harn_kernel/compiler/

state.rs

use harn_parser::{
    substitute_type_expr, BinaryOperator, Node, SNode, ShapeField, TypeExpr, TypedParam,
};
use std::collections::BTreeMap;

use crate::chunk::{Chunk, Constant, Op};
use crate::value::VmDictExt;
use crate::value::VmValue;

use super::error::CompileError;
use super::{peel_node, Compiler, CompilerOptions, FinallyEntry};

#[cfg(test)]
thread_local! {
    /// Test-only override for the value-discarding classification used by
    /// [`Compiler::compile_discarded_stmt`]. Setting it forces a
    /// `produces_value` answer regardless of the node, letting tests
    /// deliberately miswire the classification and prove the #2622 balance
    /// assertion fires (see
    /// `compiler::tests::miswired_produces_value_trips_balance_assertion`).
    pub(super) static FORCE_DISCARDED_PRODUCES_VALUE: std::cell::Cell<Option<bool>> =
        const { std::cell::Cell::new(None) };
}

impl Compiler {
    pub fn new() -> Self {
        Self::with_options(CompilerOptions::from_env())
    }

    /// Compiler for an explicitly embedder-owned host-dispatch source.
    ///
    /// This grants only privileged-wire builtin exposure. Callers must keep
    /// the resulting bytecode behind a provenance-separated runtime loader.
    pub fn new_trusted_host_dispatch() -> Self {
        Self::with_options(CompilerOptions::privileged_wire())
    }

    /// Compiler for source embedded or generated by the Harn runtime.
    ///
    /// The caller must own the complete source template. This grants private
    /// runtime implementation builtins, but never privileged host wire calls.
    #[doc(hidden)]
    pub fn new_runtime_owned_source() -> Self {
        Self::with_options(CompilerOptions::runtime_owned_source())
    }

    /// Compiler for Harn's embedder-owned stdlib sources.
    #[doc(hidden)]
    pub fn new_embedded_stdlib() -> Self {
        Self::with_options(CompilerOptions::embedded_stdlib())
    }

    /// Seed syntax-sensitive import metadata before compiling a source file.
    ///
    /// The parser intentionally keeps `Color.Ready(value)` ambiguous: it can
    /// be a method call or an enum constructor. The module graph resolves
    /// that ambiguity for imported enums, including wildcard imports, so
    /// callers that compile a file outside the module-artifact path can pass
    /// the same public export contract here.
    pub fn with_imported_enum_candidates(
        mut self,
        candidates: impl IntoIterator<Item = String>,
    ) -> Self {
        self.add_imported_enum_candidates(candidates);
        self
    }

    /// Seed callables resolved from wildcard imports by the module graph.
    ///
    /// A source declaration owns a colliding name before builtin policy is
    /// considered. Selective imports are visible in the AST, while wildcard
    /// imports require this resolved projection from the module owner.
    pub fn with_imported_source_callable_names(
        mut self,
        names: impl IntoIterator<Item = String>,
    ) -> Self {
        self.add_imported_source_callable_names(names);
        self
    }

    /// Populate every module-level compiler catalog needed by declarations
    /// compiled outside the entry pipeline. Module artifacts use this same
    /// preparation as ordinary program compilation so imported functions see
    /// the enum, struct, interface, and type-alias context of their source
    /// module.
    #[doc(hidden)]
    pub fn prepare_module_context(&mut self, program: &[SNode]) {
        self.collect_module_enum_catalog(program);
        if self.enum_names.insert("Result".to_string()) {
            Self::seed_builtin_variant_owners(&mut self.enum_variant_owners);
        }
        Self::collect_struct_layouts(program, &mut self.struct_layouts);
        Self::collect_interface_methods(program, &mut self.interface_methods);
        self.collect_type_aliases(program);
        self.collect_imported_enum_candidates(program);
        self.collect_source_callable_names(program);
        self.namespace_import_demands = harn_parser::namespace_import_demands(program);
        // Box module-level mutable `let`s that a top-level or pipeline-body
        // closure captures (harn#4479). Nested function-like bodies reseed
        // their own capture set when compiled.
        self.seed_module_captured_idents(program);
    }

    #[doc(hidden)]
    pub fn add_imported_enum_candidates(&mut self, candidates: impl IntoIterator<Item = String>) {
        self.imported_enum_candidates_authoritative = true;
        self.imported_enum_candidates.extend(candidates);
    }

    #[doc(hidden)]
    pub fn add_imported_source_callable_names(&mut self, names: impl IntoIterator<Item = String>) {
        self.source_callable_names.extend(names);
    }

    /// Compile only the declarations that form a module's initialization
    /// chunk, using the complete source program for compiler context. The
    /// caller supplies a filtered list so function and pipeline closures are
    /// materialized exactly once by the artifact's function table.
    #[doc(hidden)]
    pub fn compile_module_init(
        mut self,
        context: &[SNode],
        init_nodes: &[SNode],
        imported_enum_candidates: &[String],
        imported_source_callable_names: &[String],
    ) -> Result<Chunk, CompileError> {
        self.add_imported_enum_candidates(imported_enum_candidates.iter().cloned());
        self.add_imported_source_callable_names(imported_source_callable_names.iter().cloned());
        self.prepare_module_context(context);
        self.compile_top_level_declarations(init_nodes)?;
        self.chunk.emit(Op::Nil, self.line);
        self.chunk.emit(Op::Return, self.line);
        super::ensure_chunk_addressable(&self.chunk, "the module initialization body", self.line)?;
        Ok(self.chunk)
    }

    pub fn with_options(options: CompilerOptions) -> Self {
        // Compiler construction is the boundary that owns source-callability.
        // Install the canonical contract manifest here so every entry path
        // (programs, modules, named callables, and schema initializers) sees
        // the same typed builtin surface even before a VM exists.
        Self {
            options,
            chunk: Chunk::new(),
            line: 1,
            column: 1,
            enum_names: std::collections::HashSet::new(),
            enum_variant_owners: std::collections::HashMap::new(),
            imported_enum_candidates: std::collections::HashSet::new(),
            imported_enum_candidates_authoritative: false,
            source_callable_names: std::collections::HashSet::new(),
            predeclared_enum_declarations: std::collections::HashSet::new(),
            enum_catalog_scopes: Vec::new(),
            struct_layouts: std::collections::HashMap::new(),
            interface_methods: std::collections::HashMap::new(),
            loop_stack: Vec::new(),
            handler_depth: 0,
            declared_throw: false,
            finally_bodies: Vec::new(),
            temp_counter: 0,
            scope_depth: 0,
            type_aliases: std::collections::HashMap::new(),
            type_scopes: vec![std::collections::HashMap::new()],
            monomorphic_bindings: std::collections::HashSet::new(),
            string_constants: std::collections::HashMap::new(),
            local_scopes: vec![std::collections::HashMap::new()],
            module_level: true,
            captured_bindings: std::collections::HashSet::new(),
            namespace_import_demands: std::collections::BTreeMap::new(),
        }
    }

