>` | `is_not_empty()` | via `HasLength` | rootcause |
| `rootcause::ReportAttachments<T>` | `has_length(expected)` | via `HasLength` | rootcause |
| `rootcause::Report<C, O, T>` | `has_child_count(expected)` | | rootcause |
| `rootcause::Report<C, O, T>` | `has_attachment_count(expected)` | | rootcause |
| `rootcause::Report<C, O, T>` | `has_current_context_type::<Expected>()` | | rootcause |
| `rootcause::Report<C, O, T>` | `has_current_context_display_value(expected)` | | rootcause |
| `rootcause::Report<C, O, T>` | `has_current_context_debug_string(expected)` | | rootcause |
| `rootcause::Report<Dynamic, O, T>` | `has_current_context_satisfying::<Expected>(...)` | | rootcause |
| `rootcause::Report<Dynamic, O, T>` | `has_current_context::<Expected>()` | Panic mode only | rootcause |
| `rootcause::ReportRef<'a, C, O, T>` | Same as corresponding `rootcause::Report<C, O, T>` rows | | rootcause |
| `rootcause::ReportRef<'a, Dynamic, O, T>` | Same as corresponding `rootcause::Report<Dynamic, O, T>` rows | | rootcause |
| `jiff::SignedDuration` | `is_zero()` | | jiff |
| `jiff::SignedDuration` | `is_negative()` | | jiff |
| `jiff::SignedDuration` | `is_positive()` | | jiff |
| `jiff::SignedDuration` | `is_close_to(expected, allowed_deviation)` | | jiff |
| `jiff::Span` | `is_zero()` | | jiff |
| `jiff::Span` | `is_negative()` | | jiff |
| `jiff::Span` | `is_positive()` | | jiff |
| `jiff::Zoned` | `is_in_time_zone(expected)` | | jiff |
| `jiff::Zoned` | `is_in_time_zone_named(expected)` | | jiff |
*The generic types (`T`, `E`, ...) nearly always also require `Debug`. Otherwise the library could
not print useful failure output. We chose not to list those bounds everywhere in the table above.
For `PartialOrd` assertions, unordered comparisons fail. That matters most for floating-point values like `NaN`, where
`partial_cmp()` returns `None`.
### Conditions
- `is(condition)` / `has(condition)`: Assert that a value satisfies a reusable `Condition<T>`.
- `are(condition)` / `have(condition)`: Assert that every element of an iterable satisfies a condition.
### Derived Assertions
Use derived assertions to map the current subject to one of its fields or views and then keep
asserting on the derived value:
```rust
use assertr::prelude::*;
#[derive(Debug)]
struct Person {
age: u32,
}
#[test]
fn test() {
assert_that!(Person { age: 30 }).satisfies(
|person| person.age,
|age| age.is_greater_or_equal_to(18),
);
}
```
- `satisfies(mapper, assertions)`: Derive an owned view for nested assertions.
- `satisfies_ref(mapper, assertions)`: Derive a borrowed view for nested assertions.
## Advanced Features
### Capture Mode
Instead of immediately panicking on assertion failure, you can capture failures for later analysis:
```rust
#[test]
fn test() {
let failures = assert_that!(3)
.with_capture()
.is_equal_to(4)
.is_less_than(2)
.capture_failures();
assert_that!(failures).has_length(2);
}
```
With the `fluent` feature enabled, the same pattern can start from `.verify()` instead of
`assert_that!(...).with_capture()`.
### Partial equality assertions
You can derive a helper struct for partial equality comparisons by annotating an owned struct with
`#[derive(AssertrEq)]`.
**Make sure this crate's `derive` feature is enabled.**
```toml
assertr = { version = "0.5.6", features = ["derive"] }
```
```rust
// Deriving `AssertrEq` provides an additional `PersonAssertrEq` type.
// Deriving `Debug` is necessary because `Person` is used as an assertion subject.
#[derive(Debug, AssertrEq)]
pub struct Person {
pub name: String,
pub age: i32,
pub data: (u32, u32),
}
#[test]
fn test() {
let alice = Person {
name: "Alice".to_owned(),
age: 30,
data: (100, 998)
};
// We can still perform a standard (full) equality check.
assert_that!(&alice).is_equal_to(Person {
name: "Alice".to_owned(),
age: 30,
data: (100, 998),
});
// But we can also do a partial equality check!
assert_that!(&alice).is_equal_to(PersonAssertrEq {
name: eq("Alice".to_owned()),
age: any(), // Match any age
data: any() // Match any data
});
}
```
### Write assertions for your own types.
Good custom assertions add domain-specific value. In practice, the most maintainable way to build
them is often to delegate to existing assertions so you keep the same failure formatting,
capture-mode behavior, and chaining style.
