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.

CONTRIBUTING.md

@@ -8,6 +8,27 @@ the [instructions in the README].
 
 [instructions in the README]: README.md#building
 
+## Writing Exercises
+
+Each segment ends with an exercise. Exercises are typically structured as an
+`exercise.rs` containing the problem and solution. This is referenced from
+`exercise.md` and `solution.md`, using `{{#include exercise.rs:anchor_name}}` to
+match ANCHOR comments in the `exercise.rs` file. Each segment also has a
+`Cargo.toml` file containing a `[[bin]]` or `[lib]` section referring to
+`exercise.rs`, and that Cargo package is referenced from the workspace the root
+`Cargo.toml`. The result is that `exercise.rs` is built and tested by
+`cargo test`.
+
+For segments on day 1, exercises should use `fn main() { .. }` and `println!`,
+with students visually verifying the correct output. On subsequent days, prefer
+tests and omit `fn main() { .. }`. However, where tests would be difficult and
+visual verification is more natural (such as in the Logger exercise), using
+`fn main { .. }` is OK.
+
+Especially for exercises without tests, consider including tests in
+`exercise.rs` that do not appear in either `exercise.md` or `solution.md`, as
+these can ensure the solution is correct.
+
 ## Testing
 
 We test the course material in several ways:

src/borrowing/Cargo.toml

@@ -4,6 +4,6 @@ version = "0.1.0"
 edition = "2021"
 publish = false
 
-[[bin]]
+[lib]
 name = "borrowing"
 path = "../../third_party/rust-on-exercism/health-statistics.rs"

src/borrowing/exercise.md

@@ -9,10 +9,7 @@ minutes: 20
 Copy the code below to <https://play.rust-lang.org/> and fill in the missing
 method:
 
-```rust
-// TODO: remove this when you're done with your implementation.
-#![allow(unused_variables, dead_code)]
-
+```rust,editable
 {{#include ../../third_party/rust-on-exercism/health-statistics.rs:setup}}
 
 {{#include ../../third_party/rust-on-exercism/health-statistics.rs:User_visit_doctor}}
@@ -20,7 +17,5 @@ method:
     }
 }
 
-{{#include ../../third_party/rust-on-exercism/health-statistics.rs:main}}
-
 {{#include ../../third_party/rust-on-exercism/health-statistics.rs:tests}}
 ```

src/closures/exercise.md

@@ -4,7 +4,7 @@ Building on the generic logger from this morning, implement a `Filter` which
 uses a closure to filter log messages, sending those which pass the filtering
 predicate to an inner logger.
 
-```rust,compile_fail
+```rust,compile_fail,editable
 {{#include exercise.rs:setup}}
 
 // TODO: Define and implement `Filter`.

src/control-flow-basics/exercise.md

@@ -30,7 +30,5 @@ initial `n`.
   todo!("Implement this")
 }
 
-{{#include exercise.rs:tests}}
-
 {{#include exercise.rs:main}}
 ```

src/control-flow-basics/exercise.rs

@@ -25,15 +25,14 @@ fn collatz_length(mut n: i32) -> u32 {
     len
 }
 
-// ANCHOR: tests
+// ANCHOR: main
+fn main() {
+    println!("Length: {}", collatz_length(11)); // should be 15
+}
+// ANCHOR_END: main
+// ANCHOR_END: solution
+
 #[test]
 fn test_collatz_length() {
     assert_eq!(collatz_length(11), 15);
 }
-// ANCHOR_END: tests
-
-// ANCHOR: main
-fn main() {
-    println!("Length: {}", collatz_length(11));
-}
-// ANCHOR_END: main

src/error-handling/Cargo.toml

@@ -8,6 +8,6 @@ publish = false
 anyhow = "*"
 thiserror = "*"
 
-[[bin]]
+[lib]
 name = "parser"
 path = "exercise.rs"

src/error-handling/exercise.rs

@@ -88,25 +88,30 @@ fn eval(e: Expression) -> Result<i64, DivideByZeroError> {
 // ANCHOR_END: solution
 
 // ANCHOR: tests
-#[test]
-fn test_error() {
-    assert_eq!(
-        eval(Expression::Op {
-            op: Operation::Div,
-            left: Box::new(Expression::Value(99)),
-            right: Box::new(Expression::Value(0)),
-        }),
-        Err(DivideByZeroError)
-    );
-}
+#[cfg(test)]
+mod test {
+    use super::*;
 