    /// Compiler instance for a nested function-like body (fn, closure,
    /// tool, parallel arm, etc.). Differs from `new()` only in that
    /// `module_level` starts false — `try*` is allowed inside.
    pub(super) fn for_nested_body(options: CompilerOptions) -> Self {
        let mut c = Self::with_options(options);
        c.module_level = false;
        c
    }

    pub(super) fn nested_body(&self) -> Self {
        let mut nested = Self::for_nested_body(self.options);
        nested.declared_throw = self.declared_throw;
        nested.source_callable_names = self.source_callable_names.clone();
        nested
    }

    pub(super) fn nominal_type_names(&self) -> Vec<String> {
        let mut names: Vec<String> = self
            .struct_layouts
            .keys()
            .chain(self.enum_names.iter())
            .cloned()
            .collect();
        names.sort();
        names.dedup();
        names
    }

    pub(super) fn string_constant(&mut self, value: &str) -> u16 {
        if let Some(idx) = self.string_constants.get(value) {
            return *idx;
        }
        let owned = value.to_string();
        let idx = self.chunk.add_constant(Constant::String(owned.clone()));
        self.string_constants.insert(owned, idx);
        idx
    }

    pub(super) fn owned_string_constant(&mut self, value: String) -> u16 {
        if let Some(idx) = self.string_constants.get(value.as_str()) {
            return *idx;
        }
        let idx = self.chunk.add_constant(Constant::String(value.clone()));
        self.string_constants.insert(value, idx);
        idx
    }

    /// Populate `type_aliases` from a program's top-level `type T = ...`
    /// declarations so later lowerings can resolve alias names to their
    /// canonical `TypeExpr`.
    #[doc(hidden)]
    pub fn collect_type_aliases(&mut self, program: &[SNode]) {
        for sn in program {
            match peel_node(sn) {
                Node::SelectiveImport { names, .. } => {
                    for name in names {
                        self.type_aliases.entry(name.clone()).or_insert_with(|| {
                            super::TypeAliasDefinition {
                                type_params: Vec::new(),
                                body: None,
                            }
                        });
                    }
                }
                Node::TypeDecl {
                    name,
                    type_expr,
                    type_params,
                    is_pub: _,
                } => {
                    self.type_aliases.insert(
                        name.clone(),
                        super::TypeAliasDefinition {
                            type_params: type_params.clone(),
                            body: Some(type_expr.clone()),
                        },
                    );
                }
                _ => {}
            }
        }
    }

    /// Seed type names from the resolved import contract, including wildcard
    /// and facade exports. Bodies stay in their declaring module's schema
    /// factories; this catalog only identifies lexical schema bindings.
    pub fn with_imported_type_alias_names(
        mut self,
        names: impl IntoIterator<Item = String>,
    ) -> Self {
        for name in names {
            self.type_aliases
                .entry(name)
                .or_insert(super::TypeAliasDefinition {
                    type_params: Vec::new(),
                    body: None,
                });
        }
        self
    }

    /// Fully expand alias references, inlining every `Named(T)` whose `T` is a
    /// known alias with the alias's body. A `visiting` set breaks recursive
    /// aliases (`type Tree = {value: int, children: [Tree]}`): once an alias is
    /// already being expanded on the current path, the self-reference is left
    /// as an unexpanded `Named(T)` instead of recursing forever. This mirrors
    /// the typechecker's `resolve_alias` cycle guard so both sides agree, and
    /// keeps schema lowering (`type_expr_to_schema_value`) finite — a
    /// cycle-broken `Named(T)` lowers to no runtime constraint at that nested
    /// position rather than overflowing the stack.
    #[doc(hidden)]
    pub fn expand_alias(&self, ty: &TypeExpr) -> TypeExpr {
        let mut visiting = std::collections::HashSet::new();
        self.expand_alias_inner(ty, &mut visiting)
    }

    fn expand_alias_inner(
        &self,
        ty: &TypeExpr,
        visiting: &mut std::collections::HashSet<String>,
    ) -> TypeExpr {
        match ty {
            TypeExpr::Named(name) => {
                if let Some(target) = self
                    .type_aliases
                    .get(name)
                    .filter(|alias| alias.type_params.is_empty() && alias.body.is_some())
                {
                    if !visiting.insert(name.clone()) {
                        return TypeExpr::Named(name.clone());
                    }
                    let resolved = self.expand_alias_inner(target.body.as_ref().unwrap(), visiting);
                    visiting.remove(name);
                    resolved
                } else {
                    TypeExpr::Named(name.clone())
                }
            }
            TypeExpr::Union(types) => TypeExpr::Union(
                types
                    .iter()
                    .map(|t| self.expand_alias_inner(t, visiting))
                    .collect(),
            ),
            TypeExpr::Intersection(types) => TypeExpr::Intersection(
                types
                    .iter()
                    .map(|t| self.expand_alias_inner(t, visiting))
                    .collect(),
            ),
            TypeExpr::Shape(fields) => TypeExpr::Shape(
                fields
                    .iter()
                    .map(|field| ShapeField {
                        type_expr: self.expand_alias_inner(&field.type_expr, visiting),
                        ..field.clone()
                    })
                    .collect(),
            ),
            TypeExpr::OpenShape { fields, rests } => TypeExpr::OpenShape {
                fields: fields
                    .iter()
                    .map(|field| ShapeField {
                        type_expr: self.expand_alias_inner(&field.type_expr, visiting),
                        ..field.clone()
                    })
                    .collect(),
                rests: rests
                    .iter()
                    .map(|r| self.expand_alias_inner(r, visiting))
                    .collect(),
            },
            TypeExpr::List(inner) => {
                TypeExpr::List(Box::new(self.expand_alias_inner(inner, visiting)))
            }
            TypeExpr::Tuple(elements) => TypeExpr::Tuple(
                elements
                    .iter()
                    .map(|element| self.expand_alias_inner(element, visiting))
                    .collect(),
            ),
            TypeExpr::Iter(inner) => {
                TypeExpr::Iter(Box::new(self.expand_alias_inner(inner, visiting)))
            }
            TypeExpr::Generator(inner) => {
                TypeExpr::Generator(Box::new(self.expand_alias_inner(inner, visiting)))
            }
            TypeExpr::Stream(inner) => {
                TypeExpr::Stream(Box::new(self.expand_alias_inner(inner, visiting)))
            }
            TypeExpr::DictType(k, v) => TypeExpr::DictType(
                Box::new(self.expand_alias_inner(k, visiting)),
                Box::new(self.expand_alias_inner(v, visiting)),
            ),
            TypeExpr::FnType {
                params,
                return_type,
            } => TypeExpr::FnType {
                params: params
                    .iter()
                    .map(|p| self.expand_alias_inner(p, visiting))
                    .collect(),
                return_type: Box::new(self.expand_alias_inner(return_type, visiting)),
            },
            TypeExpr::Applied { name, args } => {
                let args = args
                    .iter()
                    .map(|arg| self.expand_alias_inner(arg, visiting))
                    .collect::<Vec<_>>();
                let Some(alias) = self.type_aliases.get(name) else {
                    return TypeExpr::Applied {
                        name: name.clone(),
                        args,
                    };
                };
                let Some(body) = alias.body.as_ref() else {
                    return TypeExpr::Applied {
                        name: name.clone(),
                        args,
                    };
                };
                if alias.type_params.len() != args.len() || !visiting.insert(name.clone()) {
                    return TypeExpr::Applied {
                        name: name.clone(),
                        args,
                    };
                }
                let bindings = alias
                    .type_params
                    .iter()
                    .zip(args.iter().cloned())
                    .map(|(param, arg)| (param.name.clone(), arg))
                    .collect();
                let instantiated = substitute_type_expr(body, &bindings);
                let resolved = self.expand_alias_inner(&instantiated, visiting);
                visiting.remove(name);
                resolved
            }
            TypeExpr::Never => TypeExpr::Never,
            TypeExpr::LitString(s) => TypeExpr::LitString(s.clone()),
            TypeExpr::LitInt(v) => TypeExpr::LitInt(*v),
            TypeExpr::Owned(inner) => {
                TypeExpr::Owned(Box::new(self.expand_alias_inner(inner, visiting)))
            }
        }
    }

