Make it easier to detect when bootstrap tries to read uncaptured stdout/stderr

If e.g. only stdout is captured, but the caller tries to read stderr, previously
they would get back an empty string. Now the code will explicitly panic when
accessing an uncaptured output stream.

src/bootstrap/src/lib.rs

@@ -968,7 +968,9 @@ impl Build {
         let mut message = String::new();
         let output: CommandOutput = match output {
             // Command has succeeded
-            Ok(output) if output.status.success() => output.into(),
+            Ok(output) if output.status.success() => {
+                CommandOutput::from_output(output, stdout, stderr)
+            }
             // Command has started, but then it failed
             Ok(output) => {
                 writeln!(
@@ -982,7 +984,7 @@ Executed at: {executed_at}"#,
                 )
                 .unwrap();
 
-                let output: CommandOutput = output.into();
+                let output: CommandOutput = CommandOutput::from_output(output, stdout, stderr);
 
                 // If the output mode is OutputMode::Capture, we can now print the output.
                 // If it is OutputMode::Print, then the output has already been printed to

src/bootstrap/src/utils/exec.rs

@@ -223,17 +223,31 @@ pub fn command<S: AsRef<OsStr>>(program: S) -> BootstrapCommand {
 }
 
 /// Represents the output of an executed process.
-#[allow(unused)]
 pub struct CommandOutput {
     status: CommandStatus,
-    stdout: Vec<u8>,
-    stderr: Vec<u8>,
+    stdout: Option<Vec<u8>>,
+    stderr: Option<Vec<u8>>,
 }
 
 impl CommandOutput {
     #[must_use]
     pub fn did_not_start() -> Self {
-        Self { status: CommandStatus::DidNotStart, stdout: vec![], stderr: vec![] }
+        Self { status: CommandStatus::DidNotStart, stdout: None, stderr: None }
+    }
+
+    #[must_use]
+    pub fn from_output(output: Output, stdout: OutputMode, stderr: OutputMode) -> Self {
+        Self {
+            status: CommandStatus::Finished(output.status),
+            stdout: match stdout {
+                OutputMode::Print => None,
+                OutputMode::Capture => Some(output.stdout),
+            },
+            stderr: match stderr {
+                OutputMode::Print => None,
+                OutputMode::Capture => Some(output.stderr),
+            },
+        }
     }
 
     #[must_use]
@@ -259,7 +273,10 @@ impl CommandOutput {
 
     #[must_use]
     pub fn stdout(&self) -> String {
-        String::from_utf8(self.stdout.clone()).expect("Cannot parse process stdout as UTF-8")
+        String::from_utf8(
+            self.stdout.clone().expect("Accessing stdout of a command that did not capture stdout"),
+        )
+        .expect("Cannot parse process stdout as UTF-8")
     }
 
     #[must_use]
@@ -269,7 +286,10 @@ impl CommandOutput {
 
     #[must_use]
     pub fn stderr(&self) -> String {
-        String::from_utf8(self.stderr.clone()).expect("Cannot parse process stderr as UTF-8")
+        String::from_utf8(
+            self.stderr.clone().expect("Accessing stderr of a command that did not capture stderr"),
+        )
+        .expect("Cannot parse process stderr as UTF-8")
     }
 }
 
@@ -277,18 +297,8 @@ impl Default for CommandOutput {
     fn default() -> Self {
         Self {
             status: CommandStatus::Finished(ExitStatus::default()),
-            stdout: vec![],
-            stderr: vec![],
-        }
-    }
-}
-
-impl From<Output> for CommandOutput {
-    fn from(output: Output) -> Self {
-        Self {
-            status: CommandStatus::Finished(output.status),
-            stdout: output.stdout,
-            stderr: output.stderr,
+            stdout: Some(vec![]),
+            stderr: Some(vec![]),
         }
     }
 }