5 files changed, 37 insertions, 8 deletions
diff --git a/clippy_dev/src/bless.rs b/clippy_dev/src/bless.rs
index b877806946c..2a869e9d449 100644
--- a/clippy_dev/src/bless.rs
+++ b/clippy_dev/src/bless.rs
@@ -24,6 +24,9 @@ static CLIPPY_BUILD_TIME: SyncLazy<Option<std::time::SystemTime>> = SyncLazy::ne
     fs::metadata(path).ok()?.modified().ok()
 });
 
+/// # Panics
+///
+/// Panics if the path to a test file is broken
 pub fn bless(ignore_timestamp: bool) {
     let test_suite_dirs = [
         clippy_project_root().join("tests").join("ui"),
diff --git a/clippy_dev/src/fmt.rs b/clippy_dev/src/fmt.rs
index 6b528d219df..4d0fdadbd85 100644
--- a/clippy_dev/src/fmt.rs
+++ b/clippy_dev/src/fmt.rs
@@ -8,7 +8,7 @@ use walkdir::WalkDir;
 
 #[derive(Debug)]
 pub enum CliError {
-    CommandFailed(String),
+    CommandFailed(String, String),
     IoError(io::Error),
     RustfmtNotInstalled,
     WalkDirError(walkdir::Error),
@@ -75,8 +75,8 @@ pub fn run(check: bool, verbose: bool) {
 
     fn output_err(err: CliError) {
         match err {
-            CliError::CommandFailed(command) => {
-                eprintln!("error: A command failed! `{}`", command);
+            CliError::CommandFailed(command, stderr) => {
+                eprintln!("error: A command failed! `{}`\nstderr: {}", command, stderr);
             },
             CliError::IoError(err) => {
                 eprintln!("error: {}", err);
@@ -136,12 +136,16 @@ fn exec(
         println!("{}", format_command(&program, &dir, args));
     }
 
-    let mut child = Command::new(&program).current_dir(&dir).args(args.iter()).spawn()?;
-    let code = child.wait()?;
-    let success = code.success();
+    let child = Command::new(&program).current_dir(&dir).args(args.iter()).spawn()?;
+    let output = child.wait_with_output()?;
+    let success = output.status.success();
 
     if !context.check && !success {
-        return Err(CliError::CommandFailed(format_command(&program, &dir, args)));
+        let stderr = std::str::from_utf8(&output.stderr).unwrap_or("");
+        return Err(CliError::CommandFailed(
+            format_command(&program, &dir, args),
+            String::from(stderr),
+        ));
     }
 
     Ok(success)
@@ -177,7 +181,10 @@ fn rustfmt_test(context: &FmtContext) -> Result<(), CliError> {
     {
         Err(CliError::RustfmtNotInstalled)
     } else {
-        Err(CliError::CommandFailed(format_command(&program, &dir, args)))
+        Err(CliError::CommandFailed(
+            format_command(&program, &dir, args),
+            std::str::from_utf8(&output.stderr).unwrap_or("").to_string(),
+        ))
     }
 }
 
diff --git a/clippy_dev/src/lib.rs b/clippy_dev/src/lib.rs
index 24d70d433f3..01d1fc9211a 100644
--- a/clippy_dev/src/lib.rs
+++ b/clippy_dev/src/lib.rs
@@ -236,6 +236,10 @@ pub struct FileChange {
 /// `path` is the relative path to the file on which you want to perform the replacement.
 ///
 /// See `replace_region_in_text` for documentation of the other options.
+///
+/// # Panics
+///
+/// Panics if the path could not read or then written
 pub fn replace_region_in_file<F>(
     path: &Path,
     start: &str,
@@ -283,6 +287,10 @@ where
 ///     .new_lines;
 /// assert_eq!("replace_start\na different\ntext\nreplace_end", result);
 /// ```
+///
+/// # Panics
+///
+/// Panics if start or end is not valid regex
 pub fn replace_region_in_text<F>(text: &str, start: &str, end: &str, replace_start: bool, replacements: F) -> FileChange
 where
     F: FnOnce() -> Vec<String>,
@@ -329,6 +337,11 @@ where
 }
 
 /// Returns the path to the Clippy project directory
+///
+/// # Panics
+///
+/// Panics if the current directory could not be retrieved, there was an error reading any of the
+/// Cargo.toml files or ancestor directory is the clippy root directory
 #[must_use]
 pub fn clippy_project_root() -> PathBuf {
     let current_dir = std::env::current_dir().unwrap();
diff --git a/clippy_dev/src/ra_setup.rs b/clippy_dev/src/ra_setup.rs
index a8a6a2cb1bd..a3c329b578b 100644
--- a/clippy_dev/src/ra_setup.rs
+++ b/clippy_dev/src/ra_setup.rs
@@ -8,6 +8,9 @@ use std::path::{Path, PathBuf};
 // This allows rust analyzer to analyze rustc internals and show proper information inside clippy
 // code. See https://github.com/rust-analyzer/rust-analyzer/issues/3517 and https://github.com/rust-lang/rust-clippy/issues/5514 for details
 
+/// # Panics
+///
+/// Panics if `rustc_path` does not lead to a rustc repo or the files could not be read
 pub fn run(rustc_path: Option<&str>) {
     // we can unwrap here because the arg is required by clap
     let rustc_path = PathBuf::from(rustc_path.unwrap());
diff --git a/clippy_dev/src/serve.rs b/clippy_dev/src/serve.rs
index a46c0e4d3f0..faa94859601 100644
--- a/clippy_dev/src/serve.rs
+++ b/clippy_dev/src/serve.rs
@@ -4,6 +4,9 @@ use std::process::Command;
 use std::thread;
 use std::time::{Duration, SystemTime};
 
+/// # Panics
+///
+/// Panics if the python commands could not be spawned
 pub fn run(port: u16, lint: Option<&str>) -> ! {
     let mut url = Some(match lint {
         None => format!("http://localhost:{}", port),