nu_protocol/engine/

engine_state.rs

use crate::{
    BlockId, Config, DeclId, FileId, GetSpan, Handlers, HistoryConfig, JobId, Module, ModuleId,
    OverlayId, ShellError, SignalAction, Signals, Signature, Span, SpanId, Type, Value, VarId,
    VirtualPathId,
    ast::{Block, Expr},
    debugger::{Debugger, NoopDebugger},
    engine::{
        CachedFile, Command, DEFAULT_OVERLAY_NAME, EnvName, EnvVars, OverlayFrame, PromptState,
        ScopeFrame, Stack, StateDelta, Variable, Visibility,
        description::{Doccomments, build_desc},
    },
    eval_const::create_nu_constant,
    report_error::ReportLog,
    shell_error::{generic::GenericError, io::IoError},
};
use fancy_regex::Regex;
use lru::LruCache;
use nu_config::NushellConfigDirs;
use nu_path::AbsolutePathBuf;
use std::{
    collections::{HashMap, HashSet},
    num::NonZeroUsize,
    path::PathBuf,
    sync::{
        Arc, Mutex, MutexGuard, PoisonError,
        atomic::{AtomicBool, AtomicU32, Ordering},
        mpsc::Sender,
        mpsc::channel,
    },
};

type PoisonDebuggerError<'a> = PoisonError<MutexGuard<'a, Box<dyn Debugger>>>;

#[cfg(feature = "plugin")]
use crate::{PluginRegistryFile, PluginRegistryItem, RegisteredPlugin};

use super::{CurrentJob, Jobs, Mail, Mailbox, ThreadJob};

/// Configure whether the current working directory may be updated when [`EngineState::merge_env`]
/// is called.
///
/// During testing, this is causing issues, so this may disable it.
pub static UPDATE_CWD: AtomicBool = AtomicBool::new(true);

#[derive(Clone, Debug)]
pub enum VirtualPath {
    File(FileId),
    Dir(Vec<VirtualPathId>),
}

#[derive(Debug, Default)]
pub struct ReplState {
    pub buffer: String,
    // A byte position, as `EditCommand::MoveToPosition` is also a byte position
    pub cursor_pos: usize,
    /// Immediately accept the buffer on the next loop.
    pub accept: bool,
}

#[derive(Debug)]
pub struct IsDebugging(AtomicBool);

impl IsDebugging {
    pub fn new(val: bool) -> Self {
        IsDebugging(AtomicBool::new(val))
    }
}

impl Clone for IsDebugging {
    fn clone(&self) -> Self {
        IsDebugging(AtomicBool::new(self.0.load(Ordering::Relaxed)))
    }
}

/// The core global engine state. This includes all global definitions as well as any global state that
/// will persist for the whole session.
///
/// Declarations, variables, blocks, and other forms of data are held in the global state and referenced
/// elsewhere using their IDs. These IDs are simply their index into the global state. This allows us to
/// more easily handle creating blocks, binding variables and callsites, and more, because each of these
/// will refer to the corresponding IDs rather than their definitions directly. At runtime, this means
/// less copying and smaller structures.
///
/// Many of the larger objects in this structure are stored within `Arc` to decrease the cost of
/// cloning `EngineState`. While `Arc`s are generally immutable, they can be modified using
/// `Arc::make_mut`, which automatically clones to a new allocation if there are other copies of
/// the `Arc` already in use, but will let us modify the `Arc` directly if we have the only
/// reference to it.
///
/// Note that the runtime stack is not part of this global state. Runtime stacks are handled differently,
/// but they also rely on using IDs rather than full definitions.
#[derive(Clone, derive_more::Debug)]
pub struct EngineState {
    files: Vec<CachedFile>,
    pub(super) virtual_paths: Vec<(String, VirtualPath)>,
    vars: Vec<Variable>,
    #[debug("{:?}", decls.iter().map(|c| c.name()).collect::<Vec<_>>())]
    decls: Arc<Vec<Box<dyn Command + 'static>>>,
    // The Vec is wrapped in Arc so that if we don't need to modify the list, we can just clone
    // the reference and not have to clone each individual Arc inside. These lists can be
    // especially long, so it helps
    #[debug("{:?}", blocks.iter().map(|b| &b.signature.name))]
    pub(super) blocks: Arc<Vec<Arc<Block>>>,
    #[debug("{:?}", modules.iter().map(|m| String::from_utf8_lossy(&m.name)))]
    pub(super) modules: Arc<Vec<Arc<Module>>>,
    pub spans: Vec<Span>,
    doccomments: Doccomments,
    pub scope: ScopeFrame,
    signals: Signals,
    pub signal_handlers: Option<Handlers>,
    pub env_vars: Arc<EnvVars>,
    pub previous_env_vars: Arc<HashMap<EnvName, Value>>,
    pub config: Arc<Config>,
    pub pipeline_externals_state: Arc<(AtomicU32, AtomicU32)>,
    pub repl_state: Arc<Mutex<ReplState>>,
    /// Shared source of truth for the interactive prompt's rendered content. The
    /// REPL fills it each cycle from `$env.PROMPT_COMMAND` and friends; a
    /// background job's `commandline set-prompt` overwrites individual segments
    /// and triggers an in-place repaint. Shared with every job that clones this
    /// engine state.
    pub prompt_state: Arc<PromptState>,
    pub table_decl_id: Option<DeclId>,
    #[cfg(feature = "plugin")]
    pub plugin_path: Option<PathBuf>,
    #[cfg(feature = "plugin")]
    #[debug("{:?}", plugins.iter().map(|rp| rp.identity().name()).collect::<Vec<_>>())]
    plugins: Vec<Arc<dyn RegisteredPlugin>>,
    /// Resolved configuration directories and file paths.
    ///
    /// Populated once at startup by `nu_config::resolve_paths()` in `main.rs`.
    /// All downstream code (config-file loading, `$nu` constant generation,
    /// history backend, etc.) reads from this struct.
    pub config_dirs: NushellConfigDirs,

    pub history_enabled: bool,
    pub history_session_id: i64,
    /// Whether the startup-only `$env.config.history.*` options are locked from further
    /// changes (currently `path`, `max_size`, `file_format`, `isolation`).
    ///
    /// Set to `true` once the REPL has finished initializing reedline's history backend.
    /// After that point, changing any of these options would have no effect on the live
    /// history, so attempts to mutate them are rejected with an error instead of being
    /// silently ignored.
    pub history_locked_after_startup: bool,
    // Path to the file Nushell is currently evaluating, or None if we're in an interactive session.
    pub file: Option<PathBuf>,
    pub regex_cache: Arc<Mutex<LruCache<String, Regex>>>,
    pub is_interactive: bool,
    /// When true with [`Self::is_interactive`], REPL user-line evaluation may store `$ans`.
    ///
    /// Set only around true REPL command evaluation (`do_run_cmd`), not config/env/banner
    /// startup, so startup scripts do not overwrite interactive last-result.
    pub capture_repl_last_result: bool,
    pub is_login: bool,
    pub is_lsp: bool,
    pub is_mcp: bool,
    startup_time: i64,
    is_debugging: IsDebugging,
    pub debugger: Arc<Mutex<Box<dyn Debugger>>>,
    pub report_log: Arc<Mutex<ReportLog>>,

    pub jobs: Arc<Mutex<Jobs>>,

    // The job being executed with this engine state, or None if main thread
    pub current_job: CurrentJob,

    pub root_job_sender: Sender<Mail>,

    // When there are background jobs running, the interactive behavior of `exit` changes depending on
    // the value of this flag:
    // - if this is false, then a warning about running jobs is shown and `exit` enables this flag
    // - if this is true, then `exit` will `std::process::exit`
    //
    // This ensures that running exit twice will terminate the program correctly
    pub exit_warning_given: Arc<AtomicBool>,
}

