Be more consistent about tests vs. main (#2644)

The content slides all use `fn main`, with the exception of the testing
segment. But with this change, where it makes sense exercises use tests
instead, and not both tests and `fn main`.

A small change in `book.js` supports running tests when a code sample
does not have `fn main` but does have `#[test]`, so these work
naturally.

Fixes #1581.

src/testing/Cargo.toml

@@ -7,6 +7,6 @@ publish = false
 [lints.rust]
 unexpected_cfgs = { level = "warn", check-cfg = ['cfg(never)'] }
 
-[[bin]]
+[lib]
 name = "luhn"
 path = "exercise.rs"

src/testing/exercise.md

@@ -27,7 +27,7 @@ correctly.
 Copy the code below to <https://play.rust-lang.org/> and write additional tests
 to uncover bugs in the provided implementation, fixing any bugs you find.
 
-```rust
+```rust,editable
 {{#include exercise.rs:luhn}}
 
 {{#include exercise.rs:unit-tests}}

src/testing/exercise.rs

@@ -11,8 +11,8 @@
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
+#![allow(dead_code)]
 
-// ANCHOR: solution
 // This is the buggy version that appears in the problem.
 #[cfg(never)]
 // ANCHOR: luhn
@@ -40,6 +40,7 @@ pub fn luhn(cc_number: &str) -> bool {
 // ANCHOR_END: luhn
 
 // This is the solution and passes all of the tests below.
+// ANCHOR: solution
 pub fn luhn(cc_number: &str) -> bool {
     let mut sum = 0;
     let mut double = false;
@@ -69,14 +70,6 @@ pub fn luhn(cc_number: &str) -> bool {
     digits >= 2 && sum % 10 == 0
 }
 
-fn main() {
-    let cc_number = "1234 5678 1234 5670";
-    println!(
-        "Is {cc_number} a valid credit card number? {}",
-        if luhn(cc_number) { "yes" } else { "no" }
-    );
-}
-
 // ANCHOR: unit-tests
 #[cfg(test)]
 mod test {

src/testing/unit-tests.md

@@ -8,7 +8,7 @@ Rust and Cargo come with a simple unit test framework. Tests are marked with
 `#[test]`. Unit tests are often put in a nested `tests` module, using
 `#[cfg(test)]` to conditionally compile them only when building tests.
 
-```rust,editable,ignore
+```rust,editable
 fn first_word(text: &str) -> &str {
     match text.find(' ') {
         Some(idx) => &text[..idx],
@@ -39,9 +39,3 @@ mod tests {
 
 - This lets you unit test private helpers.
 - The `#[cfg(test)]` attribute is only active when you run `cargo test`.
-
-<details>
-
-Run the tests in the playground in order to show their results.
-
-</details>