chore(tvix/eval): move compiler module to a new folder r/4602

Change-Id: I76157f9cf1369cd17506de1b1ded1a4fd06f004a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6268
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

1 files changed, 0 insertions, 1093 deletions

diff --git a/tvix/eval/src/compiler.rs b/tvix/eval/src/compiler.rs
deleted file mode 100644
index 5cb02a66b96a..000000000000
--- a/tvix/eval/src/compiler.rs
+++ /dev/null
@@ -1,1093 +0,0 @@
-//! This module implements a compiler for compiling the rnix AST
-//! representation to Tvix bytecode.
-//!
-//! A note on `unwrap()`: This module contains a lot of calls to
-//! `unwrap()` or `expect(...)` on data structures returned by `rnix`.
-//! The reason for this is that rnix uses the same data structures to
-//! represent broken and correct ASTs, so all typed AST variants have
-//! the ability to represent an incorrect node.
-//!
-//! However, at the time that the AST is passed to the compiler we
-//! have verified that `rnix` considers the code to be correct, so all
-//! variants are fulfilled. In cases where the invariant is guaranteed
-//! by the code in this module, `debug_assert!` has been used to catch
-//! mistakes early during development.
-
-use path_clean::PathClean;
-use rnix::ast::{self, AstToken, HasEntry};
-use rowan::ast::AstNode;
-use std::collections::{hash_map, HashMap};
-use std::path::{Path, PathBuf};
-use std::rc::Rc;
-
-use crate::chunk::Chunk;
-use crate::errors::{Error, ErrorKind, EvalResult};
-use crate::opcode::{CodeIdx, OpCode};
-use crate::value::{Lambda, Value};
-use crate::warnings::{EvalWarning, WarningKind};
-
-/// Represents the result of compiling a piece of Nix code. If
-/// compilation was successful, the resulting bytecode can be passed
-/// to the VM.
-pub struct CompilationResult {
-    pub lambda: Lambda,
-    pub warnings: Vec<EvalWarning>,
-    pub errors: Vec<Error>,
-}
-
-/// Represents a single local already known to the compiler.
-struct Local {
-    // Definition name, which can be different kinds of tokens (plain
-    // string or identifier). Nix does not allow dynamic names inside
-    // of `let`-expressions.
-    name: String,
-
-    // Syntax node at which this local was declared.
-    node: Option<rnix::SyntaxNode>,
-
-    // Scope depth of this local.
-    depth: usize,
-
-    // Phantom locals are not actually accessible by users (e.g.
-    // intermediate values used for `with`).
-    phantom: bool,
-
-    // Is this local known to have been used at all?
-    used: bool,
-}
-
-/// Represents a stack offset containing keys which are currently
-/// in-scope through a with expression.
-#[derive(Debug)]
-struct With {
-    depth: usize,
-}
-
-/// Represents a scope known during compilation, which can be resolved
-/// directly to stack indices.
-///
-/// TODO(tazjin): `with`-stack
-/// TODO(tazjin): flag "specials" (e.g. note depth if builtins are
-/// overridden)
-#[derive(Default)]
-struct Scope {
-    locals: Vec<Local>,
-
-    // How many scopes "deep" are these locals?
-    scope_depth: usize,
-
-    // Stack indices of attribute sets currently in scope through
-    // `with`.
-    with_stack: Vec<With>,
-
-    // Users are allowed to override globally defined symbols like
-    // `true`, `false` or `null` in scopes. We call this "scope
-    // poisoning", as it requires runtime resolution of those tokens.
-    //
-    // To support this efficiently, the depth at which a poisoning
-    // occured is tracked here.
-    poisoned_tokens: HashMap<&'static str, usize>,
-}
-
-impl Scope {
-    /// Mark a globally defined token as poisoned.
-    fn poison(&mut self, name: &'static str, depth: usize) {
-        match self.poisoned_tokens.entry(name) {
-            hash_map::Entry::Occupied(_) => {
-                /* do nothing, as the token is already poisoned at a
-                 * lower scope depth */
-            }
-            hash_map::Entry::Vacant(entry) => {
-                entry.insert(depth);
-            }
-        }
-    }
-
-    /// Check whether a given token is poisoned.
-    fn is_poisoned(&self, name: &str) -> bool {
-        self.poisoned_tokens.contains_key(name)
-    }
-
-    /// "Unpoison" tokens that were poisoned at a given depth. Used
-    /// when scopes are closed.
-    fn unpoison(&mut self, depth: usize) {
-        self.poisoned_tokens
-            .retain(|_, poisoned_at| *poisoned_at != depth);
-    }
-}
-
-/// Represents the lambda currently being compiled.
-struct LambdaCtx {
-    lambda: Lambda,
-    scope: Scope,
-}
-
-impl LambdaCtx {
-    fn new() -> Self {
-        LambdaCtx {
-            lambda: Lambda::new_anonymous(),
-            scope: Default::default(),
-        }
-    }
-}
-
-type GlobalsMap = HashMap<&'static str, Rc<dyn Fn(&mut Compiler)>>;
-
-struct Compiler {
-    contexts: Vec<LambdaCtx>,
-    warnings: Vec<EvalWarning>,
-    errors: Vec<Error>,
-    root_dir: PathBuf,
-
-    /// Carries all known global tokens; the full set of which is
-    /// created when the compiler is invoked.