-fn main() {
-    let expr = Expression::Op {
-        op: Operation::Sub,
-        left: Box::new(Expression::Value(20)),
-        right: Box::new(Expression::Value(10)),
-    };
-    println!("expr: {expr:?}");
-    println!("result: {:?}", eval(expr));
+    #[test]
+    fn test_error() {
+        assert_eq!(
+            eval(Expression::Op {
+                op: Operation::Div,
+                left: Box::new(Expression::Value(99)),
+                right: Box::new(Expression::Value(0)),
+            }),
+            Err(DivideByZeroError)
+        );
+    }
+
+    #[test]
+    fn test_ok() {
+        let expr = Expression::Op {
+            op: Operation::Sub,
+            left: Box::new(Expression::Value(20)),
+            right: Box::new(Expression::Value(10)),
+        };
+        assert_eq!(eval(expr), Ok(10));
+    }
 }
 // ANCHOR_END: tests

src/generics/Cargo.toml

@@ -4,6 +4,6 @@ version = "0.1.0"
 edition = "2021"
 publish = false
 
-[[bin]]
+[lib]
 name = "generics"
 path = "exercise.rs"

src/generics/exercise.md

@@ -7,12 +7,12 @@ minutes: 10
 In this short exercise, you will implement a generic `min` function that
 determines the minimum of two values, using the [`Ord`] trait.
 
-```rust,compile_fail
+```rust,editable
 use std::cmp::Ordering;
 
-// TODO: implement the `min` function used in `main`.
+// TODO: implement the `min` function used in the tests.
 
-{{#include exercise.rs:main}}
+{{#include exercise.rs:tests}}
 ```
 
 <details>

src/generics/exercise.rs

@@ -11,6 +11,7 @@
 // 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
 use std::cmp::Ordering;
@@ -22,15 +23,22 @@ fn min<T: Ord>(l: T, r: T) -> T {
     }
 }
 
-// ANCHOR: main
-fn main() {
+// ANCHOR: tests
+#[test]
+fn integers() {
     assert_eq!(min(0, 10), 0);
     assert_eq!(min(500, 123), 123);
+}
 
+#[test]
+fn chars() {
     assert_eq!(min('a', 'z'), 'a');
     assert_eq!(min('7', '1'), '1');
+}
 
+#[test]
+fn strings() {
     assert_eq!(min("hello", "goodbye"), "goodbye");
     assert_eq!(min("bat", "armadillo"), "armadillo");
 }
-// ANCHOR_END: main
+// ANCHOR_END: tests

src/iterators/Cargo.toml

@@ -4,6 +4,6 @@ version = "0.1.0"
 edition = "2021"
 publish = false
 
-[[bin]]
-name = "offset-differences"
+[lib]
+name = "offset_differences"
 path = "exercise.rs"

src/iterators/exercise.md

@@ -11,7 +11,7 @@ Copy the following code to <https://play.rust-lang.org/> and make the tests
 pass. Use an iterator expression and `collect` the result to construct the
 return value.
 
-```rust
+```rust,editable
 {{#include exercise.rs:offset_differences}}
     todo!()
 }

src/iterators/exercise.rs

@@ -51,5 +51,3 @@ fn test_degenerate_cases() {
     assert_eq!(offset_differences(1, empty), vec![]);
 }
 // ANCHOR_END: unit-tests
-
-fn main() {}

src/lifetimes/Cargo.toml

@@ -7,6 +7,6 @@ publish = false
 [dependencies]
 thiserror = "*"
 
-[[bin]]
+[lib]
 name = "protobuf"
 path = "exercise.rs"

src/lifetimes/exercise.md

@@ -83,7 +83,7 @@ What remains for you is to implement the `parse_field` function and the
 
 // TODO: Implement ProtoMessage for Person and PhoneNumber.
 
-{{#include exercise.rs:main }}
+{{#include exercise.rs:tests }}
 ```
 
 <details>

src/lifetimes/exercise.rs

@@ -11,6 +11,7 @@
 // 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
 // ANCHOR: preliminaries
@@ -193,21 +194,31 @@ impl<'a> ProtoMessage<'a> for PhoneNumber<'a> {
     }
 }
 
-// ANCHOR: main
-fn main() {
+// ANCHOR: tests
+#[test]
+fn test_id() {
     let person_id: Person = parse_message(&[0x10, 0x2a]);
     assert_eq!(person_id, Person { name: "", id: 42, phone: vec![] });
+}
 
+#[test]
+fn test_name() {
     let person_name: Person = parse_message(&[
         0x0a, 0x0e, 0x62, 0x65, 0x61, 0x75, 0x74, 0x69, 0x66, 0x75, 0x6c, 0x20,
         0x6e, 0x61, 0x6d, 0x65,
     ]);
     assert_eq!(person_name, Person { name: "beautiful name", id: 0, phone: vec![] });
+}
 