// The max number of compiled regexes to keep around in a LRU cache, arbitrarily chosen
const REGEX_CACHE_SIZE: usize = 100; // must be nonzero, otherwise will panic

pub const NU_VARIABLE_ID: VarId = VarId::new(0);
pub const IN_VARIABLE_ID: VarId = VarId::new(1);
pub const ENV_VARIABLE_ID: VarId = VarId::new(2);
/// Interactive last-result special variable.
///
/// The user-facing name is [`LAST_RESULT_VAR_NAME`] (e.g. `$ans`). Change that constant
/// if the public name should differ; keep this ID stable.
pub const LAST_VARIABLE_ID: VarId = VarId::new(3);
/// Identifier for the last-result special variable **without** the `$` sigil.
///
/// Change this single constant to rename the binding site-wide (e.g. `"ans"` → `$ans`).
/// The name is reserved (cannot be rebound with `let` / `mut` / `const`). Not user-configurable.
///
/// With a positive `max_last_result_size`, `$ans` is `{ last, exit_code, duration, command }`.
/// With size `0`, `$ans` is `{ exit_code, duration, command }` (`last` omitted). `command` is
/// the exact last REPL source (same buffer reedline stores in history). Resolved from
/// the stack's last-result slot, not from captured locals — keep capture-discovery
/// `>` checks in sync with this being the last special var id.
pub const LAST_RESULT_VAR_NAME: &str = "ans";
// NOTE: If you add more specials after LAST_VARIABLE_ID, update capture discovery
// (`var_id > LAST_VARIABLE_ID`) and any other special-id thresholds.

// The first span is unknown span
pub const UNKNOWN_SPAN_ID: SpanId = SpanId::new(0);

impl EngineState {
    pub fn new() -> Self {
        let (send, recv) = channel::<Mail>();

        Self {
            files: vec![],
            virtual_paths: vec![],
            vars: vec![
                Variable::new(Span::new(0, 0), Type::Any, false),
                Variable::new(Span::new(0, 0), Type::Any, false),
                Variable::new(Span::new(0, 0), Type::Any, false),
                Variable::new(Span::new(0, 0), Type::Any, false),
                Variable::new(Span::new(0, 0), Type::Any, false),
            ],
            decls: Arc::new(vec![]),
            blocks: Arc::new(vec![]),
            modules: Arc::new(vec![Arc::new(Module::new(
                DEFAULT_OVERLAY_NAME.as_bytes().to_vec(),
            ))]),
            spans: vec![Span::unknown()],
            doccomments: Doccomments::new(),
            // make sure we have some default overlay:
            scope: ScopeFrame::with_empty_overlay(
                DEFAULT_OVERLAY_NAME.as_bytes().to_vec(),
                ModuleId::new(0),
                false,
            ),
            signal_handlers: None,
            signals: Signals::empty(),
            env_vars: Arc::new(
                [(DEFAULT_OVERLAY_NAME.to_string(), HashMap::new())]
                    .into_iter()
                    .collect(),
            ),
            previous_env_vars: Arc::new(HashMap::new()),
            config: Arc::new(Config::default()),
            pipeline_externals_state: Arc::new((AtomicU32::new(0), AtomicU32::new(0))),
            repl_state: Arc::new(Mutex::new(ReplState {
                buffer: "".to_string(),
                cursor_pos: 0,
                accept: false,
            })),
            prompt_state: Arc::new(PromptState::new()),
            table_decl_id: None,
            #[cfg(feature = "plugin")]
            plugin_path: None,
            #[cfg(feature = "plugin")]
            plugins: vec![],
            config_dirs: NushellConfigDirs::empty(),
            history_enabled: true,
            history_session_id: 0,
            history_locked_after_startup: false,
            file: None,
            regex_cache: Arc::new(Mutex::new(LruCache::new(
                NonZeroUsize::new(REGEX_CACHE_SIZE).expect("tried to create cache of size zero"),
            ))),
            is_interactive: false,
            capture_repl_last_result: false,
            is_login: false,
            is_lsp: false,
            is_mcp: false,
            startup_time: -1,
            is_debugging: IsDebugging::new(false),
            debugger: Arc::new(Mutex::new(Box::new(NoopDebugger))),
            report_log: Arc::default(),
            jobs: Arc::new(Mutex::new(Jobs::default())),
            current_job: CurrentJob {
                id: JobId::new(0),
                background_thread_job: None,
                mailbox: Arc::new(Mutex::new(Mailbox::new(recv))),
            },
            root_job_sender: send,
            exit_warning_given: Arc::new(AtomicBool::new(false)),
        }
    }

    pub fn signals(&self) -> &Signals {
        &self.signals
    }

    /// Return a compiled regex, reusing the process-wide LRU cache when possible.
    ///
    /// On lock contention (or a poisoned mutex), compiles without touching the cache.
    pub fn get_cached_regex(&self, pattern: &str) -> Result<Regex, fancy_regex::Error> {
        match self.regex_cache.try_lock() {
            Ok(mut cache) => cache
                .try_get_or_insert_ref(pattern, || Regex::new(pattern))
                .cloned(),
            Err(_) => Regex::new(pattern),
        }
    }

    /// Compile `pattern` via [`Self::get_cached_regex`], mapping failures to
    /// [`ShellError::InvalidValue`] at `span`.
    pub fn compile_regex(&self, pattern: &str, span: Span) -> Result<Regex, ShellError> {
        self.get_cached_regex(pattern)
            .map_err(|err| invalid_regex_value(pattern, err, span))
    }

    pub fn reset_signals(&mut self) {
        self.signals.reset();
        if let Some(ref handlers) = self.signal_handlers {
            handlers.run(SignalAction::Reset);
        }
    }

    pub fn set_signals(&mut self, signals: Signals) {
        self.signals = signals;
    }

    /// Merges a `StateDelta` onto the current state. These deltas come from a system, like the parser, that
    /// creates a new set of definitions and visible symbols in the current scope. We make this transactional
    /// as there are times when we want to run the parser and immediately throw away the results (namely:
    /// syntax highlighting and completions).
    ///
    /// When we want to preserve what the parser has created, we can take its output (the `StateDelta`) and
    /// use this function to merge it into the global state.
    pub fn merge_delta(&mut self, mut delta: StateDelta) -> Result<(), ShellError> {
        // Take the mutable reference and extend the permanent state from the working set
        self.files.extend(delta.files);
        self.virtual_paths.extend(delta.virtual_paths);
        self.vars.extend(delta.vars);
        self.spans.extend(delta.spans);
        self.doccomments.merge_with(delta.doccomments);

