Plan: Embedded Nushell Engine (replace PTY-spawned binary)

Context

The current terminal spawns nushell as an external binary via PTY. This is broken — nushell doesn't produce visible output through the PTY→alacritty_terminal→sugarloaf pipeline (while zsh works fine). The root cause: nushell's interactive mode has complex terminal negotiation that conflicts with our PTY bridge.

The fix: embed nushell as a Rust library. No spawned process, no PTY, no file descriptors. Nushell evaluates commands in-process, produces ANSI output bytes, which feed directly into alacritty_terminal's grid, rendered by sugarloaf.

Obsolete plans 004-remove-egui-add-ascii-terminal.md and composed-bubbling-floyd.md will be deleted.

Architecture

Bevy KeyboardInput → LineBuffer (accumulate chars)
    │ on Enter
    ▼
nu-parser::parse() → nu-engine::eval_block() → PipelineData
    │
    ▼
Capture output as Vec<u8> (with ANSI colors from nu-table)
    │
    ▼
alacritty_terminal::Processor::advance(&mut Term, &bytes)  ← NO PTY
    │
    ▼
Term.renderable_content() → sugarloaf content builder → GPU render

Steps

Step 1: Cargo dependencies

cyb/Cargo.toml — add nushell to exclude:

exclude = ["vendor/rio-backend", "vendor/copa", "vendor/nushell"]

cyb/cyb-shell/Cargo.toml — add nu crates as path deps:

nu-protocol = { path = "../vendor/nushell/crates/nu-protocol" }
nu-engine = { path = "../vendor/nushell/crates/nu-engine" }
nu-parser = { path = "../vendor/nushell/crates/nu-parser" }
nu-command = { path = "../vendor/nushell/crates/nu-command", default-features = false }
nu-cmd-lang = { path = "../vendor/nushell/crates/nu-cmd-lang" }
nu-cmd-extra = { path = "../vendor/nushell/crates/nu-cmd-extra" }
nu-cli = { path = "../vendor/nushell/crates/nu-cli", default-features = false }
nu-std = { path = "../vendor/nushell/crates/nu-std" }
nu-utils = { path = "../vendor/nushell/crates/nu-utils" }

Remove: libc (was for PTY fd ops). Keep: alacritty_terminal, sugarloaf, bevy.

Verify: cargo check -p cyb-shell compiles.

Step 2: New structs (replace PTY-based TerminalState)

In terminal.rs, replace old state with:

struct NuShellEngine {
    engine_state: EngineState,
    stack: Stack,
}

struct LineBuffer {
    buffer: String,
    cursor_pos: usize,
    history: Vec<String>,
    history_index: Option<usize>,
}

struct TerminalState {
    nu_engine: Option<NuShellEngine>,  // Option because moved to bg thread during eval
    term: Arc<FairMutex<Term<BevyEventProxy>>>,
    processor: Processor,
    line_buffer: LineBuffer,
    cols: usize,
    rows: usize,
    rich_text_id: usize,
    eval_rx: Option<std::sync::mpsc::Receiver<(EvalResult, NuShellEngine)>>,
    eval_in_progress: bool,
}

Remove: pty_writer, _reader_handle, _pty, all PTY imports.

Step 3: Engine initialization (rewrite setup_terminal)

Keep sugarloaf setup identical. Replace PTY block with:

1. EngineState::new() + add command contexts (nu_cmd_lang, nu_command, nu_cmd_extra, nu_cli)
2. gather_parent_env_vars() + set PWD
3. load_standard_library()
4. convert_env_values()
5. Load bundled config via eval_source() (embed env.nu/config.nu with include_str!)
6. Force config.use_ansi_coloring = True
7. Create Term WITHOUT PTY (just Term::new(config, dims, BevyEventProxy))
8. Create Processor::new()
9. Evaluate prompt, feed ANSI bytes into Term
10. No reader thread, no fd duplication, no tty::new()

Step 4: Keyboard input → line buffer

Replace PTY-write with line buffer accumulation:

• Characters: insert into buffer, echo to Term via processor.advance()
• Enter: take buffer, dispatch eval
• Backspace: remove char, send \x08 \x08 to Term
• Arrow up/down: navigate history (clear line with \r\x1b[K, redraw prompt + history entry)
• Arrow left/right: move cursor in buffer, send \x1b[D/\x1b[C
• Block input while eval_in_progress

Step 5: Command evaluation with output capture

fn evaluate_and_capture(engine: &mut NuShellEngine, input: &str) -> Result<Vec<u8>, String>

1. parse() input → AST block
2. eval_block::<WithoutDebug>() → PipelineData
3. Capture output:
   • PipelineData::Empty → nothing
   • PipelineData::ByteStream → write_to(&mut buf)
   • PipelineData::Value/ListStream → pipe through table command → collect string with ANSI codes
4. engine_state.merge_env(&stack) to persist env changes (cd, export, etc.)
5. Return Vec<u8> with ANSI-colored output

Never call print_table() or print_pipeline() — they write to real stdout.

Step 6: Async eval (background thread)

For long-running commands:

1. dispatch_eval(): take NuShellEngine out of state, spawn std::thread, send result via mpsc::channel
2. New system poll_eval_results: try_recv() each frame, on result:
   • Return engine to state
   • Feed output bytes into Term
   • Evaluate and render prompt
   • Set eval_in_progress = false

Step 7: Prompt rendering

Evaluate $env.PROMPT_COMMAND` and `$env.PROMPT_INDICATOR closures:

• Use ClosureEvalOnce to run them
• Collect string output (contains ANSI codes)
• Feed \r + prompt bytes into Term

Step 8: Render system (NO CHANGES)

render_terminal stays exactly as-is. It reads term.renderable_content() and builds sugarloaf content. The only difference: Term is now fed by processor.advance() from eval results instead of a PTY reader thread.

Step 9: Makefile cleanup

• Remove nu-build target
• Remove nu binary copy from dmg target
• Remove nushell config copy (configs now embedded via include_str!)
• dmg depends only on release (not nu-build)

Step 10: Build & verify

cd cyb && make dmg — should produce smaller DMG (no 56MB nu binary).

Files

Risks

Verify

1. cargo check -p cyb-shell — compiles
2. cd cyb && make dmg — DMG builds (smaller, no nu binary)
3. Launch app → Cmd+3 → Terminal:
   • Prompt with ANSI colors visible
   • Type 1 + 1 → see 2
   • Type ls → see colored table output
   • Type "hello" | str upcase → see HELLO
   • Arrow up → history navigation
   • Cursor visible and blinking
4. Cmd+4 → Cmd+3 → no crash (state cleanup/reinit)