Rust Testing
What a Rust assertion actually claims, and where that claim is weaker than it looks. assert_eq! and its argument order, should_panic and its substring match, tests that return Result, cfg(test) modules against integration tests, doc tests, floats, and the difference between a failed assertion and a panic somewhere else.
Who it is for
Tests the code they wrote, not the code they meant.
The behaviour people get wrong after they are already productive in the language.
You will get the most out of it if you have written Rust recently enough to read it without effort, and have been surprised by it at least once. If you are picking the language up for the first time, start with Rust Essentials and come back to this one.
How long it takes
90 graded questions at roughly 45 seconds each is about 68 minutes of answering in total.
At the default five questions a day, and because the session mixes in your other courses, expect around 18 days — call it 3 weeks — to see everything once, plus a few weeks of thinning reviews after that before it is all mastered.
A question is mastered on the third correct answer, once the gap between reviews has grown past a week — so a course cannot be finished in a weekend however long you sit with it.
Question mix
- 60 multiple choice
- 10 concept cards
- 10 predict the output
- 10 put it in order
- 10 matching questions
Syllabus
What you will be asked about
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01
What a Test Asserts
Say what a Rust assertion really pins down, and what it leaves free.
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02
assert! and Its Message
Choose the assertion whose failure message will tell you what went wrong.
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03
should_panic and expected
Write a should_panic test that fails when the wrong thing panics.
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04
Testing Things That Return Result
Test a Result-returning function without turning every failure into a panic.
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05
Where Tests Live
Decide whether a test belongs beside the code or in tests/, and say what each can see.
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06
Doc Tests
Write a documentation example that is compiled and run as a real test, and say which ones are not.
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07
Comparing Floating Point
Choose a float comparison that neither passes everything nor fails on the last bit.
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08
What assert_eq! Compares
Say what equality means for the value you are asserting on, before you trust the assertion.
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09
A Failed Assertion or a Panic
Tell a failed assertion apart from a panic elsewhere in the test, and know which one is hiding a bug.
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10
Making Code Testable
Reshape a function so a test can reach the behaviour, without making everything public.
8 more topics, and all 90 questions
Rust Testing is a Pro course. One subscription opens it and every other course in the catalogue, including the ones still being written.
A question from this course
What a Test Asserts
What does this print?
fn main() {
let v = vec![1, 2, 3];
assert_eq!(v.len(), 3);
println!("assertion held");
}Show the answer
C assertion held
A passing assert_eq! produces no output at all - it only panics when the two sides differ. You may have expected it to print something because a test runner prints a line per test; that line comes from the runner, not from the macro. The macros are ordinary code and run anywhere, main included.
The rest of the track
Rust at every depth
Rust Essentials
The model underneath the syntax. Assigning a String to a second name moves it and the first name stops working, while an i32 is simply copied; borrowing, Option and exhaustive match are the same compiler insisting you say what happens in every case.
Rust Core
The compiler's side of the argument, made explicit. A lifetime constrains how long a borrow may be used rather than naming a span of time, an iterator chain does nothing at all until something consumes it, and a trait bound is the only thing a generic function is allowed to assume.
Rust Data & Collections
The containers people reach for daily and the places they bite: indexing that panics where get() hands back None, a borrow that freezes the Vec it came from, and an entry() call that writes when you only meant to read. Every snippet here runs, so the answers are what rustc really printed.
Rust Errors & Edge Cases
Failure as a value rather than an exception: Option where there is nothing to say and Result where there is, the ? operator and the From impl it quietly needs, custom error enums, Display against Debug, and the line between an error a caller can act on and a panic that ends the process. Every snippet here either runs or refuses to compile for a reason you can name.
Rust Standard Library
The parts of std that people reimplement by hand: the String and &str split, chars against bytes, the split and trim families, parse and the turbofish, the lazy iterator adaptors, collect into anything that implements FromIterator, the Option and Result combinators, the three sorts, and the Display against Debug distinction. Ten corners where a built-in already does the job and one behaviour in each is not what you assumed.
Rust Idioms & Pitfalls
Ten habits that compile and still mark the code as written by someone thinking in another language: the clone that silences the borrow checker, unwrap as punctuation, String parameters where a slice would do, index loops, a collect nobody reads, a five-line match that is one combinator, &Vec in a signature, lifetimes elision would have written, inherent methods that want to be trait impls, and drop misread. Each one is shown working, then shown costing something.
Rust Advanced
The type system and the borrow checker in earnest: elision that quietly ties a return to &self, two &mut borrows that look disjoint and are not, RefCell panicking at run time where the compiler would have refused. Every snippet here either compiles or fails to for a reason you can name.
Rust Performance
What a Rust program actually costs, read off the things you can observe: Vec::capacity, strong_count, size_of and the order drops run in. Every claim here is one the snippet proves, which is why none of them is a timing.
Rust Interview Questions
The questions a Rust interview actually turns on: where a value's owner is at each line, why two borrows that look disjoint are not, and what a lifetime annotation promises and what it cannot. Ownership and moves, aliasing, lifetimes, String against &str, Vec against slices, Option and Result, traits and dispatch, monomorphisation, Rc and RefCell, and what makes a type safe to send between threads.
Start Rust Testing
in tomorrow’s session.
Enrol and this course joins your daily five, mixed in with everything else you are studying — 90 questions spread over about 18 days rather than dropped on you at once.
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