Make the output optional

Cargo.toml

@@ -63,3 +63,7 @@ panic = "abort"
 
 [package.metadata.release]
 pre-release-hook = ["./release-hook.sh"]
+
+# TODO: Remove after the following fix is released: https://github.com/rust-lang/rust-clippy/pull/13102
+[lints.clippy]
+needless_option_as_deref = "allow"

src/app_state.rs

@@ -11,7 +11,7 @@ use std::{
 use crate::{
     clear_terminal,
     embedded::EMBEDDED_FILES,
-    exercise::{Exercise, RunnableExercise, OUTPUT_CAPACITY},
+    exercise::{Exercise, RunnableExercise},
     info_file::ExerciseInfo,
     DEBUG_PROFILE,
 };
@@ -386,8 +386,7 @@ impl AppState {
                 .iter_mut()
                 .map(|exercise| {
                     s.spawn(|| {
-                        let mut output = Vec::with_capacity(OUTPUT_CAPACITY);
+                        let success = exercise.run_exercise(None, &self.target_dir)?;
-                        let success = exercise.run_exercise(&mut output, &self.target_dir)?;
                         exercise.done = success;
                         Ok::<_, Error>(success)
                     })

src/cmd.rs

@@ -1,30 +1,42 @@
 use anyhow::{Context, Result};
-use std::{io::Read, path::Path, process::Command};
+use std::{
+    io::Read,
+    path::Path,
+    process::{Command, Stdio},
+};
 
 /// Run a command with a description for a possible error and append the merged stdout and stderr.
 /// The boolean in the returned `Result` is true if the command's exit status is success.
-pub fn run_cmd(mut cmd: Command, description: &str, output: &mut Vec<u8>) -> Result<bool> {
+pub fn run_cmd(mut cmd: Command, description: &str, output: Option<&mut Vec<u8>>) -> Result<bool> {
-    let (mut reader, writer) = os_pipe::pipe()
+    let spawn = |mut cmd: Command| {
-        .with_context(|| format!("Failed to create a pipe to run the command `{description}``"))?;
+        // NOTE: The closure drops `cmd` which prevents a pipe deadlock.
+        cmd.spawn()
+            .with_context(|| format!("Failed to run the command `{description}`"))
+    };
 
-    let writer_clone = writer.try_clone().with_context(|| {
+    let mut handle = if let Some(output) = output {
-        format!("Failed to clone the pipe writer for the command `{description}`")
+        let (mut reader, writer) = os_pipe::pipe().with_context(|| {
-    })?;
+            format!("Failed to create a pipe to run the command `{description}``")
+        })?;
 
-    let mut handle = cmd
+        let writer_clone = writer.try_clone().with_context(|| {
-        .stdout(writer_clone)
+            format!("Failed to clone the pipe writer for the command `{description}`")
-        .stderr(writer)
+        })?;
-        .spawn()
-        .with_context(|| format!("Failed to run the command `{description}`"))?;
 
-    // Prevent pipe deadlock.
+        cmd.stdout(writer_clone).stderr(writer);
-    drop(cmd);
+        let handle = spawn(cmd)?;
 
-    reader
+        reader
-        .read_to_end(output)
+            .read_to_end(output)
-        .with_context(|| format!("Failed to read the output of the command `{description}`"))?;
+            .with_context(|| format!("Failed to read the output of the command `{description}`"))?;
 
-    output.push(b'\n');
+        output.push(b'\n');
+
+        handle
+    } else {
+        cmd.stdout(Stdio::null()).stderr(Stdio::null());
+        spawn(cmd)?
+    };
 
     handle
         .wait()
@@ -42,14 +54,14 @@ pub struct CargoCmd<'a> {
     /// Added as `--target-dir` if `Self::dev` is true.
     pub target_dir: &'a Path,
     /// The output buffer to append the merged stdout and stderr.
-    pub output: &'a mut Vec<u8>,
+    pub output: Option<&'a mut Vec<u8>>,
     /// true while developing Rustlings.
     pub dev: bool,
 }
 
 impl<'a> CargoCmd<'a> {
     /// Run `cargo SUBCOMMAND --bin EXERCISE_NAME … ARGS`.
-    pub fn run(&mut self) -> Result<bool> {
+    pub fn run(self) -> Result<bool> {
         let mut cmd = Command::new("cargo");
         cmd.arg(self.subcommand);
 
@@ -86,7 +98,7 @@ mod tests {
         cmd.arg("Hello");
 
         let mut output = Vec::with_capacity(8);
-        run_cmd(cmd, "echo …", &mut output).unwrap();
+        run_cmd(cmd, "echo …", Some(&mut output)).unwrap();
 
         assert_eq!(output, b"Hello\n\n");
     }

src/dev/check.rs

@@ -184,8 +184,7 @@ fn check_exercises_unsolved(info_file: &InfoFile, target_dir: &Path) -> Result<(
                     error_occurred.store(true, atomic::Ordering::Relaxed);
                 };
 
-                let mut output = Vec::with_capacity(OUTPUT_CAPACITY);
+                match exercise_info.run_exercise(None, target_dir) {
-                match exercise_info.run_exercise(&mut output, target_dir) {
                     Ok(true) => error(b"Already solved!"),
                     Ok(false) => (),
                     Err(e) => error(e.to_string().as_bytes()),
@@ -244,7 +243,7 @@ fn check_solutions(require_solutions: bool, info_file: &InfoFile, target_dir: &P
                 }
 
                 let mut output = Vec::with_capacity(OUTPUT_CAPACITY);
-                match exercise_info.run_solution(&mut output, target_dir) {
+                match exercise_info.run_solution(Some(&mut output), target_dir) {
                     Ok(true) => {
                         paths.lock().unwrap().insert(PathBuf::from(path));
                     }

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> {
+fn run_bin(bin_name: &str, mut output: Option<&mut Vec<u8>>, target_dir: &Path) -> Result<bool> {
-    writeln!(output, "{}", "Output".underlined())?;
+    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 {
+    if let Some(output) = output {
-        // This output is important to show the user that something went wrong.
+        if !success {
-        // Otherwise, calling something like `exit(1)` in an exercise without further output
+            // This output is important to show the user that something went wrong.
-        // leaves the user confused about why the exercise isn't done yet.
+            // Otherwise, calling something like `exit(1)` in an exercise without further output
-        writeln!(
+            // leaves the user confused about why the exercise isn't done yet.
-            output,
+            writeln!(
-            "{}",
+                output,
-            "The exercise didn't run successfully (nonzero exit code)"
+                "{}",
-                .bold()
+                "The exercise didn't run successfully (nonzero exit code)"
-                .red(),
+                    .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> {
+    fn run(
-        output.clear();
+        &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);

src/run.rs

@@ -11,7 +11,7 @@ use crate::{
 pub fn run(app_state: &mut AppState) -> Result<()> {
     let exercise = app_state.current_exercise();
     let mut output = Vec::with_capacity(OUTPUT_CAPACITY);
-    let success = exercise.run_exercise(&mut output, app_state.target_dir())?;
+    let success = exercise.run_exercise(Some(&mut output), app_state.target_dir())?;
 
     let mut stdout = io::stdout().lock();
     stdout.write_all(&output)?;

src/watch/state.rs

@@ -54,7 +54,7 @@ impl<'a> WatchState<'a> {
         let success = self
             .app_state
             .current_exercise()
-            .run_exercise(&mut self.output, self.app_state.target_dir())?;
+            .run_exercise(Some(&mut self.output), self.app_state.target_dir())?;
         if success {
             self.done_status =
                 if let Some(solution_path) = self.app_state.current_solution_path()? {