Make the output optional

src/exercise.rs

@@ -19,8 +19,10 @@ pub const OUTPUT_CAPACITY: usize = 1 << 14;
 
 // Run an exercise binary and append its output to the `output` buffer.
 // Compilation must be done before calling this method.
-fn run_bin(bin_name: &str, output: &mut Vec<u8>, target_dir: &Path) -> Result<bool> {
-    writeln!(output, "{}", "Output".underlined())?;
+fn run_bin(bin_name: &str, mut output: Option<&mut Vec<u8>>, target_dir: &Path) -> Result<bool> {
+    if let Some(output) = output.as_deref_mut() {
+        writeln!(output, "{}", "Output".underlined())?;
+    }
 
     // 7 = "/debug/".len()
     let mut bin_path = PathBuf::with_capacity(target_dir.as_os_str().len() + 7 + bin_name.len());
@@ -28,19 +30,25 @@ fn run_bin(bin_name: &str, output: &mut Vec<u8>, target_dir: &Path) -> Result<bo
     bin_path.push("debug");
     bin_path.push(bin_name);
 
-    let success = run_cmd(Command::new(&bin_path), &bin_path.to_string_lossy(), output)?;
+    let success = run_cmd(
+        Command::new(&bin_path),
+        &bin_path.to_string_lossy(),
+        output.as_deref_mut(),
+    )?;
 
-    if !success {
-        // This output is important to show the user that something went wrong.
-        // Otherwise, calling something like `exit(1)` in an exercise without further output
-        // leaves the user confused about why the exercise isn't done yet.
-        writeln!(
-            output,
-            "{}",
-            "The exercise didn't run successfully (nonzero exit code)"
-                .bold()
-                .red(),
-        )?;
+    if let Some(output) = output {
+        if !success {
+            // This output is important to show the user that something went wrong.
+            // Otherwise, calling something like `exit(1)` in an exercise without further output
+            // leaves the user confused about why the exercise isn't done yet.
+            writeln!(
+                output,
+                "{}",
+                "The exercise didn't run successfully (nonzero exit code)"
+                    .bold()
+                    .red(),
+            )?;
+        }
     }
 
     Ok(success)
@@ -77,8 +85,15 @@ pub trait RunnableExercise {
 
     // Compile, check and run the exercise or its solution (depending on `bin_name´).
     // The output is written to the `output` buffer after clearing it.
-    fn run(&self, bin_name: &str, output: &mut Vec<u8>, target_dir: &Path) -> Result<bool> {
-        output.clear();
+    fn run(
+        &self,
+        bin_name: &str,
+        mut output: Option<&mut Vec<u8>>,
+        target_dir: &Path,
+    ) -> Result<bool> {
+        if let Some(output) = output.as_deref_mut() {
+            output.clear();
+        }
 
         // Developing the official Rustlings.
         let dev = DEBUG_PROFILE && in_official_repo();
@@ -90,7 +105,7 @@ pub trait RunnableExercise {
             description: "cargo build …",
             hide_warnings: false,
             target_dir,
-            output,
+            output: output.as_deref_mut(),
             dev,
         }
         .run()?;
@@ -99,7 +114,9 @@ pub trait RunnableExercise {
         }
 
         // Discard the output of `cargo build` because it will be shown again by Clippy.
-        output.clear();
+        if let Some(output) = output.as_deref_mut() {
+            output.clear();
+        }
 
         // `--profile test` is required to also check code with `[cfg(test)]`.
         let clippy_args: &[&str] = if self.strict_clippy() {
@@ -114,7 +131,7 @@ pub trait RunnableExercise {
             description: "cargo clippy …",
             hide_warnings: false,
             target_dir,
-            output,
+            output: output.as_deref_mut(),
             dev,
         }
         .run()?;
@@ -123,7 +140,7 @@ pub trait RunnableExercise {
         }
 
         if !self.test() {
-            return run_bin(bin_name, output, target_dir);
+            return run_bin(bin_name, output.as_deref_mut(), target_dir);
         }
 
         let test_success = CargoCmd {
@@ -134,12 +151,12 @@ pub trait RunnableExercise {
             // Hide warnings because they are shown by Clippy.
             hide_warnings: true,
             target_dir,
-            output,
+            output: output.as_deref_mut(),
             dev,
         }
         .run()?;
 
-        let run_success = run_bin(bin_name, output, target_dir)?;
+        let run_success = run_bin(bin_name, output.as_deref_mut(), target_dir)?;
 
         Ok(test_success && run_success)
     }
@@ -147,13 +164,13 @@ pub trait RunnableExercise {
     /// Compile, check and run the exercise.
     /// The output is written to the `output` buffer after clearing it.
     #[inline]
-    fn run_exercise(&self, output: &mut Vec<u8>, target_dir: &Path) -> Result<bool> {
+    fn run_exercise(&self, output: Option<&mut Vec<u8>>, target_dir: &Path) -> Result<bool> {
         self.run(self.name(), output, target_dir)
     }
 
     /// Compile, check and run the exercise's solution.
     /// The output is written to the `output` buffer after clearing it.
-    fn run_solution(&self, output: &mut Vec<u8>, target_dir: &Path) -> Result<bool> {
+    fn run_solution(&self, output: Option<&mut Vec<u8>>, target_dir: &Path) -> Result<bool> {
         let name = self.name();
         let mut bin_name = String::with_capacity(name.len());
         bin_name.push_str(name);