-    ///
-    /// Each global has an associated token, which when encountered as
-    /// an identifier is resolved against the scope poisoning logic,
-    /// and a function that should emit code for the token.
-    globals: GlobalsMap,
-}
-
-// Helper functions for emitting code and metadata to the internal
-// structures of the compiler.
-impl Compiler {
-    fn context(&self) -> &LambdaCtx {
-        &self.contexts[self.contexts.len() - 1]
-    }
-
-    fn context_mut(&mut self) -> &mut LambdaCtx {
-        let idx = self.contexts.len() - 1;
-        &mut self.contexts[idx]
-    }
-
-    fn chunk(&mut self) -> &mut Chunk {
-        Rc::<Chunk>::get_mut(self.context_mut().lambda.chunk())
-            .expect("compiler flaw: long-lived chunk reference")
-    }
-
-    fn scope(&self) -> &Scope {
-        &self.context().scope
-    }
-
-    fn scope_mut(&mut self) -> &mut Scope {
-        &mut self.context_mut().scope
-    }
-
-    fn emit_constant(&mut self, value: Value) {
-        let idx = self.chunk().push_constant(value);
-        self.chunk().push_op(OpCode::OpConstant(idx));
-    }
-}
-
-// Actual code-emitting AST traversal methods.
-impl Compiler {
-    fn compile(&mut self, expr: ast::Expr) {
-        match expr {
-            ast::Expr::Literal(literal) => self.compile_literal(literal),
-            ast::Expr::Path(path) => self.compile_path(path),
-            ast::Expr::Str(s) => self.compile_str(s),
-            ast::Expr::UnaryOp(op) => self.compile_unary_op(op),
-            ast::Expr::BinOp(op) => self.compile_binop(op),
-            ast::Expr::HasAttr(has_attr) => self.compile_has_attr(has_attr),
-            ast::Expr::List(list) => self.compile_list(list),
-            ast::Expr::AttrSet(attrs) => self.compile_attr_set(attrs),
-            ast::Expr::Select(select) => self.compile_select(select),
-            ast::Expr::Assert(assert) => self.compile_assert(assert),
-            ast::Expr::IfElse(if_else) => self.compile_if_else(if_else),
-            ast::Expr::LetIn(let_in) => self.compile_let_in(let_in),
-            ast::Expr::Ident(ident) => self.compile_ident(ident),
-            ast::Expr::With(with) => self.compile_with(with),
-            ast::Expr::Lambda(lambda) => self.compile_lambda(lambda),
-            ast::Expr::Apply(apply) => self.compile_apply(apply),
-
-            // Parenthesized expressions are simply unwrapped, leaving
-            // their value on the stack.
-            ast::Expr::Paren(paren) => self.compile(paren.expr().unwrap()),
-
-            ast::Expr::LegacyLet(_) => todo!("legacy let"),
-
-            ast::Expr::Root(_) => unreachable!("there cannot be more than one root"),
-            ast::Expr::Error(_) => unreachable!("compile is only called on validated trees"),
-        }
-    }
-
-    fn compile_literal(&mut self, node: ast::Literal) {
-        match node.kind() {
-            ast::LiteralKind::Float(f) => {
-                self.emit_constant(Value::Float(f.value().unwrap()));
-            }
-
-            ast::LiteralKind::Integer(i) => {
-                self.emit_constant(Value::Integer(i.value().unwrap()));
-            }
-            ast::LiteralKind::Uri(u) => {
-                self.emit_warning(node.syntax().clone(), WarningKind::DeprecatedLiteralURL);
-                self.emit_constant(Value::String(u.syntax().text().into()));
-            }
-        }
-    }
-
-    fn compile_path(&mut self, node: ast::Path) {
-        // TODO(tazjin): placeholder implementation while waiting for
-        // https://github.com/nix-community/rnix-parser/pull/96
-
-        let raw_path = node.to_string();
-        let path = if raw_path.starts_with('/') {
-            Path::new(&raw_path).to_owned()
-        } else if raw_path.starts_with('~') {
-            let mut buf = match dirs::home_dir() {
-                Some(buf) => buf,
-                None => {
-                    self.emit_error(
-                        node.syntax().clone(),
-                        ErrorKind::PathResolution("failed to determine home directory".into()),
-                    );
-                    return;
-                }
-            };
-
-            buf.push(&raw_path);
-            buf
-        } else if raw_path.starts_with('.') {
-            let mut buf = self.root_dir.clone();
-            buf.push(&raw_path);
-            buf
-        } else {
-            // TODO: decide what to do with findFile
-            todo!("other path types (e.g. <...> lookups) not yet implemented")
-        };
-
-        // TODO: Use https://github.com/rust-lang/rfcs/issues/2208
-        // once it is available
-        let value = Value::Path(path.clean());
-        self.emit_constant(value);
-    }
-
-    fn compile_str(&mut self, node: ast::Str) {
-        let mut count = 0;
-
-        // The string parts are produced in literal order, however
-        // they need to be reversed on the stack in order to
-        // efficiently create the real string in case of
-        // interpolation.
