fix(flags): add a new experimental-features flag to gate new features starting with the existing "tui" flag (#385)

Author: kallevmercury

src/cli.rs

@@ -13,6 +13,13 @@ use crate::clonable_command::ClonableCommand;
 use crate::ignore::GlobMatcher;
 use crate::normal_path::NormalPath;
 
+/// An experimental feature that can be enabled with `--experimental-features`.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, clap::ValueEnum)]
+pub enum ExperimentalFeature {
+    /// Enable TUI mode.
+    Tui,
+}
+
 /// Ghciwatch loads a GHCi session for a Haskell project and reloads it
 /// when source files change.
 #[derive(Debug, Clone, Parser)]
@@ -62,7 +69,15 @@ pub struct Opts {
     #[arg(long)]
     pub no_interrupt_reloads: bool,
 
-    /// Enable TUI mode (experimental).
+    /// Enable experimental features. These features are unsupported and may change or be removed
+    /// without notice.
+    ///
+    /// Can be given multiple times.
+    #[arg(long = "experimental-features", value_name = "FEATURE", hide = true)]
+    pub experimental_features: Vec<ExperimentalFeature>,
+
+    /// Deprecated: use `--experimental-features tui` instead.
+    // TODO: Remove after 2026-06-01.
     #[arg(long, hide = true)]
     pub tui: bool,
 
@@ -211,6 +226,11 @@ pub struct LoggingOpts {
 }
 
 impl Opts {
+    /// Check whether a given experimental feature has been enabled.
+    pub fn has_experimental_feature(&self, feature: ExperimentalFeature) -> bool {
+        self.experimental_features.contains(&feature)
+    }
+
     /// Perform late initialization of the command-line arguments. If `init` isn't called before
     /// the arguments are used, the behavior is undefined.
     pub fn init(&mut self) -> miette::Result<()> {

src/ghci/mod.rs

@@ -71,6 +71,7 @@ use loaded_module::LoadedModule;
 
 use crate::aho_corasick::AhoCorasickExt;
 use crate::buffers::LINE_BUFFER_CAPACITY;
+use crate::cli::ExperimentalFeature;
 use crate::cli::Opts;
 use crate::clonable_command::ClonableCommand;
 use crate::event_filter::FileEvent;
@@ -142,7 +143,7 @@ impl GhciOpts {
         let stderr_writer;
         let tui_reader;
 
-        if opts.tui {
+        if opts.has_experimental_feature(ExperimentalFeature::Tui) {
             let (tui_writer, tui_reader_inner) = tokio::io::duplex(GHCI_BUFFER_CAPACITY);
             let tui_writer = GhciWriter::duplex_stream(tui_writer);
             stdout_writer = tui_writer.clone();

src/main.rs

@@ -9,22 +9,37 @@ use std::time::Duration;
 use clap::CommandFactory;
 use clap::Parser;
 use ghciwatch::cli;
+use ghciwatch::cli::ExperimentalFeature;
 use ghciwatch::run_ghci;
 use ghciwatch::run_tui;
 use ghciwatch::run_watcher;
 use ghciwatch::GhciOpts;
 use ghciwatch::ShutdownManager;
 use ghciwatch::TracingOpts;
 use ghciwatch::WatcherOpts;
+use miette::miette;
 use tokio::sync::mpsc;
 
 #[tokio::main]
 async fn main() -> miette::Result<()> {
     miette::set_panic_hook();
     let mut opts = cli::Opts::parse();
     opts.init()?;
+
+    if opts.tui {
+        return Err(miette!(
+            "`--tui` has been removed. Please use `--experimental-features tui` instead."
+        ));
+    }
+
     let (maybe_tracing_reader, _tracing_guard) = TracingOpts::from_cli(&opts).install()?;
 
+    if !opts.experimental_features.is_empty() {
+        tracing::warn!(
+            "`--experimental-features` may contain bugs or change drastically in future releases."
+        );
+    }
+
     #[cfg(feature = "clap-markdown")]
     if opts.generate_markdown_help {
         println!("{}", ghciwatch::clap_markdown::help_markdown::<cli::Opts>());
@@ -58,7 +73,7 @@ async fn main() -> miette::Result<()> {
 
     let mut manager = ShutdownManager::with_timeout(Duration::from_secs(1));
 