    /// Schema-guard builtins that accept a schema as their second argument.
    /// When callers pass a type-alias identifier here, the compiler lowers
    /// it to the alias's JSON-Schema dict constant.
    pub(super) fn is_schema_guard(name: &str) -> bool {
        matches!(
            name,
            "schema_is"
                | "schema_expect"
                | "schema_parse"
                | "json_decode"
                | "schema_check"
                | "schema_report"
                | "is_type"
                | "json_validate"
        )
    }

    /// Check whether a dict-literal key node matches the given keyword
    /// (identifier or string literal form).
    pub(super) fn entry_key_is(key: &SNode, keyword: &str) -> bool {
        matches!(
            &key.node,
            Node::Identifier(name) | Node::StringLiteral(name) | Node::RawStringLiteral(name)
                if name == keyword
        )
    }

    /// Compile a program (list of top-level nodes) into a Chunk.
    /// Finds the entry pipeline and compiles its body, including inherited bodies.
    pub fn compile(mut self, program: &[SNode]) -> Result<Chunk, CompileError> {
        // Pre-scan so we can recognize EnumName.Variant as enum construction
        // even when the enum is declared inside a pipeline.
        self.prepare_module_context(program);

        for sn in program {
            match &sn.node {
                Node::ImportDecl { .. }
                | Node::SelectiveImport { .. }
                | Node::NamespaceImport { .. } => {
                    self.compile_node(sn)?;
                }
                _ => {}
            }
        }
        let main = program
            .iter()
            .find(|sn| matches!(peel_node(sn), Node::Pipeline { name, .. } if name == "default"))
            .or_else(|| {
                program
                    .iter()
                    .find(|sn| matches!(peel_node(sn), Node::Pipeline { .. }))
            });

        // When a pipeline body produces a final value, that value flows
        // out of `vm.execute()` so the CLI can map it to a process exit
        // code (int → exit n, Result::Err(msg) → stderr+exit 1).
        let mut pipeline_emits_value = false;
        if let Some(sn) = main {
            self.compile_top_level_declarations(program)?;
            if let Node::Pipeline {
                params,
                body,
                extends,
                throws,
                ..
            } = peel_node(sn)
            {
                self.compile_with_pipeline_captures(
                    program,
                    body,
                    extends.as_deref(),
                    |compiler| {
                        let saved = std::mem::replace(&mut compiler.module_level, false);
                        let saved_throw =
                            std::mem::replace(&mut compiler.declared_throw, throws.is_some());
                        if let Some(harness) = params.first().filter(|param| {
                            matches!(
                                param.type_expr.as_ref(),
                                Some(TypeExpr::Named(name)) if name == "Harness"
                            )
                        }) {
                            compiler.chunk.emit(Op::RootHarness, compiler.line);
                            compiler.emit_define_binding(&harness.name, false);
                        }
                        if let Some(parent_name) = extends {
                            compiler.compile_parent_pipeline(program, parent_name)?;
                        }
                        let result = compiler.compile_block(body);
                        compiler.module_level = saved;
                        compiler.declared_throw = saved_throw;
                        result
                    },
                )?;
                pipeline_emits_value = true;
            }
        } else {
            // Script mode: no pipeline found, treat top-level as implicit entry.
            let top_level: Vec<&SNode> = program
                .iter()
                .filter(|sn| {
                    !matches!(
                        &sn.node,
                        Node::ImportDecl { .. }
                            | Node::SelectiveImport { .. }
                            | Node::NamespaceImport { .. }
                    )
                })
                .collect();
            for sn in &top_level {
                self.compile_discarded_stmt(sn)?;
            }
            // E4.1 entrypoint convention: a top-level `fn main(harness: Harness)`
            // is invoked automatically with the runtime-provided root
            // capability. The typechecker rejects every other signature with
            // HARN-NAM-101 so we don't need to re-validate the shape here.
            if Self::has_top_level_fn_main(program) {
                self.chunk.emit(Op::RootHarness, self.line);
                self.emit_named_call("main", 1);
                pipeline_emits_value = true;
            }
        }

        self.drain_finallys_to_floor(0)?;
        if !pipeline_emits_value {
            self.chunk.emit(Op::Nil, self.line);
        }
        self.chunk.emit(Op::Return, self.line);
        super::ensure_chunk_addressable(&self.chunk, "the program body", self.line)?;
        Ok(self.chunk)
    }

    /// True when the program declares a top-level `fn main(...)`. Drives the
    /// auto-call wired by `compile()` for the new `main(harness: Harness)`
    /// entrypoint convention.
    fn has_top_level_fn_main(program: &[SNode]) -> bool {
        program
            .iter()
            .any(|sn| matches!(peel_node(sn), Node::FnDecl { name, .. } if name == "main"))
    }