-        for part in node.normalized_parts().into_iter().rev() {
-            count += 1;
-
-            match part {
-                // Interpolated expressions are compiled as normal and
-                // dealt with by the VM before being assembled into
-                // the final string.
-                ast::InterpolPart::Interpolation(node) => self.compile(node.expr().unwrap()),
-
-                ast::InterpolPart::Literal(lit) => {
-                    self.emit_constant(Value::String(lit.into()));
-                }
-            }
-        }
-
-        if count != 1 {
-            self.chunk().push_op(OpCode::OpInterpolate(count));
-        }
-    }
-
-    fn compile_unary_op(&mut self, op: ast::UnaryOp) {
-        self.compile(op.expr().unwrap());
-
-        let opcode = match op.operator().unwrap() {
-            ast::UnaryOpKind::Invert => OpCode::OpInvert,
-            ast::UnaryOpKind::Negate => OpCode::OpNegate,
-        };
-
-        self.chunk().push_op(opcode);
-    }
-
-    fn compile_binop(&mut self, op: ast::BinOp) {
-        use ast::BinOpKind;
-
-        // Short-circuiting and other strange operators, which are
-        // under the same node type as NODE_BIN_OP, but need to be
-        // handled separately (i.e. before compiling the expressions
-        // used for standard binary operators).
-
-        match op.operator().unwrap() {
-            BinOpKind::And => return self.compile_and(op),
-            BinOpKind::Or => return self.compile_or(op),
-            BinOpKind::Implication => return self.compile_implication(op),
-            _ => {}
-        };
-
-        // For all other operators, the two values need to be left on
-        // the stack in the correct order before pushing the
-        // instruction for the operation itself.
-        self.compile(op.lhs().unwrap());
-        self.compile(op.rhs().unwrap());
-
-        match op.operator().unwrap() {
-            BinOpKind::Add => self.chunk().push_op(OpCode::OpAdd),
-            BinOpKind::Sub => self.chunk().push_op(OpCode::OpSub),
-            BinOpKind::Mul => self.chunk().push_op(OpCode::OpMul),
-            BinOpKind::Div => self.chunk().push_op(OpCode::OpDiv),
-            BinOpKind::Update => self.chunk().push_op(OpCode::OpAttrsUpdate),
-            BinOpKind::Equal => self.chunk().push_op(OpCode::OpEqual),
-            BinOpKind::Less => self.chunk().push_op(OpCode::OpLess),
-            BinOpKind::LessOrEq => self.chunk().push_op(OpCode::OpLessOrEq),
-            BinOpKind::More => self.chunk().push_op(OpCode::OpMore),
-            BinOpKind::MoreOrEq => self.chunk().push_op(OpCode::OpMoreOrEq),
-            BinOpKind::Concat => self.chunk().push_op(OpCode::OpConcat),
-
-            BinOpKind::NotEqual => {
-                self.chunk().push_op(OpCode::OpEqual);
-                self.chunk().push_op(OpCode::OpInvert)
-            }
-
-            // Handled by separate branch above.
-            BinOpKind::And | BinOpKind::Implication | BinOpKind::Or => {
-                unreachable!()
-            }
-        };
-    }
-
-    fn compile_and(&mut self, node: ast::BinOp) {
-        debug_assert!(
-            matches!(node.operator(), Some(ast::BinOpKind::And)),
-            "compile_and called with wrong operator kind: {:?}",
-            node.operator(),
-        );
-
-        // Leave left-hand side value on the stack.
-        self.compile(node.lhs().unwrap());
-
-        // If this value is false, jump over the right-hand side - the
-        // whole expression is false.
-        let end_idx = self.chunk().push_op(OpCode::OpJumpIfFalse(0));
-
-        // Otherwise, remove the previous value and leave the
-        // right-hand side on the stack. Its result is now the value
-        // of the whole expression.
-        self.chunk().push_op(OpCode::OpPop);
-        self.compile(node.rhs().unwrap());
-
-        self.patch_jump(end_idx);
-        self.chunk().push_op(OpCode::OpAssertBool);
-    }
-
-    fn compile_or(&mut self, node: ast::BinOp) {
-        debug_assert!(
-            matches!(node.operator(), Some(ast::BinOpKind::Or)),
-            "compile_or called with wrong operator kind: {:?}",
-            node.operator(),
-        );
-
-        // Leave left-hand side value on the stack
-        self.compile(node.lhs().unwrap());
-
-        // Opposite of above: If this value is **true**, we can
-        // short-circuit the right-hand side.
