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src/testing/doc-tests.md

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+# Documentation Tests
+
+Rust has built-in support for documentation tests:
+
+```rust
+/// Shorten string will trip the string to the given length.
+///
+/// ```
+/// use playground::shorten_string;
+/// assert_eq!(shorten_string("Hello World", 5), "Hello");
+/// assert_eq!(shorten_string("Hello World", 20), "Hello World");
+/// ```
+pub fn shorten_string(s: &str, length: usize) -> &str {
+    &s[..std::cmp::min(length, s.len())]
+}
+```
+
+* Code blocks in `///` comments are automatically seen as Rust code.
+* The code will be compiled and executed as part of `cargo test`.
+* Test the above code on the [Rust Playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=c8ce535a3778218fed50c2b4c317d15d).

src/testing/integration-tests.md

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+# Integration Tests
+
+If you want to test your library as a client, use an integration test.
+
+Create a `.rs` file under `tests/`:
+
+```rust,ignore
+use my_library::init;
+
+#[test]
+fn test_init() {
+    assert!(init().is_ok());
+}
+```
+
+These tests only have access to the public API of your crate.

src/testing/test-modules.md

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+# Test Modules
+
+Unit tests are often put in a nested module (run tests on the
+[Playground](https://play.rust-lang.org/)):
+
+```rust,editable
+fn helper(a: &str, b: &str) -> String {
+    format!("{a} {b}")
+}
+
+pub fn main() {
+    println!("{}", helper("Hello", "World"));
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_helper() {
+        assert_eq!(helper("foo", "bar"), "foo bar");
+    }
+}
+```
+
+* This lets you unit test private helpers.
+* The `#[cfg(test)]` attribute is only active when you run `cargo test`.

src/testing/unit-tests.md

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+# Unit Tests
+
+Mark unit tests with `#[test]`:
+
+```rust,editable
+fn first_word(text: &str) -> &str {
+    match text.find(' ') {
+        Some(idx) => &text[..idx],
+        None => &text,
+    }
+}
+
+#[test]
+fn test_empty() {
+    assert_eq!(first_word(""), "");
+}
+
+#[test]
+fn test_single_word() {
+    assert_eq!(first_word("Hello"), Some("Hello"));
+}
+
+#[test]
+fn test_multiple_words() {
+    assert_eq!(first_word("Hello World"), Some("Hello"));
+}
+```
+
+Use `cargo test` to find and run the unit tests.