        // Avoid potentially cloning the Arcs if we aren't adding anything
        if !delta.decls.is_empty() {
            Arc::make_mut(&mut self.decls).extend(delta.decls);
        }
        if !delta.blocks.is_empty() {
            Arc::make_mut(&mut self.blocks).extend(delta.blocks);
        }
        if !delta.modules.is_empty() {
            Arc::make_mut(&mut self.modules).extend(delta.modules);
        }

        let first = delta.scope.remove(0);

        for (delta_name, delta_overlay) in first.clone().overlays {
            if let Some((_, existing_overlay)) = self
                .scope
                .overlays
                .iter_mut()
                .find(|(name, _)| name == &delta_name)
            {
                // Updating existing overlay
                for item in delta_overlay.decls.into_iter() {
                    existing_overlay.decls.insert(item.0, item.1);
                }
                for item in delta_overlay.vars.into_iter() {
                    existing_overlay.insert_variable(item.0, item.1);
                }
                for item in delta_overlay.modules.into_iter() {
                    existing_overlay.modules.insert(item.0, item.1);
                }

                existing_overlay
                    .visibility
                    .merge_with(delta_overlay.visibility);
            } else {
                // New overlay was added to the delta
                self.scope.overlays.push((delta_name, delta_overlay));
            }
        }

        let mut activated_ids = self.translate_overlay_ids(&first);

        let mut removed_ids = vec![];

        for name in &first.removed_overlays {
            if let Some(overlay_id) = self.find_overlay(name) {
                removed_ids.push(overlay_id);
            }
        }

        // Remove overlays removed in delta
        self.scope
            .active_overlays
            .retain(|id| !removed_ids.contains(id));

        // Move overlays activated in the delta to be first
        self.scope
            .active_overlays
            .retain(|id| !activated_ids.contains(id));
        self.scope.active_overlays.append(&mut activated_ids);

        #[cfg(feature = "plugin")]
        if !delta.plugins.is_empty() {
            for plugin in std::mem::take(&mut delta.plugins) {
                // Connect plugins to the signal handlers
                if let Some(handlers) = &self.signal_handlers {
                    plugin.clone().configure_signal_handler(handlers)?;
                }

                // Replace plugins that overlap in identity.
                if let Some(existing) = self
                    .plugins
                    .iter_mut()
                    .find(|p| p.identity().name() == plugin.identity().name())
                {
                    // Stop the existing plugin, so that the new plugin definitely takes over
                    existing.stop()?;
                    *existing = plugin;
                } else {
                    self.plugins.push(plugin);
                }
            }
        }

        #[cfg(feature = "plugin")]
        if !delta.plugin_registry_items.is_empty() {
            // Update the plugin file with the new signatures.
            if self.plugin_path.is_some() {
                self.update_plugin_file(std::mem::take(&mut delta.plugin_registry_items))?;
            }
        }

        Ok(())
    }

    /// Merge the environment from the runtime Stack into the engine state
    pub fn merge_env(&mut self, stack: &mut Stack) -> Result<(), ShellError> {
        for mut scope in stack.env_vars.drain(..) {
            for (overlay_name, mut env) in Arc::make_mut(&mut scope).drain() {
                if let Some(env_vars) = Arc::make_mut(&mut self.env_vars).get_mut(&overlay_name) {
                    // Updating existing overlay
                    env_vars.extend(env.drain());
                } else {
                    // Pushing a new overlay
                    Arc::make_mut(&mut self.env_vars).insert(overlay_name, env);
                }
            }
        }

        if UPDATE_CWD.load(Ordering::Relaxed) {
            let cwd = self.cwd(Some(stack))?;
            std::env::set_current_dir(cwd)
                .map_err(|err| IoError::new_internal(err, "Could not set current dir"))?;
        }

        if let Some(config) = stack.config.take() {
            // If config was updated in the stack, replace it.
            self.config = config;

            // Make plugin GC config changes take effect immediately.
            #[cfg(feature = "plugin")]
            self.update_plugin_gc_configs(&self.config.plugin_gc);
        }

        Ok(())
    }

    /// Clean up unused variables from a Stack to prevent memory leaks.
    /// This removes variables that are no longer referenced by any overlay or alias.
    pub fn cleanup_stack_variables(&mut self, stack: &mut Stack) {
        let mut shadowed_vars = HashSet::new();
        for (_, frame) in self.scope.overlays.iter_mut() {
            shadowed_vars.extend(frame.shadowed_vars.drain(..));
        }

        if shadowed_vars.is_empty() {
            return;
        }

        // Collect VarIds still referenced by alias definitions so we don't
        // remove them — doing so would cause a VariableNotFoundAtRuntime error
        // when the alias is invoked after the variable has been re-bound.
        let mut alias_var_ids = HashSet::new();
        for decl in self.decls.iter() {
            if let Some(alias) = decl.as_alias() {
                collect_alias_var_ids(&alias.wrapped_call, &mut alias_var_ids);
            }
        }

        // Remove variables from stack that are shadowed and not referenced by any alias
        stack.vars.retain(|(var_id, _)| {
            !shadowed_vars.contains(var_id) || alias_var_ids.contains(var_id)
        });
    }

    pub fn active_overlay_ids<'a, 'b>(
        &'b self,
        removed_overlays: &'a [Vec<u8>],
    ) -> impl DoubleEndedIterator<Item = &'b OverlayId> + 'a
    where
        'b: 'a,
    {
        self.scope.active_overlays.iter().filter(|id| {
            !removed_overlays
                .iter()
                .any(|name| name == self.get_overlay_name(**id))
        })
    }

    pub fn active_overlays<'a, 'b>(
        &'b self,
        removed_overlays: &'a [Vec<u8>],
    ) -> impl DoubleEndedIterator<Item = &'b OverlayFrame> + 'a
    where
        'b: 'a,
    {
        self.active_overlay_ids(removed_overlays)
            .map(|id| self.get_overlay(*id))
    }

    pub fn active_overlay_names<'a, 'b>(
        &'b self,
        removed_overlays: &'a [Vec<u8>],
    ) -> impl DoubleEndedIterator<Item = &'b [u8]> + 'a
    where
        'b: 'a,
    {
        self.active_overlay_ids(removed_overlays)
            .map(|id| self.get_overlay_name(*id))
    }