-        let end_idx = self.chunk().push_op(OpCode::OpJumpIfTrue(0));
-        self.chunk().push_op(OpCode::OpPop);
-        self.compile(node.rhs().unwrap());
-        self.patch_jump(end_idx);
-        self.chunk().push_op(OpCode::OpAssertBool);
-    }
-
-    fn compile_implication(&mut self, node: ast::BinOp) {
-        debug_assert!(
-            matches!(node.operator(), Some(ast::BinOpKind::Implication)),
-            "compile_implication called with wrong operator kind: {:?}",
-            node.operator(),
-        );
-
-        // Leave left-hand side value on the stack and invert it.
-        self.compile(node.lhs().unwrap());
-        self.chunk().push_op(OpCode::OpInvert);
-
-        // Exactly as `||` (because `a -> b` = `!a || b`).
-        let end_idx = self.chunk().push_op(OpCode::OpJumpIfTrue(0));
-        self.chunk().push_op(OpCode::OpPop);
-        self.compile(node.rhs().unwrap());
-        self.patch_jump(end_idx);
-        self.chunk().push_op(OpCode::OpAssertBool);
-    }
-
-    fn compile_has_attr(&mut self, node: ast::HasAttr) {
-        // Put the attribute set on the stack.
-        self.compile(node.expr().unwrap());
-        let mut count = 0;
-
-        // Push all path fragments with an operation for fetching the
-        // next nested element, for all fragments except the last one.
-        for fragment in node.attrpath().unwrap().attrs() {
-            if count > 0 {
-                self.chunk().push_op(OpCode::OpAttrOrNotFound);
-            }
-            count += 1;
-            self.compile_attr(fragment);
-        }
-
-        // After the last fragment, emit the actual instruction that
-        // leaves a boolean on the stack.
-        self.chunk().push_op(OpCode::OpAttrsIsSet);
-    }
-
-    fn compile_attr(&mut self, node: ast::Attr) {
-        match node {
-            ast::Attr::Dynamic(dynamic) => self.compile(dynamic.expr().unwrap()),
-            ast::Attr::Str(s) => self.compile_str(s),
-            ast::Attr::Ident(ident) => self.emit_literal_ident(&ident),
-        }
-    }
-
-    // Compile list literals into equivalent bytecode. List
-    // construction is fairly simple, consisting of pushing code for
-    // each literal element and an instruction with the element count.
-    //
-    // The VM, after evaluating the code for each element, simply
-    // constructs the list from the given number of elements.
-    fn compile_list(&mut self, node: ast::List) {
-        let mut count = 0;
-
-        for item in node.items() {
-            count += 1;
-            self.compile(item);
-        }
-
-        self.chunk().push_op(OpCode::OpList(count));
-    }
-
-    // Compile attribute set literals into equivalent bytecode.
-    //
-    // This is complicated by a number of features specific to Nix
-    // attribute sets, most importantly:
-    //
-    // 1. Keys can be dynamically constructed through interpolation.
-    // 2. Keys can refer to nested attribute sets.
-    // 3. Attribute sets can (optionally) be recursive.
-    fn compile_attr_set(&mut self, node: ast::AttrSet) {
-        if node.rec_token().is_some() {
-            todo!("recursive attribute sets are not yet implemented")
-        }
-
-        let mut count = 0;
-
-        // Inherits have to be evaluated before entering the scope of
-        // a potentially recursive attribute sets (i.e. we always
-        // inherit "from the outside").
-        for inherit in node.inherits() {
-            match inherit.from() {
-                Some(from) => {
-                    for ident in inherit.idents() {
-                        count += 1;
-
-                        // First emit the identifier itself
-                        self.emit_literal_ident(&ident);
-
-                        // Then emit the node that we're inheriting
-                        // from.
-                        //
-                        // TODO: Likely significant optimisation
-                        // potential in having a multi-select
-                        // instruction followed by a merge, rather
-                        // than pushing/popping the same attrs
-                        // potentially a lot of times.
-                        self.compile(from.expr().unwrap());
-                        self.emit_literal_ident(&ident);
-                        self.chunk().push_op(OpCode::OpAttrsSelect);
-                    }
-                }
-
-                None => {
-                    for ident in inherit.idents() {
-                        count += 1;
-                        self.emit_literal_ident(&ident);
-
-                        match self.resolve_local(ident.ident_token().unwrap().text()) {
-                            Some(idx) => self.chunk().push_op(OpCode::OpGetLocal(idx)),
-                            None => {
-                                self.emit_error(
-                                    ident.syntax().clone(),
-                                    ErrorKind::UnknownStaticVariable,
-                                );
-                                continue;
-                            }
-                        };
-                    }
-                }
-            }
-        }
-
-        for kv in node.attrpath_values() {
-            count += 1;
-
-            // Because attribute set literals can contain nested keys,
-            // there is potentially more than one key fragment. If
-            // this is the case, a special operation to construct a
-            // runtime value representing the attribute path is
-            // emitted.
-            let mut key_count = 0;
-            for fragment in kv.attrpath().unwrap().attrs() {
-                key_count += 1;
-                self.compile_attr(fragment);
-            }
-
-            // We're done with the key if there was only one fragment,
-            // otherwise we need to emit an instruction to construct
-            // the attribute path.