    /// Compile a specific named pipeline (for test runners).
    pub fn compile_named(
        self,
        program: &[SNode],
        pipeline_name: &str,
    ) -> Result<Chunk, CompileError> {
        self.compile_named_inner(program, pipeline_name)
    }

    fn compile_named_inner(
        mut self,
        program: &[SNode],
        pipeline_name: &str,
    ) -> Result<Chunk, CompileError> {
        self.prepare_module_context(program);

        for sn in program {
            if matches!(
                &sn.node,
                Node::ImportDecl { .. }
                    | Node::SelectiveImport { .. }
                    | Node::NamespaceImport { .. }
            ) {
                self.compile_node(sn)?;
            }
        }
        let target = program.iter().find(
            |sn| matches!(peel_node(sn), Node::Pipeline { name, .. } if name == pipeline_name),
        );

        if let Some(sn) = target {
            self.compile_top_level_declarations(program)?;
            if let Node::Pipeline {
                body,
                extends,
                params,
                throws,
                ..
            } = peel_node(sn)
            {
                self.compile_with_pipeline_captures(
                    program,
                    body,
                    extends.as_deref(),
                    |compiler| {
                        let saved = std::mem::replace(&mut compiler.module_level, false);
                        let saved_throw =
                            std::mem::replace(&mut compiler.declared_throw, throws.is_some());
                        if let Some(harness) = params.first().filter(|param| {
                            matches!(
                                param.type_expr.as_ref(),
                                Some(TypeExpr::Named(name)) if name == "Harness"
                            )
                        }) {
                            compiler.chunk.emit(Op::RootHarness, compiler.line);
                            compiler.emit_define_binding(&harness.name, false);
                        }
                        if let Some(parent_name) = extends {
                            compiler.compile_parent_pipeline(program, parent_name)?;
                        }
                        let result = compiler.compile_block(body);
                        compiler.module_level = saved;
                        compiler.declared_throw = saved_throw;
                        result
                    },
                )?;
            }
        }

        self.drain_finallys_to_floor(0)?;
        self.chunk.emit(Op::Nil, self.line);
        self.chunk.emit(Op::Return, self.line);
        super::ensure_chunk_addressable(&self.chunk, "the pipeline body", self.line)?;
        Ok(self.chunk)
    }

    /// Emit bytecode preamble for default parameter values.
    /// For each param with a default at index i, emits:
    ///   GetArgc; PushInt (i+1); GreaterEqual; JumpIfTrue <skip>;
    ///   [compile default expr]; DefLet param_name; <skip>:
    pub(super) fn emit_default_preamble(
        &mut self,
        params: &[TypedParam],
    ) -> Result<(), CompileError> {
        for (i, param) in params.iter().enumerate() {
            if let Some(default_expr) = &param.default_value {
                self.chunk.emit(Op::GetArgc, self.line);
                let threshold_idx = self.chunk.add_constant(Constant::Int((i + 1) as i64));
                self.chunk.emit_u16(Op::Constant, threshold_idx, self.line);
                self.chunk.emit(Op::GreaterEqual, self.line);
                let skip_jump = self.chunk.emit_jump(Op::JumpIfTrue, self.line);
                // JumpIfTrue doesn't pop its boolean operand.
                self.chunk.emit(Op::Pop, self.line);
                // Compile the default with this param and all *later* params
                // hidden from local resolution. A default is evaluated left to
                // right at call time: it may reference an earlier parameter,
                // but a mention of its own name (or a later, not-yet-bound
                // parameter) must resolve to the enclosing scope — e.g.
                // `let n = 7; fn f(n = n * 2)` reads the outer `n`. Without the
                // mask, `n` bound to the param's own unset slot and threw at
                // runtime. Earlier params stay visible.
                let masked = self.mask_param_names(&params[i..]);
                let result = self.compile_node(default_expr);
                self.restore_param_names(masked);
                result?;
                self.emit_init_or_define_binding(&param.name, false);
                let end_jump = self.chunk.emit_jump(Op::Jump, self.line);
                self.chunk.patch_jump(skip_jump);
                self.chunk.emit(Op::Pop, self.line);
                self.chunk.patch_jump(end_jump);
            }
        }
        Ok(())
    }

    /// Emit body-local type checks that call-site validation cannot cover.
    /// Ordinary supplied arguments are validated by precomputed
    /// [`crate::chunk::ParamSlot`] guards before the frame is entered. The
    /// bytecode preamble still checks interface parameters, because interface
    /// satisfaction depends on compiler-collected method metadata, and checks
    /// defaulted schema parameters only when the caller omitted that argument.
    pub(super) fn emit_type_checks(&mut self, params: &[TypedParam]) {
        for (param_index, param) in params.iter().enumerate() {
            if let Some(type_expr) = &param.type_expr {
                let check_type = if param.rest {
                    harn_parser::TypeExpr::List(Box::new(type_expr.clone()))
                } else {
                    type_expr.clone()
                };

                if let harn_parser::TypeExpr::Named(name) = &check_type {
                    if let Some(methods) = self.interface_methods.get(name).cloned() {
                        let fn_idx = self.string_constant("__assert_interface");
                        self.chunk.emit_u16(Op::Constant, fn_idx, self.line);
                        self.emit_get_binding(&param.name);
                        let name_idx = self.string_constant(&param.name);
                        self.chunk.emit_u16(Op::Constant, name_idx, self.line);
                        let iface_idx = self.string_constant(name);
                        self.chunk.emit_u16(Op::Constant, iface_idx, self.line);
                        let methods_str = methods.join(",");
                        let methods_idx = self.owned_string_constant(methods_str);
                        self.chunk.emit_u16(Op::Constant, methods_idx, self.line);
                        self.chunk.emit_u8(Op::Call, 4, self.line);
                        self.chunk.emit(Op::Pop, self.line);
                        continue;
                    }
                }

                if param.default_value.is_some() {
                    if let Some(schema) = Self::type_expr_to_schema_value(&check_type) {
                        self.emit_default_param_schema_check(param_index, param, &schema);
                    }
                }
            }
        }
    }

    fn emit_default_param_schema_check(
        &mut self,
        param_index: usize,
        param: &TypedParam,
        schema: &VmValue,
    ) {
        self.chunk.emit(Op::GetArgc, self.line);
        let threshold_idx = self
            .chunk
            .add_constant(Constant::Int((param_index + 1) as i64));
        self.chunk.emit_u16(Op::Constant, threshold_idx, self.line);
        self.chunk.emit(Op::GreaterEqual, self.line);
        let supplied_jump = self.chunk.emit_jump(Op::JumpIfTrue, self.line);
        self.chunk.emit(Op::Pop, self.line);
        self.emit_schema_assert_call(param, schema);
        let end_jump = self.chunk.emit_jump(Op::Jump, self.line);
        self.chunk.patch_jump(supplied_jump);
        self.chunk.emit(Op::Pop, self.line);
        self.chunk.patch_jump(end_jump);
    }

    fn emit_schema_assert_call(&mut self, param: &TypedParam, schema: &VmValue) {
        let fn_idx = self.string_constant("__assert_schema");
        self.chunk.emit_u16(Op::Constant, fn_idx, self.line);
        self.emit_get_binding(&param.name);
        let name_idx = self.string_constant(&param.name);
        self.chunk.emit_u16(Op::Constant, name_idx, self.line);
        self.emit_vm_value_literal(schema);
        self.chunk.emit_u8(Op::Call, 3, self.line);
        self.chunk.emit(Op::Pop, self.line);
    }