    /// Translate overlay IDs from other to IDs in self
    fn translate_overlay_ids(&self, other: &ScopeFrame) -> Vec<OverlayId> {
        let other_names = other.active_overlays.iter().map(|other_id| {
            &other
                .overlays
                .get(other_id.get())
                .expect("internal error: missing overlay")
                .0
        });

        other_names
            .map(|other_name| {
                self.find_overlay(other_name)
                    .expect("internal error: missing overlay")
            })
            .collect()
    }

    pub fn last_overlay_name(&self, removed_overlays: &[Vec<u8>]) -> &[u8] {
        self.active_overlay_names(removed_overlays)
            .last()
            .expect("internal error: no active overlays")
    }

    pub fn last_overlay(&self, removed_overlays: &[Vec<u8>]) -> &OverlayFrame {
        self.active_overlay_ids(removed_overlays)
            .last()
            .map(|id| self.get_overlay(*id))
            .expect("internal error: no active overlays")
    }

    pub fn get_overlay_name(&self, overlay_id: OverlayId) -> &[u8] {
        &self
            .scope
            .overlays
            .get(overlay_id.get())
            .expect("internal error: missing overlay")
            .0
    }

    pub fn get_overlay(&self, overlay_id: OverlayId) -> &OverlayFrame {
        &self
            .scope
            .overlays
            .get(overlay_id.get())
            .expect("internal error: missing overlay")
            .1
    }

    pub fn render_env_vars(&self) -> HashMap<&str, &Value> {
        let mut result: HashMap<&str, &Value> = HashMap::new();

        for overlay_name in self.active_overlay_names(&[]) {
            let name = String::from_utf8_lossy(overlay_name);
            if let Some(env_vars) = self.env_vars.get(name.as_ref()) {
                result.extend(env_vars.iter().map(|(k, v)| (k.as_str(), v)));
            }
        }

        result
    }

    pub fn add_env_var(&mut self, name: String, val: Value) {
        let overlay_name = String::from_utf8_lossy(self.last_overlay_name(&[])).to_string();

        if let Some(env_vars) = Arc::make_mut(&mut self.env_vars).get_mut(&overlay_name) {
            env_vars.insert(EnvName::from(name), val);
        } else {
            Arc::make_mut(&mut self.env_vars).insert(
                overlay_name,
                [(EnvName::from(name), val)].into_iter().collect(),
            );
        }
    }

    pub fn get_env_var(&self, name: &str) -> Option<&Value> {
        for overlay_id in self.scope.active_overlays.iter().rev() {
            let overlay_name = String::from_utf8_lossy(self.get_overlay_name(*overlay_id));
            if let Some(env_vars) = self.env_vars.get(overlay_name.as_ref())
                && let Some(val) = env_vars.get(&EnvName::from(name))
            {
                return Some(val);
            }
        }

        None
    }

    #[cfg(feature = "plugin")]
    pub fn plugins(&self) -> &[Arc<dyn RegisteredPlugin>] {
        &self.plugins
    }

    #[cfg(feature = "plugin")]
    fn update_plugin_file(&self, updated_items: Vec<PluginRegistryItem>) -> Result<(), ShellError> {
        // Updating the signatures plugin file with the added signatures
        use std::fs::File;

        let plugin_path = self.plugin_path.as_ref().ok_or_else(|| {
            ShellError::Generic(
                GenericError::new_internal("Plugin file path not set", "")
                    .with_help("you may be running nu with --no-config-file"),
            )
        })?;

        // Read the current contents of the plugin file if it exists
        let mut contents = match File::open(plugin_path.as_path()) {
            Ok(mut plugin_file) => PluginRegistryFile::read_from(&mut plugin_file, None),
            Err(err) => {
                if err.kind() == std::io::ErrorKind::NotFound {
                    Ok(PluginRegistryFile::default())
                } else {
                    Err(ShellError::Io(IoError::new_internal_with_path(
                        err,
                        "Failed to open plugin file",
                        PathBuf::from(plugin_path),
                    )))
                }
            }
        }?;

        // Update the given signatures
        for item in updated_items {
            contents.upsert_plugin(item);
        }

        // Write it to the same path
        let plugin_file = File::create(plugin_path.as_path()).map_err(|err| {
            IoError::new_internal_with_path(
                err,
                "Failed to write plugin file",
                PathBuf::from(plugin_path),
            )
        })?;

        contents.write_to(plugin_file, None)
    }

    /// Update plugins with new garbage collection config
    #[cfg(feature = "plugin")]
    fn update_plugin_gc_configs(&self, plugin_gc: &crate::PluginGcConfigs) {
        for plugin in &self.plugins {
            plugin.set_gc_config(plugin_gc.get(plugin.identity().name()));
        }
    }

    pub fn num_files(&self) -> usize {
        self.files.len()
    }

    pub fn num_virtual_paths(&self) -> usize {
        self.virtual_paths.len()
    }

    pub fn num_vars(&self) -> usize {
        self.vars.len()
    }

    pub fn num_decls(&self) -> usize {
        self.decls.len()
    }

    pub fn num_blocks(&self) -> usize {
        self.blocks.len()
    }

    pub fn num_modules(&self) -> usize {
        self.modules.len()
    }

    pub fn num_spans(&self) -> usize {
        self.spans.len()
    }
    pub fn print_vars(&self) {
        for var in self.vars.iter().enumerate() {
            println!("var{}: {:?}", var.0, var.1);
        }
    }

    pub fn print_decls(&self) {
        for decl in self.decls.iter().enumerate() {
            println!("decl{}: {:?}", decl.0, decl.1.signature());
        }
    }

    pub fn print_blocks(&self) {
        for block in self.blocks.iter().enumerate() {
            println!("block{}: {:?}", block.0, block.1);
        }
    }

    pub fn print_contents(&self) {
        for cached_file in self.files.iter() {
            let string = String::from_utf8_lossy(&cached_file.content);
            println!("{string}");
        }
    }

    /// Find the [`DeclId`](crate::DeclId) corresponding to a declaration with `name`.
    ///
    /// Searches within active overlays, and filtering out overlays in `removed_overlays`.
    pub fn find_decl(&self, name: &[u8], removed_overlays: &[Vec<u8>]) -> Option<DeclId> {
        let mut visibility: Visibility = Visibility::new();

        for overlay_frame in self.active_overlays(removed_overlays).rev() {
            visibility.append(&overlay_frame.visibility);

            if let Some(decl_id) = overlay_frame.get_decl(name)
                && visibility.is_decl_id_visible(&decl_id)
            {
                return Some(decl_id);
            }
        }

        None
    }

    /// Find the name of the declaration corresponding to `decl_id`.
    ///
    /// Searches within active overlays, and filtering out overlays in `removed_overlays`.
    pub fn find_decl_name(&self, decl_id: DeclId, removed_overlays: &[Vec<u8>]) -> Option<&[u8]> {
        let mut visibility: Visibility = Visibility::new();

        for overlay_frame in self.active_overlays(removed_overlays).rev() {
            visibility.append(&overlay_frame.visibility);

            if visibility.is_decl_id_visible(&decl_id) {
                for (name, id) in overlay_frame.decls.iter() {
                    if id == &decl_id {
                        return Some(name);
                    }
                }
            }
        }

        None
    }

    /// Find the [`OverlayId`](crate::OverlayId) corresponding to `name`.
    ///
    /// Searches all overlays, not just active overlays. To search only in active overlays, use [`find_active_overlay`](EngineState::find_active_overlay)
    pub fn find_overlay(&self, name: &[u8]) -> Option<OverlayId> {
        self.scope.find_overlay(name)
    }

    /// Find the [`OverlayId`](crate::OverlayId) of the active overlay corresponding to `name`.
    ///
    /// Searches only active overlays. To search in all overlays, use [`find_overlay`](EngineState::find_active_overlay)
    pub fn find_active_overlay(&self, name: &[u8]) -> Option<OverlayId> {
        self.scope.find_active_overlay(name)
    }