-            if key_count > 1 {
-                self.chunk().push_op(OpCode::OpAttrPath(key_count));
-            }
-
-            // The value is just compiled as normal so that its
-            // resulting value is on the stack when the attribute set
-            // is constructed at runtime.
-            self.compile(kv.value().unwrap());
-        }
-
-        self.chunk().push_op(OpCode::OpAttrs(count));
-    }
-
-    fn compile_select(&mut self, node: ast::Select) {
-        let set = node.expr().unwrap();
-        let path = node.attrpath().unwrap();
-
-        if node.or_token().is_some() {
-            self.compile_select_or(set, path, node.default_expr().unwrap());
-            return;
-        }
-
-        // Push the set onto the stack
-        self.compile(set);
-
-        // Compile each key fragment and emit access instructions.
-        //
-        // TODO: multi-select instruction to avoid re-pushing attrs on
-        // nested selects.
-        for fragment in path.attrs() {
-            self.compile_attr(fragment);
-            self.chunk().push_op(OpCode::OpAttrsSelect);
-        }
-    }
-
-    /// Compile an `or` expression into a chunk of conditional jumps.
-    ///
-    /// If at any point during attribute set traversal a key is
-    /// missing, the `OpAttrOrNotFound` instruction will leave a
-    /// special sentinel value on the stack.
-    ///
-    /// After each access, a conditional jump evaluates the top of the
-    /// stack and short-circuits to the default value if it sees the
-    /// sentinel.
-    ///
-    /// Code like `{ a.b = 1; }.a.c or 42` yields this bytecode and
-    /// runtime stack:
-    ///
-    /// ```notrust
-    ///            Bytecode                     Runtime stack
-    ///  ┌────────────────────────────┐   ┌─────────────────────────┐
-    ///  │    ...                     │   │ ...                     │
-    ///  │ 5  OP_ATTRS(1)             │ → │ 5  [ { a.b = 1; }     ] │
-    ///  │ 6  OP_CONSTANT("a")        │ → │ 6  [ { a.b = 1; } "a" ] │
-    ///  │ 7  OP_ATTR_OR_NOT_FOUND    │ → │ 7  [ { b = 1; }       ] │
-    ///  │ 8  JUMP_IF_NOT_FOUND(13)   │ → │ 8  [ { b = 1; }       ] │
-    ///  │ 9  OP_CONSTANT("C")        │ → │ 9  [ { b = 1; } "c"   ] │
-    ///  │ 10 OP_ATTR_OR_NOT_FOUND    │ → │ 10 [ NOT_FOUND        ] │
-    ///  │ 11 JUMP_IF_NOT_FOUND(13)   │ → │ 11 [                  ] │
-    ///  │ 12 JUMP(14)                │   │ ..     jumped over      │
-    ///  │ 13 CONSTANT(42)            │ → │ 12 [ 42 ]               │
-    ///  │ 14 ...                     │   │ ..   ....               │
-    ///  └────────────────────────────┘   └─────────────────────────┘
-    /// ```
-    fn compile_select_or(&mut self, set: ast::Expr, path: ast::Attrpath, default: ast::Expr) {
-        self.compile(set);
-        let mut jumps = vec![];
-
-        for fragment in path.attrs() {
-            self.compile_attr(fragment);
-            self.chunk().push_op(OpCode::OpAttrOrNotFound);
-            jumps.push(self.chunk().push_op(OpCode::OpJumpIfNotFound(0)));
-        }
-
-        let final_jump = self.chunk().push_op(OpCode::OpJump(0));
-
-        for jump in jumps {
-            self.patch_jump(jump);
-        }
-
-        // Compile the default value expression and patch the final
-        // jump to point *beyond* it.
-        self.compile(default);
-        self.patch_jump(final_jump);
-    }
-
-    fn compile_assert(&mut self, node: ast::Assert) {
-        // Compile the assertion condition to leave its value on the stack.
-        self.compile(node.condition().unwrap());
-        self.chunk().push_op(OpCode::OpAssert);
-
-        // The runtime will abort evaluation at this point if the
-        // assertion failed, if not the body simply continues on like
-        // normal.
-        self.compile(node.body().unwrap());
-    }
-
-    // Compile conditional expressions using jumping instructions in the VM.
-    //
-    //                        ┌────────────────────┐
-    //                        │ 0  [ conditional ] │
-    //                        │ 1   JUMP_IF_FALSE →┼─┐
-    //                        │ 2  [  main body  ] │ │ Jump to else body if
-    //                       ┌┼─3─←     JUMP       │ │ condition is false.