+#[test]
+fn test_just_person() {
     let person_name_id: Person =
         parse_message(&[0x0a, 0x04, 0x45, 0x76, 0x61, 0x6e, 0x10, 0x16]);
     assert_eq!(person_name_id, Person { name: "Evan", id: 22, phone: vec![] });
+}
 
+#[test]
+fn test_phone() {
     let phone: Person = parse_message(&[
         0x0a, 0x00, 0x10, 0x00, 0x1a, 0x16, 0x0a, 0x0e, 0x2b, 0x31, 0x32, 0x33,
         0x34, 0x2d, 0x37, 0x37, 0x37, 0x2d, 0x39, 0x30, 0x39, 0x30, 0x12, 0x04,
@@ -221,8 +232,11 @@ fn main() {
             phone: vec![PhoneNumber { number: "+1234-777-9090", type_: "home" },],
         }
     );
+}
 
-    // Put that all together into a single parse.
+// Put that all together into a single parse.
+#[test]
+fn test_full_person() {
     let person: Person = parse_message(&[
         0x0a, 0x07, 0x6d, 0x61, 0x78, 0x77, 0x65, 0x6c, 0x6c, 0x10, 0x2a, 0x1a,
         0x16, 0x0a, 0x0e, 0x2b, 0x31, 0x32, 0x30, 0x32, 0x2d, 0x35, 0x35, 0x35,
@@ -243,4 +257,4 @@ fn main() {
         }
     );
 }
-// ANCHOR_END: main
+// ANCHOR_END: tests

src/methods-and-traits/exercise.md

@@ -18,7 +18,7 @@ implementing that same trait, adding behavior in the process. In the "Generics"
 segment this afternoon, we will see how to make the wrapper generic over the
 wrapped type.
 
-```rust,compile_fail
+```rust,compile_fail,editable
 {{#include exercise.rs:setup}}
 
 // TODO: Implement the `Logger` trait for `VerbosityFilter`.

src/modules/exercise.md

@@ -29,7 +29,7 @@ files in the `src` directory.
 
 Here's the single-module implementation of the GUI library:
 
-```rust
+```rust,editable
 {{#include exercise.rs:single-module}}
 ```
 

src/pattern-matching/Cargo.toml

@@ -4,6 +4,6 @@ version = "0.1.0"
 edition = "2021"
 publish = false
 
-[[bin]]
+[lib]
 name = "eval"
 path = "exercise.rs"

src/pattern-matching/exercise.md

@@ -46,7 +46,7 @@ evaluate to `85`. We represent this as a much bigger tree:
 
 In code, we will represent the tree with two types:
 
-```rust,editable
+```rust
 {{#include exercise.rs:Operation}}
 
 {{#include exercise.rs:Expression}}

src/pattern-matching/exercise.rs

@@ -127,13 +127,3 @@ fn test_zeros() {
     );
 }
 // ANCHOR_END: tests
-
-fn main() {
-    let expr = Expression::Op {
-        op: Operation::Div,
-        left: Box::new(Expression::Value(10)),
-        right: Box::new(Expression::Value(2)),
-    };
-    println!("expr: {expr:?}");
-    println!("result: {:?}", eval(expr));
-}

src/references/exercise.md

@@ -7,7 +7,7 @@ minutes: 20
 We will create a few utility functions for 3-dimensional geometry, representing
 a point as `[f64;3]`. It is up to you to determine the function signatures.
 
-```rust,compile_fail
+```rust,compile_fail,editable
 // Calculate the magnitude of a vector by summing the squares of its coordinates
 // and taking the square root. Use the `sqrt()` method to calculate the square
 // root, like `v.sqrt()`.

src/smart-pointers/Cargo.toml

@@ -4,6 +4,6 @@ version = "0.1.0"
 edition = "2021"
 publish = false
 
-[[bin]]
-name = "binary-tree"
+[lib]
+name = "binary_tree"
 path = "exercise.rs"

src/smart-pointers/exercise.md

@@ -14,7 +14,7 @@ Implement the following types, so that the given tests pass.
 Extra Credit: implement an iterator over a binary tree that returns the values
 in order.
 
-```rust,editable,ignore
+```rust,compile_fail,editable
 {{#include exercise.rs:types}}
 
 // Implement `new`, `insert`, `len`, and `has` for `Subtree`.

src/smart-pointers/exercise.rs

@@ -11,6 +11,7 @@
 // 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
 use std::cmp::Ordering;
@@ -96,14 +97,6 @@ impl<T: Ord> Node<T> {
     }
 }
 
-fn main() {
-    let mut tree = BinaryTree::new();
-    tree.insert("foo");
-    assert_eq!(tree.len(), 1);
-    tree.insert("bar");
-    assert!(tree.has(&"foo"));
-}
-
 // ANCHOR: tests
 #[cfg(test)]
 mod tests {

src/std-traits/Cargo.toml

@@ -4,6 +4,6 @@ version = "0.1.0"
 edition = "2021"
 publish = false
 
-[[bin]]
-name = "std-traits"
+[lib]
+name = "std_traits"
 path = "exercise.rs"

src/std-traits/exercise.md

@@ -9,12 +9,12 @@ In this example, you will implement the classic
 playground, and implement the missing bits. Only rotate ASCII alphabetic
 characters, to ensure the result is still valid UTF-8.
 
-```rust,compile_fail
+```rust,editable
 {{#include exercise.rs:head }}
 
 // Implement the `Read` trait for `RotDecoder`.
 
-{{#include exercise.rs:main }}
+{{#include exercise.rs:tests }}
 ```
 