-    if opts.tui {
+    if opts.has_experimental_feature(ExperimentalFeature::Tui) {
         let tracing_reader =
             maybe_tracing_reader.expect("`tracing_reader` must be present if `tui` is given");
         let ghci_reader =

src/tracing/mod.rs

@@ -15,6 +15,7 @@ use tracing_subscriber::util::SubscriberInitExt;
 use tracing_subscriber::EnvFilter;
 use tracing_subscriber::Layer;
 
+use crate::cli::ExperimentalFeature;
 use crate::cli::Opts;
 
 /// Options for initializing the [`tracing`] logging framework. This is like a lower-effort builder
@@ -40,7 +41,7 @@ impl<'opts> TracingOpts<'opts> {
             filter_directives: &opts.logging.log_filter,
             trace_spans: &opts.logging.trace_spans,
             json_log_path: opts.logging.log_json.as_deref(),
-            tui: if opts.tui {
+            tui: if opts.has_experimental_feature(ExperimentalFeature::Tui) {
                 Some(tokio::io::duplex(crate::buffers::TRACING_BUFFER_CAPACITY))
             } else {
                 None

src/tui/mod.rs

@@ -227,8 +227,6 @@ pub async fn run_tui(
 
     let mut event_stream = EventStream::new();
 
-    tracing::warn!("`--tui` mode is experimental and may contain bugs or change drastically in future releases.");
-
     while !tui.quit {
         tui.render()?;
 

tests/clap_markdown.rs

@@ -35,9 +35,9 @@ fn test_clap_markdown() {
             #[arg(long, alias = "allow-eval")]
             pub enable_eval: bool,
 
-            /// Enable TUI mode (experimental).
-            #[arg(long, hide = true)]
-            pub tui: bool,
+            /// Enable experimental features.
+            #[arg(long = "experimental-features", value_name = "FEATURE", hide = true)]
+            pub experimental_features: Vec<ExperimentalFeature>,
 
             /// Options to modify file watching.
             #[command(flatten)]
@@ -48,6 +48,11 @@ fn test_clap_markdown() {
             pub logging: LoggingOpts,
         }
 
+        #[derive(Debug, Clone, Copy, PartialEq, Eq, clap::ValueEnum)]
+        pub enum ExperimentalFeature {
+            Tui,
+        }
+
         /// Options for watching files.
         #[derive(Debug, Clone, clap::Args)]
         #[clap(next_help_heading = "File watching options")]

tests/experimental_features.rs

@@ -0,0 +1,46 @@
+use std::path::PathBuf;
+
+use test_harness::test;
+use test_harness::GhciWatchBuilder;
+use tokio::process::Command;
+
+/// Invalid experimental feature values should produce an error immediately.
+#[test]
+async fn invalid_experimental_feature_errors() {
+    let result = GhciWatchBuilder::new("tests/data/simple")
+        .with_args(["--experimental-features", "asdflkj"])
+        .start()
+        .await;
+    assert!(
+        result.is_err(),
+        "ghciwatch should error on invalid experimental feature"
+    );
+}
+
+/// Runs ghciwatch directly (not via `GhciWatchBuilder`) because TUI mode
+/// crashes without a terminal, exiting before the test harness can connect.
+#[tokio::test]
+async fn experimental_features_emits_warning() {
+    let log_dir = PathBuf::from(env!("CARGO_TARGET_TMPDIR")).join("experimental-warning");
+    std::fs::create_dir_all(&log_dir).expect("can create log dir");
+    let log_path = log_dir.join("ghciwatch.json");
+
+    let _output = Command::new(env!("CARGO_BIN_EXE_ghciwatch"))
+        .args(["--experimental-features", "tui"])
+        .arg("--log-json")
+        .arg(&log_path)
+        .args(["--watch", "src"])
+        .current_dir("tests/data/simple")
+        .output()
+        .await
+        .expect("can run ghciwatch");
+
+    let log_contents = std::fs::read_to_string(&log_path)
+        .unwrap_or_else(|e| panic!("can read log file at {}: {e}", log_path.display()));
+    assert!(
+        log_contents.contains("--experimental-features"),
+        "warning about experimental features should appear in JSON log"
+    );
+
+    let _ = std::fs::remove_dir_all(&log_dir);
+}