-    //  Jump over else body  ││ 4  [  else body  ]←┼─┘
-    //  if condition is true.└┼─5─→     ...        │
-    //                        └────────────────────┘
-    fn compile_if_else(&mut self, node: ast::IfElse) {
-        self.compile(node.condition().unwrap());
-
-        let then_idx = self.chunk().push_op(OpCode::OpJumpIfFalse(0));
-
-        self.chunk().push_op(OpCode::OpPop); // discard condition value
-        self.compile(node.body().unwrap());
-
-        let else_idx = self.chunk().push_op(OpCode::OpJump(0));
-
-        self.patch_jump(then_idx); // patch jump *to* else_body
-        self.chunk().push_op(OpCode::OpPop); // discard condition value
-        self.compile(node.else_body().unwrap());
-
-        self.patch_jump(else_idx); // patch jump *over* else body
-    }
-
-    // Compile an `inherit` node of a `let`-expression.
-    fn compile_let_inherit<I: Iterator<Item = ast::Inherit>>(&mut self, inherits: I) {
-        for inherit in inherits {
-            match inherit.from() {
-                // Within a `let` binding, inheriting from the outer
-                // scope is a no-op *if* the identifier can be
-                // statically resolved.
-                None if self.scope().with_stack.is_empty() => {
-                    self.emit_warning(inherit.syntax().clone(), WarningKind::UselessInherit);
-                    continue;
-                }
-
-                None => {
-                    for ident in inherit.idents() {
-                        // If the identifier resolves statically, it
-                        // has precedence over dynamic bindings, and
-                        // the inherit is useless.
-                        if self
-                            .resolve_local(ident.ident_token().unwrap().text())
-                            .is_some()
-                        {
-                            self.emit_warning(ident.syntax().clone(), WarningKind::UselessInherit);
-                            continue;
-                        }
-
-                        self.compile_ident(ident.clone());
-                        self.declare_local(
-                            ident.syntax().clone(),
-                            ident.ident_token().unwrap().text(),
-                        );
-                    }
-                }
-
-                Some(from) => {
-                    for ident in inherit.idents() {
-                        self.compile(from.expr().unwrap());
-                        self.emit_literal_ident(&ident);
-                        self.chunk().push_op(OpCode::OpAttrsSelect);
-                        self.declare_local(
-                            ident.syntax().clone(),
-                            ident.ident_token().unwrap().text(),
-                        );
-                    }
-                }
-            }
-        }
-    }
-
-    // Compile a standard `let ...; in ...` statement.
-    //
-    // Unless in a non-standard scope, the encountered values are
-    // simply pushed on the stack and their indices noted in the
-    // entries vector.
-    fn compile_let_in(&mut self, node: ast::LetIn) {
-        self.begin_scope();
-
-        self.compile_let_inherit(node.inherits());
-
-        for entry in node.attrpath_values() {
-            let mut path = match normalise_ident_path(entry.attrpath().unwrap().attrs()) {
-                Ok(p) => p,
-                Err(err) => {
-                    self.errors.push(err);
-                    continue;
-                }
-            };
-
-            if path.len() != 1 {
-                todo!("nested bindings in let expressions :(")
-            }
-
-            self.compile(entry.value().unwrap());
-            self.declare_local(
-                entry.attrpath().unwrap().syntax().clone(),
-                path.pop().unwrap(),
-            );
-        }
-
-        // Deal with the body, then clean up the locals afterwards.
-        self.compile(node.body().unwrap());
-        self.end_scope();
-    }
-
-    fn compile_ident(&mut self, node: ast::Ident) {
-        let ident = node.ident_token().unwrap();
-
-        // If the identifier is a global, and it is not poisoned, emit
-        // the global directly.
-        if let Some(global) = self.globals.get(ident.text()) {
-            if !self.scope().is_poisoned(ident.text()) {
-                global.clone()(self);
-                return;
-            }
-        }
-
-        match self.resolve_local(ident.text()) {
-            Some(idx) => self.chunk().push_op(OpCode::OpGetLocal(idx)),
-            None => {
-                if self.scope().with_stack.is_empty() {
-                    self.emit_error(node.syntax().clone(), ErrorKind::UnknownStaticVariable);
-                    return;
-                }
-
-                // Variable needs to be dynamically resolved
-                // at runtime.
-                self.emit_constant(Value::String(ident.text().into()));
-                self.chunk().push_op(OpCode::OpResolveWith)
-            }
-        };
-    }
-
-    // Compile `with` expressions by emitting instructions that
-    // pop/remove the indices of attribute sets that are implicitly in
-    // scope through `with` on the "with-stack".
-    fn compile_with(&mut self, node: ast::With) {
-        // TODO: Detect if the namespace is just an identifier, and
-        // resolve that directly (thus avoiding duplication on the
-        // stack).
-        self.compile(node.namespace().unwrap());
-
-        self.declare_phantom();
-        let depth = self.scope().scope_depth;
-        self.scope_mut().with_stack.push(With { depth });
-
-        let with_idx = self.scope().locals.len() - 1;
-        self.chunk().push_op(OpCode::OpPushWith(with_idx));
-
-        self.compile(node.body().unwrap());
-    }
-
-    fn compile_lambda(&mut self, node: ast::Lambda) {
-        // Open new lambda context in compiler, which has its own
-        // scope etc.