 What happens if you chain two `RotDecoder` instances together, each rotating by

src/std-traits/exercise.rs

@@ -36,15 +36,7 @@ impl<R: Read> Read for RotDecoder<R> {
     }
 }
 
-// ANCHOR: main
-fn main() {
-    let mut rot =
-        RotDecoder { input: "Gb trg gb gur bgure fvqr!".as_bytes(), rot: 13 };
-    let mut result = String::new();
-    rot.read_to_string(&mut result).unwrap();
-    println!("{}", result);
-}
-
+// ANCHOR: tests
 #[cfg(test)]
 mod test {
     use super::*;
@@ -72,4 +64,4 @@ mod test {
         }
     }
 }
-// ANCHOR_END: main
+// ANCHOR_END: tests

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>

src/tuples-and-arrays/exercise.md

@@ -26,15 +26,10 @@ transpose a matrix (turn rows into columns):
 Copy the code below to <https://play.rust-lang.org/> and implement the function.
 This function only operates on 3x3 matrices.
 
-```rust,should_panic
-// TODO: remove this when you're done with your implementation.
-#![allow(unused_variables, dead_code)]
-
+```rust,should_panic,editable
 {{#include exercise.rs:transpose}}
     todo!()
 }
 
-{{#include exercise.rs:tests}}
-
 {{#include exercise.rs:main}}
 ```

src/tuples-and-arrays/exercise.rs

@@ -25,7 +25,23 @@ fn transpose(matrix: [[i32; 3]; 3]) -> [[i32; 3]; 3] {
     result
 }
 
-// ANCHOR: tests
+// ANCHOR: main
+fn main() {
+    let matrix = [
+        [101, 102, 103], // <-- the comment makes rustfmt add a newline
+        [201, 202, 203],
+        [301, 302, 303],
+    ];
+
+    println!("matrix: {:#?}", matrix);
+    let transposed = transpose(matrix);
+    println!("transposed: {:#?}", transposed);
+}
+// ANCHOR_END: main
+// ANCHOR_END: solution
+
+// This test does not appear in the exercise, as this is very early in the course, but it verifies
+// that the solution is correct.
 #[test]
 fn test_transpose() {
     let matrix = [
@@ -43,18 +59,3 @@ fn test_transpose() {
         ]
     );
 }
-// ANCHOR_END: tests
-
-// ANCHOR: main
-fn main() {
-    let matrix = [
-        [101, 102, 103], // <-- the comment makes rustfmt add a newline
-        [201, 202, 203],
-        [301, 302, 303],
-    ];
-
-    println!("matrix: {:#?}", matrix);
-    let transposed = transpose(matrix);
-    println!("transposed: {:#?}", transposed);
-}
-// ANCHOR_END: main

src/unsafe-rust/exercise.md

@@ -51,7 +51,7 @@ The [Nomicon] also has a very useful chapter about FFI.
 Copy the code below to <https://play.rust-lang.org/> and fill in the missing
 functions and methods:
 
-```rust,should_panic
+```rust,should_panic,editable
 // TODO: remove this when you're done with your implementation.
 #![allow(unused_imports, unused_variables, dead_code)]
 

theme/book.js

@@ -148,6 +148,11 @@ function playground_text(playground, hidden = true) {
             crateType: "bin",
         };
 
+        // If the code block has no `main` but does have tests, run those.
+        if (text.indexOf("fn main") === -1 && text.indexOf("#[test]") !== -1) {
+            params.tests = true;
+        }
+
         if (text.indexOf("#![feature") !== -1) {
             params.version = "nightly";
         }

third_party/rust-on-exercism/health-statistics.rs

@@ -50,13 +50,6 @@ impl User {
     }
 }
 
-// ANCHOR: main
-fn main() {
-    let bob = User::new(String::from("Bob"), 32, 155.2);
-    println!("I'm {} and my age is {}", bob.name, bob.age);
-}
-// ANCHOR_END: main
-
 // ANCHOR: tests
 #[test]
 fn test_visit() {