cli: print source chain of internal error in a similar way to anyhow

Multi-line output should be easier to follow than lengthy line separated by
colon.

cli/src/cli_util.rs

@@ -105,7 +105,7 @@ pub enum CommandError {
 
 /// Wraps error with user-visible message.
 #[derive(Debug, Error)]
-#[error("{message}: {source}")]
+#[error("{message}")]
 struct ErrorWithMessage {
     message: String,
     source: Box<dyn std::error::Error + Send + Sync>,
@@ -160,6 +160,33 @@ fn format_similarity_hint<S: AsRef<str>>(candidates: &[S]) -> Option<String> {
     }
 }
 
+fn print_error_sources(ui: &Ui, source: Option<&dyn std::error::Error>) -> io::Result<()> {
+    let Some(err) = source else {
+        return Ok(());
+    };
+    if err.source().is_none() {
+        writeln!(ui.stderr(), "Caused by: {err}")?;
+    } else {
+        writeln!(ui.stderr(), "Caused by:")?;
+        for (i, err) in iter::successors(Some(err), |err| err.source()).enumerate() {
+            let message = strip_error_source(err);
+            writeln!(ui.stderr(), "{n}: {message}", n = i + 1)?;
+        }
+    }
+    Ok(())
+}
+
+// TODO: remove ": {source}" from error types and drop this hack
+fn strip_error_source(err: &dyn std::error::Error) -> String {
+    let mut message = err.to_string();
+    if let Some(source) = err.source() {
+        if let Some(s) = message.strip_suffix(&format!(": {source}")) {
+            message.truncate(s.len());
+        }
+    }
+    message
+}
+
 impl From<std::io::Error> for CommandError {
     fn from(err: std::io::Error) -> Self {
         if err.kind() == std::io::ErrorKind::BrokenPipe {
@@ -2793,7 +2820,7 @@ pub fn handle_command_result(
     ui: &mut Ui,
     result: Result<(), CommandError>,
 ) -> std::io::Result<ExitCode> {
-    match result {
+    match &result {
         Ok(()) => Ok(ExitCode::SUCCESS),
         Err(CommandError::UserError { message, hint }) => {
             writeln!(ui.error(), "Error: {message}")?;
@@ -2846,7 +2873,8 @@ pub fn handle_command_result(
             Ok(ExitCode::from(BROKEN_PIPE_EXIT_CODE))
         }
         Err(CommandError::InternalError(err)) => {
-            writeln!(ui.error(), "Internal error: {err}")?;
+            writeln!(ui.error(), "Internal error: {}", strip_error_source(err))?;
+            print_error_sources(ui, err.source())?;
             Ok(ExitCode::from(255))
         }
     }

cli/tests/test_edit_command.rs

@@ -104,7 +104,11 @@ fn test_edit_current_wc_commit_missing() {
         .assert()
         .code(255);
     insta::assert_snapshot!(get_stderr_string(&assert), @r###"
-    Internal error: Failed to edit a commit: Current working-copy commit not found: Object 69542c1984c1f9d91f7c6c9c9e6941782c944bd9 of type commit not found: An object with id 69542c1984c1f9d91f7c6c9c9e6941782c944bd9 could not be found
+    Internal error: Failed to edit a commit
+    Caused by:
+    1: Current working-copy commit not found
+    2: Object 69542c1984c1f9d91f7c6c9c9e6941782c944bd9 of type commit not found
+    3: An object with id 69542c1984c1f9d91f7c6c9c9e6941782c944bd9 could not be found
     "###);
 }
 

cli/tests/test_global_opts.rs

@@ -219,15 +219,18 @@ fn test_broken_repo_structure() {
     // Test the error message when the git repository can't be located.
     std::fs::remove_file(store_path.join("git_target")).unwrap();
     let stderr = test_env.jj_cmd_internal_error(&repo_path, &["log"]);
-    insta::assert_snapshot!(stderr, @r###"
-    Internal error: The repository appears broken or inaccessible: Cannot access $TEST_ENV/repo/.jj/repo/store/git_target
+    insta::assert_snapshot!(strip_last_line(&stderr), @r###"
+    Internal error: The repository appears broken or inaccessible
+    Caused by:
+    1: Cannot access $TEST_ENV/repo/.jj/repo/store/git_target
     "###);
 
     // Test the error message when the commit backend is of unknown type.
     std::fs::write(&store_type_path, "unknown").unwrap();
     let stderr = test_env.jj_cmd_internal_error(&repo_path, &["log"]);
     insta::assert_snapshot!(stderr, @r###"
-    Internal error: This version of the jj binary doesn't support this type of repo: Unsupported commit backend type 'unknown'
+    Internal error: This version of the jj binary doesn't support this type of repo
+    Caused by: Unsupported commit backend type 'unknown'
     "###);
 
     // Test the error message when the file indicating the commit backend type
@@ -235,8 +238,11 @@ fn test_broken_repo_structure() {
     std::fs::remove_file(&store_type_path).unwrap();
     std::fs::create_dir(&store_type_path).unwrap();
     let stderr = test_env.jj_cmd_internal_error(&repo_path, &["log"]);
-    insta::assert_snapshot!(stderr, @r###"
-    Internal error: The repository appears broken or inaccessible: Failed to read commit backend type: Cannot access $TEST_ENV/repo/.jj/repo/store/type
+    insta::assert_snapshot!(strip_last_line(&stderr), @r###"
+    Internal error: The repository appears broken or inaccessible
+    Caused by:
+    1: Failed to read commit backend type
+    2: Cannot access $TEST_ENV/repo/.jj/repo/store/type
     "###);
 
     // Test when the .jj directory is empty. The error message is identical to
@@ -244,8 +250,11 @@ fn test_broken_repo_structure() {
     std::fs::remove_dir_all(repo_path.join(".jj")).unwrap();
     std::fs::create_dir(repo_path.join(".jj")).unwrap();
     let stderr = test_env.jj_cmd_internal_error(&repo_path, &["log"]);
-    insta::assert_snapshot!(stderr, @r###"
-    Internal error: The repository appears broken or inaccessible: Failed to read commit backend type: Cannot access $TEST_ENV/repo/.jj/repo/store/type
+    insta::assert_snapshot!(strip_last_line(&stderr), @r###"
+    Internal error: The repository appears broken or inaccessible
+    Caused by:
+    1: Failed to read commit backend type
+    2: Cannot access $TEST_ENV/repo/.jj/repo/store/type
     "###);
 }
 
@@ -456,3 +465,9 @@ fn test_verbose_logging_enabled() {
     // Luckily, insta will print this in colour when reviewing.
     insta::assert_snapshot!(log_line, @" INFO jj_cli::cli_util: verbose logging enabled");
 }
+
+fn strip_last_line(s: &str) -> &str {
+    s.trim_end_matches('\n')
+        .rsplit_once('\n')
+        .map_or(s, |(h, _)| &s[..h.len() + 1])
+}