-        self.contexts.push(LambdaCtx::new());
-        self.begin_scope();
-
-        // Compile the function itself
-        match node.param().unwrap() {
-            ast::Param::Pattern(_) => todo!("formals function definitions"),
-            ast::Param::IdentParam(param) => {
-                let name = param
-                    .ident()
-                    .unwrap()
-                    .ident_token()
-                    .unwrap()
-                    .text()
-                    .to_string();
-
-                self.declare_local(param.syntax().clone(), name);
-            }
-        }
-
-        self.compile(node.body().unwrap());
-        self.end_scope();
-
-        // TODO: determine and insert enclosing name, if available.
-
-        // Pop the lambda context back off, and emit the finished
-        // lambda as a constant.
-        let compiled = self.contexts.pop().unwrap();
-
-        #[cfg(feature = "disassembler")]
-        {
-            crate::disassembler::disassemble_chunk(&compiled.lambda.chunk);
-        }
-
-        self.emit_constant(Value::Lambda(compiled.lambda));
-    }
-
-    fn compile_apply(&mut self, node: ast::Apply) {
-        // To call a function, we leave its arguments on the stack,
-        // followed by the function expression itself, and then emit a
-        // call instruction. This way, the stack is perfectly laid out
-        // to enter the function call straight away.
-        self.compile(node.argument().unwrap());
-        self.compile(node.lambda().unwrap());
-        self.chunk().push_op(OpCode::OpCall);
-    }
-
-    /// Emit the literal string value of an identifier. Required for
-    /// several operations related to attribute sets, where
-    /// identifiers are used as string keys.
-    fn emit_literal_ident(&mut self, ident: &ast::Ident) {
-        self.emit_constant(Value::String(ident.ident_token().unwrap().text().into()));
-    }
-
-    fn patch_jump(&mut self, idx: CodeIdx) {
-        let offset = self.chunk().code.len() - 1 - idx.0;
-
-        match &mut self.chunk().code[idx.0] {
-            OpCode::OpJump(n)
-            | OpCode::OpJumpIfFalse(n)
-            | OpCode::OpJumpIfTrue(n)
-            | OpCode::OpJumpIfNotFound(n) => {
-                *n = offset;
-            }
-
-            op => panic!("attempted to patch unsupported op: {:?}", op),
-        }
-    }
-
-    fn begin_scope(&mut self) {
-        self.scope_mut().scope_depth += 1;
-    }
-
-    fn end_scope(&mut self) {
-        debug_assert!(self.scope().scope_depth != 0, "can not end top scope");
-
-        // If this scope poisoned any builtins or special identifiers,
-        // they need to be reset.
-        let depth = self.scope().scope_depth;
-        self.scope_mut().unpoison(depth);
-
-        self.scope_mut().scope_depth -= 1;
-
-        // When ending a scope, all corresponding locals need to be
-        // removed, but the value of the body needs to remain on the
-        // stack. This is implemented by a separate instruction.
-        let mut pops = 0;
-
-        // TL;DR - iterate from the back while things belonging to the
-        // ended scope still exist.
-        while !self.scope().locals.is_empty()
-            && self.scope().locals[self.scope().locals.len() - 1].depth > self.scope().scope_depth
-        {
-            pops += 1;
-
-            // While removing the local, analyse whether it has been
-            // accessed while it existed and emit a warning to the
-            // user otherwise.
-            if let Some(Local {
-                node: Some(node),
-                used,
-                ..
-            }) = self.scope_mut().locals.pop()
-            {
-                if !used {
-                    self.emit_warning(node, WarningKind::UnusedBinding);
-                }
-            }
-        }
-
-        if pops > 0 {
-            self.chunk().push_op(OpCode::OpCloseScope(pops));
-        }
-
-        while !self.scope().with_stack.is_empty()
-            && self.scope().with_stack[self.scope().with_stack.len() - 1].depth
-                > self.scope().scope_depth
-        {
-            self.chunk().push_op(OpCode::OpPopWith);
-            self.scope_mut().with_stack.pop();
-        }
-    }
-
-    /// Declare a local variable known in the scope that is being
-    /// compiled by pushing it to the locals. This is used to
-    /// determine the stack offset of variables.
-    fn declare_local<S: Into<String>>(&mut self, node: rnix::SyntaxNode, name: S) {
-        let name = name.into();
-        let depth = self.scope().scope_depth;
-
-        // Do this little dance to get ahold of the *static* key and
-        // use it for poisoning if required.