```rust
use assertr::prelude::*;
#[derive(Debug)]
struct Person {
age: u32,
}
trait PersonAssertions {
fn is_adult(self) -> Self;
}
impl<M: Mode> PersonAssertions for AssertThat<'_, Person, M> {
#[track_caller]
fn is_adult(self) -> Self {
self.satisfies(|person| person.age, |age| {
age.is_greater_or_equal_to(18);
})
}
}
#[test]
fn test() {
assert_that!(Person { age: 30 })
.is_adult();
}
```
### Type Testing
Test properties of types:
```rust
#[test]
fn test() {
assert_that_type::<MyType>()
.needs_drop()
.satisfies(|it| it.size(), |size| {
size.is_equal_to(32);
});
}
```
## Examples
```rust
#[test]
fn test() {
// Assertions that read like English.
assert_that!("foobar").starts_with("foo").contains("ooba");
assert_that!(vec![1, 2, 3]).has_length(3).contains(2);
assert_that!(Ok(42)).is_ok().is_equal_to(42);
assert_that!(Some(42)).has_debug_string("Some(42)");
// Chainable.
assert_that!("foobar")
.is_not_empty()
.starts_with("foo")
.ends_with("bar")
.has_length(6);
}
```
- Partial equality assertions (meaning that only some fields of a struct are compared, while some are ignored).
Add the `AssertrEq` annotation to one of your structs to enable this.
## Compared to other assertion styles
One other style of assertions in Rust is the "individual macros" approach.
The standard library already comes with a few of them, like the `assert_eq!` macro, many libraries provide a more
exhaustive list of macros specifically tailored for specific types and operations.
Let me point out a few benefits of fluent assertions compared to individual assert macros.
#### Chainability and Readability
The fluent interface allows you to chain multiple assertions naturally, following the way we think about validating
properties. Instead of writing multiple separate assertions, you can express related checks in a single, flowing
statement that reads almost like natural language.
Additionally, having a concrete entrance into the assertion context using the `assert_that!` macro with assertions
coming after makes it totally obvious which value is the "actual" and which is the "expected" value. This provides
a clear schema for how assertions are written, compared to an assertion macro, like std's `assert_eq!`, in which the
order of arguments can be chosen freely, making it non-obvious when coming into a new codebase which style
was chosen.
#### Better Error Messages
Fluent assertions can provide more detailed and structured error messages out of the box. Rather than just showing
the values that didn't match, they can include context about what specific check failed within the chain and clearer
descriptions of the expected vs actual values. Descriptive messages can be collected throughout the call chain.
#### Reduced Code Duplication
With traditional assert macros, you often need to reference the same value multiple times:
```rust
#[test]
fn test() {
let vec = vec![1, 2, 3];
assert_eq!(vec.len(), 3);
assert!(vec.contains(&2));
}
```
Versus the fluent style:
```rust
#[test]
fn test() {
assert_that!(vec![1, 2, 3]).has_length(3).contains(2);
}
```
## Technical decisions
- Derived assertions are not allowed to control whether the location is printed.
- Detail messages are collected from the current assertion upwards, taking the messages of all parents into account.
- Failures are stored at the root assertion.
- Failures can only be extracted from the root assertion.
- Assertions to be defined on common traits as often as possible. Allowing, for example, all types implementing `Eq`
to allow `is_equal_to`, `PartialOrd` types to allow `is_greater_than` assertions and all types implementing the
`HasLength` trait to support the `has_length` assertion.
- A single `assert_that!(...)` macro suffices to get into an assertion context. It handles both owned values
and references automatically — pass `assert_that!(value)` for owned values or `assert_that!(&value)` to borrow.
- One import should be enough to access all possible assertions through **autocomplete**.
`use assertr::prelude::*;`
## Dev
Run the full maintenance pipeline, including formatting, checks, clippy, tests, and docs with:
(This will run checks and tests with default features, no default features and all features enabled.)
```bash
just tidy
```
### Testing
To test all crates from the workspace root:
```bash
cargo test --all
```
Some assertions are feature-gated. To run the full test suite, enable all features:
```bash
cargo test --all-features
```
## MSRV
Current MSRV:
- `assertr`: `1.89.0`
- `assertr-derive`: `1.85.0`
Previous MSRV values:
- As of `0.1.0`, the MSRV was `1.76.0`
- As of `0.2.0`, the MSRV was `1.85.0`
- As of `0.4.0`, the MSRV was `1.89.0`
## Open questions
- Many assertions require `std::fmt::Debug`, limiting usability to types implementing Debug.
Can we implement fallback rendering? Will probably require the currently unstable specialization feature.
## Contributing
Contributions are welcome. Feel free to open a PR with your suggested changes.
## Acknowledgements
Midway through implementing this, I found out that "spectral" already exists, which uses a very similar style of
assertions.
After looking into it, I took the concept of generally relying on `*Assertions` trait definitions instead of directly
implementing Assertions with multiple impl blocks on the `AssertThat` type.