    #[doc(hidden)]
    pub fn type_expr_to_schema_value(type_expr: &harn_parser::TypeExpr) -> Option<VmValue> {
        match type_expr {
            harn_parser::TypeExpr::Named(name) => match name.as_str() {
                "any" | "unknown" => Some(VmValue::dict(BTreeMap::<String, VmValue>::new())),
                "int" | "float" | "string" | "bool" | "list" | "dict" | "set" | "nil"
                | "closure" | "bytes" => Some(VmValue::dict(BTreeMap::from([(
                    "type".to_string(),
                    VmValue::String(arcstr::ArcStr::from(name.as_str())),
                )]))),
                _ => None,
            },
            harn_parser::TypeExpr::Shape(fields) => {
                let mut properties = BTreeMap::new();
                let mut required = Vec::new();
                for field in fields {
                    let mut field_schema = Self::type_expr_to_schema_value(&field.type_expr)?;
                    if field.optional {
                        field_schema = VmValue::dict(BTreeMap::from([(
                            "union".to_string(),
                            VmValue::List(std::sync::Arc::new(vec![
                                field_schema,
                                VmValue::dict(BTreeMap::from([(
                                    "type".to_string(),
                                    VmValue::String(arcstr::ArcStr::from("nil")),
                                )])),
                            ])),
                        )]));
                    }
                    properties.insert(field.name.clone(), field_schema);
                    if !field.optional {
                        required.push(VmValue::String(arcstr::ArcStr::from(field.name.as_str())));
                    }
                }
                let mut out = BTreeMap::new();
                out.put_str("type", "dict");
                out.insert("properties".to_string(), VmValue::dict(properties));
                if !required.is_empty() {
                    out.insert(
                        "required".to_string(),
                        VmValue::List(std::sync::Arc::new(required)),
                    );
                }
                Some(VmValue::dict(out))
            }
            harn_parser::TypeExpr::OpenShape { .. } => None,
            harn_parser::TypeExpr::List(inner) => {
                let mut out = BTreeMap::new();
                out.put_str("type", "list");
                let item_schema = Self::type_expr_to_schema_value(inner)?;
                out.insert("items".to_string(), item_schema);
                Some(VmValue::dict(out))
            }
            // The canonical Harn schema vocabulary currently has homogeneous
            // `items` but no positional-items contract. Returning `None`
            // deliberately selects the compiled TypeExpr runtime guard, which
            // preserves exact arity and slot types instead of weakening a
            // tuple to a homogeneous list schema.
            harn_parser::TypeExpr::Tuple(_) => None,
            harn_parser::TypeExpr::DictType(key, value) => {
                let mut out = BTreeMap::new();
                out.put_str("type", "dict");
                if matches!(key.as_ref(), harn_parser::TypeExpr::Named(name) if name == "string") {
                    let value_schema = Self::type_expr_to_schema_value(value)?;
                    out.insert("additional_properties".to_string(), value_schema);
                }
                Some(VmValue::dict(out))
            }
            harn_parser::TypeExpr::Union(members) => {
                // Special-case unions of literals: emit as `enum: [...]`
                // so the schema round-trips as canonical JSON Schema and
                // is ACP-/OpenAPI-compatible. Mixed unions fall back to
                // the `union:` key that validators recognize.
                if !members.is_empty()
                    && members
                        .iter()
                        .all(|m| matches!(m, harn_parser::TypeExpr::LitString(_)))
                {
                    let values = members
                        .iter()
                        .map(|m| match m {
                            harn_parser::TypeExpr::LitString(s) => {
                                VmValue::String(arcstr::ArcStr::from(s.as_str()))
                            }
                            _ => unreachable!(),
                        })
                        .collect::<Vec<_>>();
                    return Some(VmValue::dict(BTreeMap::from([
                        (
                            "type".to_string(),
                            VmValue::String(arcstr::ArcStr::from("string")),
                        ),
                        (
                            "enum".to_string(),
                            VmValue::List(std::sync::Arc::new(values)),
                        ),
                    ])));
                }
                if !members.is_empty()
                    && members
                        .iter()
                        .all(|m| matches!(m, harn_parser::TypeExpr::LitInt(_)))
                {
                    let values = members
                        .iter()
                        .map(|m| match m {
                            harn_parser::TypeExpr::LitInt(v) => VmValue::Int(*v),
                            _ => unreachable!(),
                        })
                        .collect::<Vec<_>>();
                    return Some(VmValue::dict(BTreeMap::from([
                        (
                            "type".to_string(),
                            VmValue::String(arcstr::ArcStr::from("int")),
                        ),
                        (
                            "enum".to_string(),
                            VmValue::List(std::sync::Arc::new(values)),
                        ),
                    ])));
                }
                let branches = members
                    .iter()
                    .map(Self::type_expr_to_schema_value)
                    .collect::<Option<Vec<_>>>()?;
                if branches.is_empty() {
                    None
                } else {
                    Some(VmValue::dict(BTreeMap::from([(
                        "union".to_string(),
                        VmValue::List(std::sync::Arc::new(branches)),
                    )])))
                }
            }
            harn_parser::TypeExpr::Intersection(members) => {
                // Encode `A & B` as JSON-Schema `allOf` (the runtime
                // accepts the snake_case `all_of` key directly). The
                // value must validate against every branch.
                let branches = members
                    .iter()
                    .map(Self::type_expr_to_schema_value)
                    .collect::<Option<Vec<_>>>()?;
                if branches.is_empty() {
                    None
                } else {
                    Some(VmValue::dict(BTreeMap::from([(
                        "all_of".to_string(),
                        VmValue::List(std::sync::Arc::new(branches)),
                    )])))
                }
            }
            harn_parser::TypeExpr::FnType { .. } => Some(VmValue::dict(BTreeMap::from([(
                "type".to_string(),
                VmValue::String(arcstr::ArcStr::from("closure")),
            )]))),
            harn_parser::TypeExpr::Applied { .. } => None,
            harn_parser::TypeExpr::Iter(_)
            | harn_parser::TypeExpr::Generator(_)
            | harn_parser::TypeExpr::Stream(_) => None,
            harn_parser::TypeExpr::Never => None,
            harn_parser::TypeExpr::LitString(s) => Some(VmValue::dict(BTreeMap::from([
                (
                    "type".to_string(),
                    VmValue::String(arcstr::ArcStr::from("string")),
                ),
                (
                    "const".to_string(),
                    VmValue::String(arcstr::ArcStr::from(s.as_str())),
                ),
            ]))),
            harn_parser::TypeExpr::LitInt(v) => Some(VmValue::dict(BTreeMap::from([
                (
                    "type".to_string(),
                    VmValue::String(arcstr::ArcStr::from("int")),
                ),
                ("const".to_string(), VmValue::Int(*v)),
            ]))),
            harn_parser::TypeExpr::Owned(inner) => Self::type_expr_to_schema_value(inner),
        }
    }