-        let key: Option<&'static str> = match self.globals.get_key_value(name.as_str()) {
-            Some((key, _)) => Some(*key),
-            None => None,
-        };
-
-        if let Some(global_ident) = key {
-            self.emit_warning(node.clone(), WarningKind::ShadowedGlobal(global_ident));
-            self.scope_mut().poison(global_ident, depth);
-        }
-
-        self.scope_mut().locals.push(Local {
-            depth,
-            name: name.into(),
-            node: Some(node),
-            phantom: false,
-            used: false,
-        });
-    }
-
-    fn declare_phantom(&mut self) {
-        let depth = self.scope().scope_depth;
-        self.scope_mut().locals.push(Local {
-            depth,
-            name: "".into(),
-            node: None,
-            phantom: true,
-            used: true,
-        });
-    }
-
-    fn resolve_local(&mut self, name: &str) -> Option<usize> {
-        let scope = self.scope_mut();
-
-        for (idx, local) in scope.locals.iter_mut().enumerate().rev() {
-            if !local.phantom && local.name == name {
-                local.used = true;
-                return Some(idx);
-            }
-        }
-
-        None
-    }
-
-    fn emit_warning(&mut self, node: rnix::SyntaxNode, kind: WarningKind) {
-        self.warnings.push(EvalWarning { node, kind })
-    }
-
-    fn emit_error(&mut self, node: rnix::SyntaxNode, kind: ErrorKind) {
-        self.errors.push(Error {
-            node: Some(node),
-            kind,
-        })
-    }
-}
-
-/// Convert a non-dynamic string expression to a string if possible,
-/// or raise an error.
-fn expr_str_to_string(expr: ast::Str) -> EvalResult<String> {
-    if expr.normalized_parts().len() == 1 {
-        if let ast::InterpolPart::Literal(s) = expr.normalized_parts().pop().unwrap() {
-            return Ok(s);
-        }
-    }
-
-    return Err(Error {
-        node: Some(expr.syntax().clone()),
-        kind: ErrorKind::DynamicKeyInLet(expr.syntax().clone()),
-    });
-}
-
-/// Convert a single identifier path fragment to a string if possible,
-/// or raise an error about the node being dynamic.
-fn attr_to_string(node: ast::Attr) -> EvalResult<String> {
-    match node {
-        ast::Attr::Ident(ident) => Ok(ident.ident_token().unwrap().text().into()),
-        ast::Attr::Str(s) => expr_str_to_string(s),
-
-        // The dynamic node type is just a wrapper. C++ Nix does not
-        // care about the dynamic wrapper when determining whether the
-        // node itself is dynamic, it depends solely on the expression
-        // inside (i.e. `let ${"a"} = 1; in a` is valid).
-        ast::Attr::Dynamic(ref dynamic) => match dynamic.expr().unwrap() {
-            ast::Expr::Str(s) => expr_str_to_string(s),
-            _ => Err(ErrorKind::DynamicKeyInLet(node.syntax().clone()).into()),
-        },
-    }
-}
-
-// Normalises identifier fragments into a single string vector for
-// `let`-expressions; fails if fragments requiring dynamic computation
-// are encountered.
-fn normalise_ident_path<I: Iterator<Item = ast::Attr>>(path: I) -> EvalResult<Vec<String>> {
-    path.map(attr_to_string).collect()
-}
-
-/// Prepare the full set of globals from additional globals supplied
-/// by the caller of the compiler, as well as the built-in globals
-/// that are always part of the language.
-///
-/// Note that all builtin functions are *not* considered part of the
-/// language in this sense and MUST be supplied as additional global
-/// values, including the `builtins` set itself.
-fn prepare_globals(additional: HashMap<&'static str, Value>) -> GlobalsMap {
-    let mut globals: GlobalsMap = HashMap::new();
-
-    globals.insert(
-        "true",
-        Rc::new(|compiler| {
-            compiler.chunk().push_op(OpCode::OpTrue);
-        }),
-    );
-
-    globals.insert(
-        "false",
-        Rc::new(|compiler| {
-            compiler.chunk().push_op(OpCode::OpFalse);
-        }),
-    );
-
-    globals.insert(
-        "null",
-        Rc::new(|compiler| {
-            compiler.chunk().push_op(OpCode::OpNull);
-        }),
-    );
-
-    for (ident, value) in additional.into_iter() {
-        globals.insert(
-            ident,
-            Rc::new(move |compiler| compiler.emit_constant(value.clone())),
-        );
-    }
-
-    globals
-}
-
-pub fn compile(
-    expr: ast::Expr,
-    location: Option<PathBuf>,
-    globals: HashMap<&'static str, Value>,
-) -> EvalResult<CompilationResult> {
-    let mut root_dir = match location {
-        Some(dir) => Ok(dir),
-        None => std::env::current_dir().map_err(|e| {
-            ErrorKind::PathResolution(format!("could not determine current directory: {}", e))
-        }),
-    }?;
-
-    // If the path passed from the caller points to a file, the
-    // filename itself needs to be truncated as this must point to a
-    // directory.
-    if root_dir.is_file() {
-        root_dir.pop();
-    }
-
-    let mut c = Compiler {
-        root_dir,
-        globals: prepare_globals(globals),
-        contexts: vec![LambdaCtx::new()],
-        warnings: vec![],
-        errors: vec![],
-    };
-
-    c.compile(expr);
-
-    Ok(CompilationResult {
-        lambda: c.contexts.pop().unwrap().lambda,
-        warnings: c.warnings,
-        errors: c.errors,
-    })
-}