    /// Find the [`ModuleId`](crate::ModuleId) corresponding to `name`.
    ///
    /// Searches within active overlays, and filtering out overlays in `removed_overlays`.
    pub fn find_module(&self, name: &[u8], removed_overlays: &[Vec<u8>]) -> Option<ModuleId> {
        for overlay_frame in self.active_overlays(removed_overlays).rev() {
            if let Some(module_id) = overlay_frame.modules.get(name) {
                return Some(*module_id);
            }
        }

        None
    }

    pub fn get_module_comments(&self, module_id: ModuleId) -> Option<&[Span]> {
        self.doccomments.get_module_comments(module_id)
    }

    #[cfg(feature = "plugin")]
    pub fn plugin_decls(&self) -> impl Iterator<Item = &Box<dyn Command + 'static>> {
        let mut unique_plugin_decls = HashMap::new();

        // Make sure there are no duplicate decls: Newer one overwrites the older one
        for decl in self.decls.iter().filter(|d| d.is_plugin()) {
            unique_plugin_decls.insert(decl.name(), decl);
        }

        let mut plugin_decls: Vec<(&str, &Box<dyn Command>)> =
            unique_plugin_decls.into_iter().collect();

        // Sort the plugins by name so we don't end up with a random plugin file each time
        plugin_decls.sort_by(|a, b| a.0.cmp(b.0));
        plugin_decls.into_iter().map(|(_, decl)| decl)
    }

    pub fn which_module_has_decl(
        &self,
        decl_name: &[u8],
        removed_overlays: &[Vec<u8>],
    ) -> Option<&[u8]> {
        for overlay_frame in self.active_overlays(removed_overlays).rev() {
            for (module_name, module_id) in overlay_frame.modules.iter() {
                let module = self.get_module(*module_id);
                if module.has_decl(decl_name) {
                    return Some(module_name);
                }
            }
        }

        None
    }

    /// Apply a function to all commands. The function accepts a command name and its DeclId
    pub fn traverse_commands(&self, mut f: impl FnMut(&[u8], DeclId)) {
        for overlay_frame in self.active_overlays(&[]).rev() {
            for (name, decl_id) in &overlay_frame.decls {
                if overlay_frame.visibility.is_decl_id_visible(decl_id) {
                    f(name, *decl_id);
                }
            }
        }
    }

    pub fn get_span_contents(&self, span: Span) -> &[u8] {
        self.try_get_file_contents(span).unwrap_or(&[0u8; 0])
    }

    pub fn try_get_file_contents(&self, span: Span) -> Option<&[u8]> {
        self.files.iter().find_map(|file| {
            if file.covered_span.contains_span(span) {
                let start = span.start - file.covered_span.start;
                let end = span.end - file.covered_span.start;
                Some(&file.content[start..end])
            } else {
                None
            }
        })
    }

    /// If the span's content starts with the given prefix, return two subspans
    /// corresponding to this prefix, and the rest of the content.
    pub fn span_match_prefix(&self, span: Span, prefix: &[u8]) -> Option<(Span, Span)> {
        let contents = self.get_span_contents(span);

        if contents.starts_with(prefix) {
            span.split_at(prefix.len())
        } else {
            None
        }
    }

    /// If the span's content ends with the given postfix, return two subspans
    /// corresponding to the rest of the content, and this postfix.
    pub fn span_match_postfix(&self, span: Span, prefix: &[u8]) -> Option<(Span, Span)> {
        let contents = self.get_span_contents(span);

        if contents.ends_with(prefix) {
            span.split_at(span.len() - prefix.len())
        } else {
            None
        }
    }

    /// Get the global config from the engine state.
    ///
    /// Use [`Stack::get_config()`] instead whenever the `Stack` is available, as it takes into
    /// account local changes to `$env.config`.
    pub fn get_config(&self) -> &Arc<Config> {
        &self.config
    }

    pub fn set_config(&mut self, conf: impl Into<Arc<Config>>) {
        let conf = conf.into();

        #[cfg(feature = "plugin")]
        if conf.plugin_gc != self.config.plugin_gc {
            // Make plugin GC config changes take effect immediately.
            self.update_plugin_gc_configs(&conf.plugin_gc);
        }

        self.config = conf;
    }

    /// Fetch the configuration for a plugin
    ///
    /// The `plugin` must match the registered name of a plugin.  For `plugin add
    /// nu_plugin_example` the plugin name to use will be `"example"`
    pub fn get_plugin_config(&self, plugin: &str) -> Option<&Value> {
        self.config.plugins.get(plugin)
    }

    /// Returns the configuration settings for command history or `None` if history is disabled
    pub fn history_config(&self) -> Option<HistoryConfig> {
        self.history_enabled.then(|| self.config.history.clone())
    }

    /// Resolve the history file path using the already-resolved config dirs.
    ///
    /// Returns `None` when history is disabled or the history path is set to
    /// [`HistoryPath::Disabled`].
    pub fn history_path(&self) -> Option<std::path::PathBuf> {
        self.history_config()?
            .file_path(&self.config_dirs.config_home)
    }

    pub fn get_var(&self, var_id: VarId) -> &Variable {
        self.vars
            .get(var_id.get())
            .expect("internal error: missing variable")
    }

    pub fn get_constant(&self, var_id: VarId) -> Option<&Value> {
        let var = self.get_var(var_id);
        var.const_val.as_ref()
    }

    pub fn generate_nu_constant(&mut self) {
        self.vars[NU_VARIABLE_ID.get()].const_val = Some(create_nu_constant(self, Span::unknown()));
    }

    pub fn get_decl(&self, decl_id: DeclId) -> &dyn Command {
        self.decls
            .get(decl_id.get())
            .expect("internal error: missing declaration")
            .as_ref()
    }

    /// Get all commands within scope, sorted by the commands' names
    pub fn get_decls_sorted(&self, include_hidden: bool) -> Vec<(Vec<u8>, DeclId)> {
        let mut decls_map = HashMap::new();

        for overlay_frame in self.active_overlays(&[]) {
            let new_decls = if include_hidden {
                overlay_frame.decls.clone()
            } else {
                overlay_frame
                    .decls
                    .clone()
                    .into_iter()
                    .filter(|(_, id)| overlay_frame.visibility.is_decl_id_visible(id))
                    .collect()
            };

            decls_map.extend(new_decls);
        }

        let mut decls: Vec<(Vec<u8>, DeclId)> = decls_map.into_iter().collect();

        decls.sort_by(|a, b| a.0.cmp(&b.0));
        decls
    }

    pub fn get_signature(&self, decl: &dyn Command) -> Signature {
        if let Some(block_id) = decl.block_id() {
            *self.blocks[block_id.get()].signature.clone()
        } else {
            decl.signature()
        }
    }

    /// Get signatures of all commands within scope with their decl ids.
    pub fn get_signatures_and_declids(&self, include_hidden: bool) -> Vec<(Signature, DeclId)> {
        self.get_decls_sorted(include_hidden)
            .into_iter()
            .map(|(_, id)| {
                let decl = self.get_decl(id);