    pub(super) fn emit_vm_value_literal(&mut self, value: &VmValue) {
        match value {
            VmValue::String(text) => {
                let idx = self.string_constant(text);
                self.chunk.emit_u16(Op::Constant, idx, self.line);
            }
            VmValue::Int(number) => {
                let idx = self.chunk.add_constant(Constant::Int(*number));
                self.chunk.emit_u16(Op::Constant, idx, self.line);
            }
            VmValue::Float(number) => {
                let idx = self.chunk.add_constant(Constant::Float(*number));
                self.chunk.emit_u16(Op::Constant, idx, self.line);
            }
            VmValue::Bool(value) => {
                let idx = self.chunk.add_constant(Constant::Bool(*value));
                self.chunk.emit_u16(Op::Constant, idx, self.line);
            }
            VmValue::Nil => self.chunk.emit(Op::Nil, self.line),
            VmValue::List(items) => {
                for item in items.iter() {
                    self.emit_vm_value_literal(item);
                }
                self.chunk
                    .emit_u16(Op::BuildList, items.len() as u16, self.line);
            }
            VmValue::Dict(entries) => {
                for (key, item) in entries.iter() {
                    let key_idx = self.string_constant(key);
                    self.chunk.emit_u16(Op::Constant, key_idx, self.line);
                    self.emit_vm_value_literal(item);
                }
                self.chunk
                    .emit_u16(Op::BuildDict, entries.len() as u16, self.line);
            }
            _ => {}
        }
    }

    /// Emit the extra u16 type name index after a TryCatchSetup jump.
    pub(super) fn emit_type_name_extra(&mut self, type_name_idx: u16) {
        let hi = (type_name_idx >> 8) as u8;
        let lo = type_name_idx as u8;
        self.chunk.code.push(hi);
        self.chunk.code.push(lo);
        self.chunk.lines.push(self.line);
        self.chunk.columns.push(self.column);
        self.chunk.lines.push(self.line);
        self.chunk.columns.push(self.column);
    }

    /// Compile a try/catch body block (produces a value on the stack).
    pub(super) fn compile_try_body(&mut self, body: &[SNode]) -> Result<(), CompileError> {
        if body.is_empty() {
            self.chunk.emit(Op::Nil, self.line);
        } else {
            self.compile_scoped_block(body)?;
        }
        Ok(())
    }

    /// Compile catch error binding (error value is on stack from handler).
    pub(super) fn compile_catch_binding(
        &mut self,
        error_var: &Option<String>,
    ) -> Result<(), CompileError> {
        if let Some(var_name) = error_var {
            self.emit_define_binding(var_name, false);
        } else {
            self.chunk.emit(Op::Pop, self.line);
        }
        Ok(())
    }

    /// True if there are any pending cleanup bodies.
    pub(super) fn has_pending_finally(&self) -> bool {
        !self.finally_bodies.is_empty()
    }

    /// Register a cleanup body for the region that follows and install the
    /// exception handler that runs it when an error leaves that region.
    pub(super) fn push_cleanup(&mut self, body: Vec<SNode>) {
        let handler_depth = self.handler_depth;
        self.handler_depth += 1;
        let error_jump = self.chunk.emit_jump(Op::TryCatchPreserve, self.line);
        let empty_type = self.string_constant("");
        self.emit_type_name_extra(empty_type);
        self.finally_bodies.push(FinallyEntry {
            body,
            handler_depth,
            declared_throw: self.declared_throw,
            error_jump,
        });
    }

    /// Close the innermost cleanup region: on the normal path pop its handler
    /// and run the body; on the exception path run the body and rethrow.
    ///
    /// The entry is removed before either copy of the body is compiled, so a
    /// `return`/`break`/`continue` inside the body runs only the cleanups
    /// outside it. The runtime pops the handler before delivering an error,
    /// so a throw from the body replaces the original error and escapes.
    fn pop_cleanup(&mut self) -> Result<(), CompileError> {
        let entry = self.finally_bodies.pop().expect("pending cleanup");
        debug_assert_eq!(self.handler_depth, entry.handler_depth + 1);
        self.handler_depth = entry.handler_depth;
        self.chunk.emit(Op::PopHandler, self.line);
        self.compile_finally_inline(&entry.body, entry.declared_throw)?;
        let end_jump = self.chunk.emit_jump(Op::Jump, self.line);

        self.chunk.patch_jump(entry.error_jump);
        self.temp_counter += 1;
        let temp_name = format!("__finally_err_{}__", self.temp_counter);
        self.emit_define_binding(&temp_name, true);
        self.compile_finally_inline(&entry.body, entry.declared_throw)?;
        self.emit_get_binding(&temp_name);
        self.chunk.emit(Op::Rethrow, self.line);

        self.chunk.patch_jump(end_jump);
        Ok(())
    }

    pub(super) fn declare_param_slots(&mut self, params: &[TypedParam]) {
        for param in params {
            self.define_local_slot(&param.name, false);
        }
    }

    /// Temporarily remove the given parameters' names from the innermost local
    /// scope so that, while compiling a default-value expression, references to
    /// them resolve to the enclosing scope instead of their not-yet-bound param
    /// slots. Returns the removed bindings so [`Self::restore_param_names`] can
    /// reinstate them afterward. See [`Self::emit_default_preamble`].
    pub(super) fn mask_param_names(
        &mut self,
        params: &[TypedParam],
    ) -> Vec<(String, super::LocalBinding)> {
        let mut removed = Vec::new();
        if let Some(scope) = self.local_scopes.last_mut() {
            for param in params {
                if let Some(binding) = scope.remove(&param.name) {
                    removed.push((param.name.clone(), binding));
                }
            }
        }
        removed
    }

    /// Reinstate parameter names removed by [`Self::mask_param_names`].
    pub(super) fn restore_param_names(&mut self, removed: Vec<(String, super::LocalBinding)>) {
        if let Some(scope) = self.local_scopes.last_mut() {
            for (name, binding) in removed {
                scope.insert(name, binding);
            }
        }
    }

    /// Seed exact source bindings captured by nested callables in the body
    /// about to be compiled. Parser-owned lexical analysis accounts for
    /// parameters, patterns, blocks, loops, catches, selects, and nested
    /// callable boundaries before the VM decides whether to use `DefCell`.
    pub(super) fn seed_captured_idents(&mut self, body: &[SNode]) {
        let match_patterns = self.lexical_match_pattern_catalog();
        self.captured_bindings =
            harn_parser::lexical::captured_bindings_in_nested_callables(body, &match_patterns);
    }

    fn seed_module_captured_idents(&mut self, body: &[SNode]) {
        let match_patterns = self.lexical_match_pattern_catalog();
        self.captured_bindings =
            harn_parser::lexical::captured_bindings_in_compiled_module(body, &match_patterns);
    }

    pub(super) fn lexical_match_pattern_catalog(
        &self,
    ) -> harn_parser::lexical::MatchPatternCatalog {
        if self.imported_enum_candidates.is_empty() {
            return harn_parser::lexical::MatchPatternCatalog::new(
                &self.enum_names,
                &self.enum_variant_owners,
            );
        }
        let mut enum_names = self.enum_names.clone();
        enum_names.extend(self.imported_enum_candidates.iter().cloned());
        harn_parser::lexical::MatchPatternCatalog::new(&enum_names, &self.enum_variant_owners)
    }

    pub(super) fn begin_scope(&mut self) {
        self.chunk.emit(Op::PushScope, self.line);
        self.scope_depth += 1;
        let enum_catalog = self.enum_catalog_snapshot();
        self.enum_catalog_scopes.push(enum_catalog);
        self.type_scopes.push(std::collections::HashMap::new());
        self.local_scopes.push(std::collections::HashMap::new());
    }

    pub(super) fn end_scope(&mut self) {
        if self.scope_depth > 0 {
            self.chunk.emit(Op::PopScope, self.line);
            self.scope_depth -= 1;
            if let Some(snapshot) = self.enum_catalog_scopes.pop() {
                self.restore_enum_catalog(snapshot);
            }
            self.type_scopes.pop();
            self.local_scopes.pop();
        }
    }

    /// Emit cleanup for an abrupt control-flow path without changing the
    /// compiler's lexical scope stacks for the source path that follows it.
    pub(super) fn emit_scope_unwind_to(&mut self, target_depth: usize) {
        for _ in target_depth..self.scope_depth {
            self.chunk.emit(Op::PopScope, self.line);
        }
    }

    pub(super) fn compile_scoped_block(&mut self, stmts: &[SNode]) -> Result<(), CompileError> {
        self.begin_scope();
        let finally_floor = self.finally_bodies.len();
        if stmts.is_empty() {
            self.chunk.emit(Op::Nil, self.line);
        } else {
            self.compile_block(stmts)?;
        }
        self.drain_finallys_to_floor(finally_floor)?;
        self.end_scope();
        Ok(())
    }

    pub(super) fn compile_scoped_statements(
        &mut self,
        stmts: &[SNode],
    ) -> Result<(), CompileError> {
        self.begin_scope();
        self.record_monomorphic_var_bindings(stmts);
        let finally_floor = self.finally_bodies.len();
        for sn in stmts {
            self.compile_discarded_stmt(sn)?;
        }
        self.drain_finallys_to_floor(finally_floor)?;
        self.end_scope();
        Ok(())
    }

    /// Close pending cleanup regions down to a saved floor in LIFO order.
    /// See [`Self::pop_cleanup`].
    pub(super) fn drain_finallys_to_floor(&mut self, floor: usize) -> Result<(), CompileError> {
        while self.finally_bodies.len() > floor {
            self.pop_cleanup()?;
        }
        Ok(())
    }

    /// Run the pending cleanup bodies a non-local transfer (`return`,
    /// `break`, `continue`) crosses on its way down to `floor`, innermost
    /// first, then restore the pending stack.
    ///
    /// Before each body runs, the handlers above it, including its own, are
    /// popped, so a throw from the body reaches only the handlers outside it.
    /// `handler_floor` is the handler depth the transfer lands at; `None`
    /// leaves the remaining handlers to the frame teardown of a `return`.
    ///
    /// Each body is removed from the stack *before* it is inlined, so a
    /// `return`/`break`/`continue` inside a finally body runs only the
    /// finallys *outside* it instead of re-running the one it is in — which
    /// otherwise recursed forever at compile time and aborted the process
    /// with a stack overflow. The stack is restored afterward because a
    /// transfer is a branch: the code the compiler emits after it still needs
    /// the pending cleanups for the fall-through and sibling paths.
    pub(super) fn run_pending_finallys_for_transfer(
        &mut self,
        floor: usize,
        handler_floor: Option<usize>,
    ) -> Result<(), CompileError> {
        let saved_entries = self.finally_bodies[floor..].to_vec();
        let saved_handler_depth = self.handler_depth;
        let result = self.unwind_for_transfer(floor, handler_floor);
        self.finally_bodies.truncate(floor);
        self.finally_bodies.extend(saved_entries);
        self.handler_depth = saved_handler_depth;
        result
    }

    fn unwind_for_transfer(
        &mut self,
        floor: usize,
        handler_floor: Option<usize>,
    ) -> Result<(), CompileError> {
        while self.finally_bodies.len() > floor {
            let entry = self.finally_bodies.pop().expect("non-empty by guard");
            self.emit_pop_handlers_to(entry.handler_depth);
            self.compile_finally_inline(&entry.body, entry.declared_throw)?;
        }
        if let Some(handler_floor) = handler_floor {
            self.emit_pop_handlers_to(handler_floor);
        }
        Ok(())
    }

    fn emit_pop_handlers_to(&mut self, depth: usize) {
        while self.handler_depth > depth {
            self.chunk.emit(Op::PopHandler, self.line);
            self.handler_depth -= 1;
        }
    }

    /// Register an auto-drop defer for an `owned<T>` binding. The drop runs
    /// at scope exit alongside any user-written `defer { ... }` blocks (LIFO
    /// order) and on `return` / `break` / `continue` / `throw` via the
    /// existing finally-unwinding machinery.
    pub(super) fn maybe_register_owned_drop(
        &mut self,
        pattern: &harn_parser::BindingPattern,
        type_ann: Option<&TypeExpr>,
        span: harn_lexer::Span,
    ) {
        // Auto-drop only fires when the user explicitly opted in via
        // `owned<T>` on a single-identifier binding. Destructured patterns
        // (`{a, b}`, `[a, b]`, pairs) aren't auto-dropped: ownership of a
        // composite isn't well-defined, and users can wrap individual fields
        // with `owned<T>` and bind them separately if needed.
        let Some(ty) = type_ann else {
            return;
        };
        if !matches!(ty, TypeExpr::Owned(_)) {
            return;
        }
        let harn_parser::BindingPattern::Identifier(name) = pattern else {
            return;
        };
        if harn_parser::is_discard_name(name) {
            return;
        }
        let call = harn_parser::spanned(
            Node::FunctionCall {
                name: "drop".to_string(),
                args: vec![harn_parser::spanned(Node::Identifier(name.clone()), span)],
                type_args: Vec::new(),
            },
            span,
        );
        self.push_cleanup(vec![call]);
    }