                (self.get_signature(decl).update_from_command(decl), id)
            })
            .collect()
    }

    pub fn get_block(&self, block_id: BlockId) -> &Arc<Block> {
        self.blocks
            .get(block_id.get())
            .expect("internal error: missing block")
    }

    /// Optionally get a block by id, if it exists
    ///
    /// Prefer to use [`.get_block()`](Self::get_block) in most cases - `BlockId`s that don't exist
    /// are normally a compiler error. This only exists to stop plugins from crashing the engine if
    /// they send us something invalid.
    pub fn try_get_block(&self, block_id: BlockId) -> Option<&Arc<Block>> {
        self.blocks.get(block_id.get())
    }

    pub fn get_module(&self, module_id: ModuleId) -> &Module {
        self.modules
            .get(module_id.get())
            .expect("internal error: missing module")
    }

    pub fn get_virtual_path(&self, virtual_path_id: VirtualPathId) -> &(String, VirtualPath) {
        self.virtual_paths
            .get(virtual_path_id.get())
            .expect("internal error: missing virtual path")
    }

    pub fn next_span_start(&self) -> usize {
        if let Some(cached_file) = self.files.last() {
            cached_file.covered_span.end
        } else {
            0
        }
    }

    pub fn files(
        &self,
    ) -> impl DoubleEndedIterator<Item = &CachedFile> + ExactSizeIterator<Item = &CachedFile> {
        self.files.iter()
    }

    pub fn add_file(&mut self, filename: Arc<str>, content: Arc<[u8]>) -> FileId {
        let next_span_start = self.next_span_start();
        let next_span_end = next_span_start + content.len();

        let covered_span = Span::new(next_span_start, next_span_end);

        self.files.push(CachedFile {
            name: filename,
            content,
            covered_span,
        });

        FileId::new(self.num_files() - 1)
    }

    pub fn build_desc(&self, spans: &[Span]) -> (String, String) {
        let comment_lines: Vec<&[u8]> = spans
            .iter()
            .map(|span| self.get_span_contents(*span))
            .collect();
        build_desc(&comment_lines)
    }

    pub fn build_module_desc(&self, module_id: ModuleId) -> Option<(String, String)> {
        self.get_module_comments(module_id)
            .map(|comment_spans| self.build_desc(comment_spans))
    }

    /// Returns the current working directory, which is guaranteed to be canonicalized.
    ///
    /// Returns an empty String if $env.PWD doesn't exist.
    #[deprecated(since = "0.92.3", note = "please use `EngineState::cwd()` instead")]
    pub fn current_work_dir(&self) -> String {
        self.cwd(None)
            .map(|path| path.to_string_lossy().to_string())
            .unwrap_or_default()
    }

    /// Returns the current working directory, which is guaranteed to be an
    /// absolute path without trailing slashes (unless it's the root path), but
    /// might contain symlink components.
    ///
    /// If `stack` is supplied, also considers modifications to the working
    /// directory on the stack that have yet to be merged into the engine state.
    pub fn cwd(&self, stack: Option<&Stack>) -> Result<AbsolutePathBuf, ShellError> {
        // Helper function to create a simple generic error.
        fn error(msg: &str, cwd: impl AsRef<nu_path::Path>) -> ShellError {
            ShellError::Generic(
                GenericError::new_internal(
                    msg.to_string(),
                    format!("$env.PWD = {}", cwd.as_ref().display()),
                )
                .with_help("Use `cd` to reset $env.PWD into a good state"),
            )
        }

        // Retrieve $env.PWD from the stack or the engine state.
        let pwd = if let Some(stack) = stack {
            stack.get_env_var(self, "PWD")
        } else {
            self.get_env_var("PWD")
        };

        let pwd = pwd.ok_or_else(|| error("$env.PWD not found", ""))?;

        if let Ok(pwd) = pwd.as_str() {
            let path = AbsolutePathBuf::try_from(pwd)
                .map_err(|_| error("$env.PWD is not an absolute path", pwd))?;

            // Technically, a root path counts as "having trailing slashes", but
            // for the purpose of PWD, a root path is acceptable.
            if path.parent().is_some() && nu_path::has_trailing_slash(path.as_ref()) {
                Err(error("$env.PWD contains trailing slashes", &path))
            } else if !path.exists() {
                Err(error("$env.PWD points to a non-existent directory", &path))
            } else if !path.is_dir() {
                Err(error("$env.PWD points to a non-directory", &path))
            } else {
                Ok(path)
            }
        } else {
            Err(error("$env.PWD is not a string", format!("{pwd:?}")))
        }
    }

    /// Like `EngineState::cwd()`, but returns a String instead of a PathBuf for convenience.
    pub fn cwd_as_string(&self, stack: Option<&Stack>) -> Result<String, ShellError> {
        let cwd = self.cwd(stack)?;
        cwd.into_os_string()
            .into_string()
            .map_err(|err| ShellError::NonUtf8Custom {
                msg: format!("The current working directory is not a valid utf-8 string: {err:?}"),
                span: Span::unknown(),
            })
    }

    // TODO: see if we can completely get rid of this
    pub fn get_file_contents(&self) -> &[CachedFile] {
        &self.files
    }

    pub fn get_startup_time(&self) -> i64 {
        self.startup_time
    }

    pub fn set_startup_time(&mut self, startup_time: i64) {
        self.startup_time = startup_time;
    }

    pub fn activate_debugger(
        &self,
        debugger: Box<dyn Debugger>,
    ) -> Result<(), PoisonDebuggerError<'_>> {
        let mut locked_debugger = self.debugger.lock()?;
        *locked_debugger = debugger;
        locked_debugger.activate();
        self.is_debugging.0.store(true, Ordering::Relaxed);
        Ok(())
    }

    pub fn deactivate_debugger(&self) -> Result<Box<dyn Debugger>, PoisonDebuggerError<'_>> {
        let mut locked_debugger = self.debugger.lock()?;
        locked_debugger.deactivate();
        let ret = std::mem::replace(&mut *locked_debugger, Box::new(NoopDebugger));
        self.is_debugging.0.store(false, Ordering::Relaxed);
        Ok(ret)
    }

    pub fn is_debugging(&self) -> bool {
        self.is_debugging.0.load(Ordering::Relaxed)
    }

    pub fn recover_from_panic(&mut self) {
        if Mutex::is_poisoned(&self.repl_state) {
            self.repl_state = Arc::new(Mutex::new(ReplState {
                buffer: "".to_string(),
                cursor_pos: 0,
                accept: false,
            }));
        }
        if Mutex::is_poisoned(&self.jobs) {
            self.jobs = Arc::new(Mutex::new(Jobs::default()));
        }
        if Mutex::is_poisoned(&self.regex_cache) {
            self.regex_cache = Arc::new(Mutex::new(LruCache::new(
                NonZeroUsize::new(REGEX_CACHE_SIZE).expect("tried to create cache of size zero"),
            )));
        }
    }