    /// Compile a statement that appears in a value-discarding sequence —
    /// the script-mode module body, an inherited pipeline body, and block
    /// interiors — then pop its value when `produces_value` says it left
    /// one.
    ///
    /// In debug builds this also asserts the operand stack stayed balanced
    /// across the statement: a straight-line statement must net exactly one
    /// value when `produces_value` is true and zero otherwise. That turns a
    /// `produces_value` misclassification — like the attributed-decl gap
    /// fixed in #2610, where the loop popped against an empty stack — from a
    /// latent runtime "Stack underflow" (often masked further by the
    /// bytecode cache, #2621) into a loud compile-time failure in tests/CI.
    /// Statements containing branches or other non-linearly-modeled opcodes
    /// can't be summed by the lightweight model, so the assertion skips them
    /// (see [`Chunk::balance_delta_since`]).
    pub(super) fn compile_discarded_stmt(&mut self, sn: &SNode) -> Result<(), CompileError> {
        #[cfg(debug_assertions)]
        let probe = self.chunk.balance_probe();
        self.compile_node(sn)?;
        #[allow(unused_mut)]
        let mut produces = Self::produces_value(&sn.node);
        // Test-only hook: deliberately miswire the classification to prove
        // the balance assertion below trips on a `produces_value` gap (the
        // #2622 verification). No-op in non-test builds.
        #[cfg(test)]
        if let Some(forced) = FORCE_DISCARDED_PRODUCES_VALUE.with(std::cell::Cell::get) {
            produces = forced;
        }
        #[cfg(debug_assertions)]
        if let Some(delta) = self.chunk.balance_delta_since(probe) {
            let expected = i32::from(produces);
            debug_assert_eq!(
                delta, expected,
                "operand-stack imbalance at line {}: produces_value={produces} but the \
                 node's emitted bytecode netted {delta} (expected {expected}). A \
                 `produces_value` arm is out of sync with this node's codegen — see #2622.\n\
                 node: {:?}",
                self.line, sn.node,
            );
        }
        if produces {
            self.chunk.emit(Op::Pop, self.line);
        }
        Ok(())
    }

    pub(super) fn compile_block(&mut self, stmts: &[SNode]) -> Result<(), CompileError> {
        self.record_monomorphic_var_bindings(stmts);
        let callable_declarations = stmts
            .iter()
            .enumerate()
            .filter_map(|(index, node)| {
                harn_parser::lexical::hoisted_callable_name(node)
                    .map(|name| (index, name.to_string()))
            })
            .collect::<Vec<_>>();
        let callable_names = callable_declarations
            .iter()
            .map(|(_, name)| name.as_str())
            .collect::<std::collections::HashSet<_>>();
        let mut emitted_callables = std::collections::HashSet::new();

        for (i, snode) in stmts.iter().enumerate() {
            if harn_parser::lexical::hoisted_callable_name(snode).is_some() {
                if emitted_callables.insert(i) {
                    self.compile_discarded_stmt(snode)?;
                }
                if i == stmts.len() - 1 {
                    self.chunk.emit(Op::Nil, self.line);
                }
                continue;
            }

            // Function-like declarations are visible throughout their block.
            // Materialize only the forward declarations reachable from this
            // statement (including mutual-recursion dependencies), at the
            // latest safe point. Earlier data bindings have therefore run and
            // remain available to the closure, while a forward call cannot
            // reach an uninitialized callable slot.
            let mut pending = Vec::new();
            Self::collect_callable_references(snode, &callable_names, &mut pending);
            let mut reachable = std::collections::HashSet::new();
            while let Some(name) = pending.pop() {
                if !reachable.insert(name.clone()) {
                    continue;
                }
                for (index, declaration_name) in &callable_declarations {
                    if declaration_name == &name {
                        Self::collect_callable_references(
                            &stmts[*index],
                            &callable_names,
                            &mut pending,
                        );
                    }
                }
            }
            for (index, name) in &callable_declarations {
                if reachable.contains(name) && emitted_callables.insert(*index) {
                    self.compile_discarded_stmt(&stmts[*index])?;
                }
            }

            if i == stmts.len() - 1 {
                // The block's value is its last statement's. Backfill a `Nil`
                // when that statement produced none, so the block always
                // leaves exactly one value on the stack.
                self.compile_node(snode)?;
                if !Self::produces_value(&snode.node) {
                    self.chunk.emit(Op::Nil, self.line);
                }
            } else {
                self.compile_discarded_stmt(snode)?;
            }
        }
        Ok(())
    }

    fn collect_callable_references(
        node: &SNode,
        callable_names: &std::collections::HashSet<&str>,
        out: &mut Vec<String>,
    ) {
        match &node.node {
            Node::Identifier(name) | Node::FunctionCall { name, .. }
                if callable_names.contains(name.as_str()) =>
            {
                out.push(name.clone());
            }
            _ => {}
        }
        for child in harn_parser::visit::immediate_children(node) {
            Self::collect_callable_references(child, callable_names, out);
        }
    }

    /// Compile a match arm body, ensuring it always pushes exactly one value.
    pub(super) fn compile_match_body(&mut self, body: &[SNode]) -> Result<(), CompileError> {
        self.begin_scope();
        let finally_floor = self.finally_bodies.len();
        if body.is_empty() {
            self.chunk.emit(Op::Nil, self.line);
        } else {
            self.compile_block(body)?;
            if !Self::produces_value(&body.last().unwrap().node) {
                self.chunk.emit(Op::Nil, self.line);
            }
        }
        self.drain_finallys_to_floor(finally_floor)?;
        self.end_scope();
        Ok(())
    }

    /// Emit the binary op instruction for a compound assignment operator.
    pub(super) fn emit_compound_op(&mut self, op: BinaryOperator) -> Result<(), CompileError> {
        match op {
            BinaryOperator::Add => self.chunk.emit(Op::Add, self.line),
            BinaryOperator::Sub => self.chunk.emit(Op::Sub, self.line),
            BinaryOperator::Mul => self.chunk.emit(Op::Mul, self.line),
            BinaryOperator::Div => self.chunk.emit(Op::Div, self.line),
            BinaryOperator::Mod => self.chunk.emit(Op::Mod, self.line),
            _ => {
                return Err(CompileError {
                    message: format!("Unknown compound operator: {op}"),
                    line: self.line,
                })
            }
        }
        Ok(())
    }

    /// Check if a node produces a value on the stack that needs to be popped.
    pub(super) fn produces_value(node: &Node) -> bool {
        harn_parser::node_produces_value(node)
    }
}

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