    /// Reset mutable per-session state after cloning a shared engine template.
    pub fn make_session_state_unique(&mut self) {
        let (send, recv) = channel();

        self.pipeline_externals_state = Arc::new((AtomicU32::new(0), AtomicU32::new(0)));
        self.repl_state = Default::default();
        self.report_log = Default::default();
        self.prompt_state = Arc::new(PromptState::new());
        self.jobs = Default::default();
        self.current_job = CurrentJob {
            id: JobId::new(0),
            background_thread_job: None,
            mailbox: Arc::new(Mutex::new(Mailbox::new(recv))),
        };
        self.root_job_sender = send;
        self.exit_warning_given = Default::default();
        self.regex_cache = Arc::new(Mutex::new(LruCache::new(
            NonZeroUsize::new(REGEX_CACHE_SIZE).expect("tried to create cache of size zero"),
        )));
        self.is_debugging = IsDebugging::new(false);
        self.debugger = Arc::new(Mutex::new(Box::new(NoopDebugger)));
    }

    /// Add new span and return its ID
    pub fn add_span(&mut self, span: Span) -> SpanId {
        self.spans.push(span);
        SpanId::new(self.num_spans() - 1)
    }

    /// Find ID of a span (should be avoided if possible)
    pub fn find_span_id(&self, span: Span) -> Option<SpanId> {
        self.spans
            .iter()
            .position(|sp| sp == &span)
            .map(SpanId::new)
    }

    // Determines whether the current state is being held by a background job
    pub fn is_background_job(&self) -> bool {
        self.current_job.background_thread_job.is_some()
    }

    // Gets the thread job entry
    pub fn current_thread_job(&self) -> Option<&ThreadJob> {
        self.current_job.background_thread_job.as_ref()
    }
}

/// Collect all `VarId`s referenced by `Expr::Var` nodes in `expr`.
/// This is used by [`EngineState::cleanup_stack_variables`] to avoid removing
/// variables that are still referenced by alias definitions.
fn collect_alias_var_ids(expr: &crate::ast::Expression, var_ids: &mut HashSet<VarId>) {
    let mut queue = vec![expr];

    while let Some(e) = queue.pop() {
        match &e.expr {
            Expr::Var(id) => {
                var_ids.insert(*id);
            }
            Expr::Call(call) => {
                for arg in &call.arguments {
                    if let Some(sub_expr) = arg.expr() {
                        queue.push(sub_expr);
                    }
                }
            }
            Expr::ExternalCall(head, args) => {
                queue.push(head);
                for arg in args.iter() {
                    queue.push(arg.expr());
                }
            }
            Expr::FullCellPath(fcp) => queue.push(&fcp.head),
            _ => {}
        }
    }
}

impl GetSpan for &EngineState {
    /// Get existing span
    fn get_span(&self, span_id: SpanId) -> Span {
        *self
            .spans
            .get(span_id.get())
            .expect("internal error: missing span")
    }
}

impl Default for EngineState {
    fn default() -> Self {
        Self::new()
    }
}

/// Map a fancy-regex compile failure to [`ShellError::InvalidValue`].
///
/// Shared by [`EngineState::compile_regex`] and call sites that compile with
/// options that bypass the LRU cache (for example a custom backtrack limit).
pub fn invalid_regex_value(pattern: &str, err: fancy_regex::Error, span: Span) -> ShellError {
    ShellError::InvalidValue {
        valid: "a valid regular expression".into(),
        actual: format!("'{pattern}' ({err})"),
        span,
    }
}

#[cfg(test)]
mod engine_state_tests {
    use crate::engine::StateWorkingSet;
    use std::str::{Utf8Error, from_utf8};

    use super::*;

    #[test]
    fn add_file_gives_id() {
        let engine_state = EngineState::new();
        let mut engine_state = StateWorkingSet::new(&engine_state);
        let id = engine_state.add_file("test.nu", &[]);

        assert_eq!(id, FileId::new(0));
    }

    #[test]
    fn get_cached_regex_reuses_compiled_patterns() {
        let engine_state = EngineState::new();
        let pattern = r"[^\w]+";

        let first = engine_state
            .get_cached_regex(pattern)
            .expect("pattern should compile");
        assert!(first.is_match("!!!").unwrap_or(false));

        {
            let cache = engine_state.regex_cache.lock().expect("cache lock");
            assert_eq!(cache.len(), 1);
            assert!(cache.peek(pattern).is_some());
        }

        let second = engine_state
            .get_cached_regex(pattern)
            .expect("pattern should compile from cache");
        assert!(second.is_match("!!!").unwrap_or(false));

        {
            let cache = engine_state.regex_cache.lock().expect("cache lock");
            assert_eq!(cache.len(), 1, "second lookup should not grow the cache");
        }
    }

    #[test]
    fn get_cached_regex_does_not_store_invalid_patterns() {
        let engine_state = EngineState::new();

        assert!(engine_state.get_cached_regex("(").is_err());

        let cache = engine_state.regex_cache.lock().expect("cache lock");
        assert_eq!(cache.len(), 0);
    }

    #[test]
    fn add_file_gives_id_including_parent() {
        let mut engine_state = EngineState::new();
        let parent_id = engine_state.add_file("test.nu".into(), Arc::new([]));

        let mut working_set = StateWorkingSet::new(&engine_state);
        let working_set_id = working_set.add_file("child.nu", &[]);

        assert_eq!(parent_id, FileId::new(0));
        assert_eq!(working_set_id, FileId::new(1));
    }

    #[test]
    fn merge_states() -> Result<(), ShellError> {
        let mut engine_state = EngineState::new();
        engine_state.add_file("test.nu".into(), Arc::new([]));

        let delta = {
            let mut working_set = StateWorkingSet::new(&engine_state);
            let _ = working_set.add_file("child.nu", &[]);
            working_set.render()
        };

        engine_state.merge_delta(delta)?;

        assert_eq!(engine_state.num_files(), 2);
        assert_eq!(&*engine_state.files[0].name, "test.nu");
        assert_eq!(&*engine_state.files[1].name, "child.nu");

        Ok(())
    }

    #[test]
    fn list_variables() -> Result<(), Utf8Error> {
        let varname = "something";
        let varname_with_sigil = "$".to_owned() + varname;
        let engine_state = EngineState::new();
        let mut working_set = StateWorkingSet::new(&engine_state);
        working_set.add_variable(
            varname.as_bytes().into(),
            Span { start: 0, end: 1 },
            Type::Int,
            false,
        );
        let variables = working_set
            .list_variables()
            .into_iter()
            .map(from_utf8)
            .collect::<Result<Vec<&str>, Utf8Error>>()?;
        assert_eq!(variables, vec![varname_with_sigil]);
        Ok(())
    }

    #[test]
    fn get_plugin_config() {
        let mut engine_state = EngineState::new();

        assert!(
            engine_state.get_plugin_config("example").is_none(),
            "Unexpected plugin configuration"
        );

        let mut plugins = HashMap::new();
        plugins.insert("example".into(), Value::string("value", Span::test_data()));

        let mut config = Config::clone(engine_state.get_config());
        config.plugins = plugins;

        engine_state.set_config(config);

        assert!(
            engine_state.get_plugin_config("example").is_some(),
            "Plugin configuration not found"
        );
    }
}

#[cfg(test)]
mod test_cwd {
    //! Here're the test cases we need to cover:
    //!
    //! `EngineState::cwd()` computes the result from `self.env_vars["PWD"]` and
    //! optionally `stack.env_vars["PWD"]`.
    //!
    //! PWD may be unset in either `env_vars`.
    //! PWD should NOT be an empty string.
    //! PWD should NOT be a non-string value.
    //! PWD should NOT be a relative path.
    //! PWD should NOT contain trailing slashes.
    //! PWD may point to a directory or a symlink to directory.
    //! PWD should NOT point to a file or a symlink to file.
    //! PWD should NOT point to non-existent entities in the filesystem.

    use crate::{
        Value,
        engine::{EngineState, Stack},
    };
    use nu_path::{AbsolutePath, Path, assert_path_eq};
    use tempfile::{NamedTempFile, TempDir};

    /// Creates a symlink. Works on both Unix and Windows.
    #[cfg(any(unix, windows))]
    fn symlink(
        original: impl AsRef<AbsolutePath>,
        link: impl AsRef<AbsolutePath>,
    ) -> std::io::Result<()> {
        let original = original.as_ref();
        let link = link.as_ref();

        #[cfg(unix)]
        {
            std::os::unix::fs::symlink(original, link)
        }
        #[cfg(windows)]
        {
            if original.is_dir() {
                std::os::windows::fs::symlink_dir(original, link)
            } else {
                std::os::windows::fs::symlink_file(original, link)
            }
        }
    }

    /// Create an engine state initialized with the given PWD.
    fn engine_state_with_pwd(path: impl AsRef<Path>) -> EngineState {
        let mut engine_state = EngineState::new();
        engine_state.add_env_var(
            "PWD".into(),
            Value::test_string(path.as_ref().to_str().unwrap()),
        );
        engine_state
    }

    /// Create a stack initialized with the given PWD.
    fn stack_with_pwd(path: impl AsRef<Path>) -> Stack {
        let mut stack = Stack::new();
        stack.add_env_var(
            "PWD".into(),
            Value::test_string(path.as_ref().to_str().unwrap()),
        );
        stack
    }

    #[test]
    fn pwd_not_set() {
        let engine_state = EngineState::new();
        engine_state.cwd(None).unwrap_err();
    }

    #[test]
    fn pwd_is_empty_string() {
        let engine_state = engine_state_with_pwd("");
        engine_state.cwd(None).unwrap_err();
    }

    #[test]
    fn pwd_is_non_string_value() {
        let mut engine_state = EngineState::new();
        engine_state.add_env_var("PWD".into(), Value::test_glob("*"));
        engine_state.cwd(None).unwrap_err();
    }

    #[test]
    fn pwd_is_relative_path() {
        let engine_state = engine_state_with_pwd("./foo");

        engine_state.cwd(None).unwrap_err();
    }

    #[test]
    fn pwd_has_trailing_slash() {
        let dir = TempDir::new().unwrap();
        let engine_state = engine_state_with_pwd(dir.path().join(""));

        engine_state.cwd(None).unwrap_err();
    }

    #[test]
    fn pwd_points_to_root() {
        #[cfg(windows)]
        let root = Path::new(r"C:\");
        #[cfg(not(windows))]
        let root = Path::new("/");

        let engine_state = engine_state_with_pwd(root);
        let cwd = engine_state.cwd(None).unwrap();
        assert_path_eq!(cwd, root);
    }

    #[test]
    fn pwd_points_to_normal_file() {
        let file = NamedTempFile::new().unwrap();
        let engine_state = engine_state_with_pwd(file.path());

        engine_state.cwd(None).unwrap_err();
    }

    #[test]
    fn pwd_points_to_normal_directory() {
        let dir = TempDir::new().unwrap();
        let engine_state = engine_state_with_pwd(dir.path());

        let cwd = engine_state.cwd(None).unwrap();
        assert_path_eq!(cwd, dir.path());
    }

    #[test]
    fn pwd_points_to_symlink_to_file() {
        let file = NamedTempFile::new().unwrap();
        let temp_file = AbsolutePath::try_new(file.path()).unwrap();
        let dir = TempDir::new().unwrap();
        let temp = AbsolutePath::try_new(dir.path()).unwrap();

        let link = temp.join("link");
        symlink(temp_file, &link).unwrap();
        let engine_state = engine_state_with_pwd(&link);

        engine_state.cwd(None).unwrap_err();
    }

    #[test]
    fn pwd_points_to_symlink_to_directory() {
        let dir = TempDir::new().unwrap();
        let temp = AbsolutePath::try_new(dir.path()).unwrap();

        let link = temp.join("link");
        symlink(temp, &link).unwrap();
        let engine_state = engine_state_with_pwd(&link);

        let cwd = engine_state.cwd(None).unwrap();
        assert_path_eq!(cwd, link);
    }

    #[test]
    fn pwd_points_to_broken_symlink() {
        let dir = TempDir::new().unwrap();
        let temp = AbsolutePath::try_new(dir.path()).unwrap();
        let other_dir = TempDir::new().unwrap();
        let other_temp = AbsolutePath::try_new(other_dir.path()).unwrap();

        let link = temp.join("link");
        symlink(other_temp, &link).unwrap();
        let engine_state = engine_state_with_pwd(&link);

        drop(other_dir);
        engine_state.cwd(None).unwrap_err();
    }

    #[test]
    fn pwd_points_to_nonexistent_entity() {
        let engine_state = engine_state_with_pwd(TempDir::new().unwrap().path());

        engine_state.cwd(None).unwrap_err();
    }

    #[test]
    fn stack_pwd_not_set() {
        let dir = TempDir::new().unwrap();
        let engine_state = engine_state_with_pwd(dir.path());
        let stack = Stack::new();

        let cwd = engine_state.cwd(Some(&stack)).unwrap();
        assert_eq!(cwd, dir.path());
    }

    #[test]
    fn stack_pwd_is_empty_string() {
        let dir = TempDir::new().unwrap();
        let engine_state = engine_state_with_pwd(dir.path());
        let stack = stack_with_pwd("");

        engine_state.cwd(Some(&stack)).unwrap_err();
    }

    #[test]
    fn stack_pwd_points_to_normal_directory() {
        let dir1 = TempDir::new().unwrap();
        let dir2 = TempDir::new().unwrap();
        let engine_state = engine_state_with_pwd(dir1.path());
        let stack = stack_with_pwd(dir2.path());

        let cwd = engine_state.cwd(Some(&stack)).unwrap();
        assert_path_eq!(cwd, dir2.path());
    }

    #[test]
    fn stack_pwd_points_to_normal_directory_with_symlink_components() {
        let dir = TempDir::new().unwrap();
        let temp = AbsolutePath::try_new(dir.path()).unwrap();

        // `/tmp/dir/link` points to `/tmp/dir`, then we set PWD to `/tmp/dir/link/foo`
        let link = temp.join("link");
        symlink(temp, &link).unwrap();
        let foo = link.join("foo");
        std::fs::create_dir(temp.join("foo")).unwrap();
        let engine_state = EngineState::new();
        let stack = stack_with_pwd(&foo);

        let cwd = engine_state.cwd(Some(&stack)).unwrap();
        assert_path_eq!(cwd, foo);
    }
}