svelte/src/compiler/phases/scope.js

Cybernetically enhanced web apps

/** @import { ArrowFunctionExpression, BinaryOperator, ClassDeclaration, Expression, FunctionDeclaration, FunctionExpression, Identifier, ImportDeclaration, MemberExpression, LogicalOperator, Node, Pattern, UnaryOperator, VariableDeclarator, Super } from 'estree' */
/** @import { Context, Visitor } from 'zimmerframe' */
/** @import { AST, BindingKind, DeclarationKind } from '#compiler' */
import is_reference from 'is-reference';
import { walk } from 'zimmerframe';
import { create_expression_metadata } from './nodes.js';
import * as b from '#compiler/builders';
import * as e from '../errors.js';
import {
	extract_identifiers,
	extract_identifiers_from_destructuring,
	object,
	unwrap_pattern
} from '../utils/ast.js';
import { is_reserved, is_rune } from '../../utils.js';
import { determine_slot } from '../utils/slot.js';
import { validate_identifier_name } from './2-analyze/visitors/shared/utils.js';

const UNKNOWN = Symbol('unknown');
/** Includes `BigInt` */
export const NUMBER = Symbol('number');
export const STRING = Symbol('string');

/** @type {Record<string, [type: NUMBER | STRING | UNKNOWN, fn?: Function]>} */
const globals = {
	BigInt: [NUMBER, BigInt],
	'Math.min': [NUMBER, Math.min],
	'Math.max': [NUMBER, Math.max],
	'Math.random': [NUMBER],
	'Math.floor': [NUMBER, Math.floor],
	// @ts-expect-error
	'Math.f16round': [NUMBER, Math.f16round],
	'Math.round': [NUMBER, Math.round],
	'Math.abs': [NUMBER, Math.abs],
	'Math.acos': [NUMBER, Math.acos],
	'Math.asin': [NUMBER, Math.asin],
	'Math.atan': [NUMBER, Math.atan],
	'Math.atan2': [NUMBER, Math.atan2],
	'Math.ceil': [NUMBER, Math.ceil],
	'Math.cos': [NUMBER, Math.cos],
	'Math.sin': [NUMBER, Math.sin],
	'Math.tan': [NUMBER, Math.tan],
	'Math.exp': [NUMBER, Math.exp],
	'Math.log': [NUMBER, Math.log],
	'Math.pow': [NUMBER, Math.pow],
	'Math.sqrt': [NUMBER, Math.sqrt],
	'Math.clz32': [NUMBER, Math.clz32],
	'Math.imul': [NUMBER, Math.imul],
	'Math.sign': [NUMBER, Math.sign],
	'Math.log10': [NUMBER, Math.log10],
	'Math.log2': [NUMBER, Math.log2],
	'Math.log1p': [NUMBER, Math.log1p],
	'Math.expm1': [NUMBER, Math.expm1],
	'Math.cosh': [NUMBER, Math.cosh],
	'Math.sinh': [NUMBER, Math.sinh],
	'Math.tanh': [NUMBER, Math.tanh],
	'Math.acosh': [NUMBER, Math.acosh],
	'Math.asinh': [NUMBER, Math.asinh],
	'Math.atanh': [NUMBER, Math.atanh],
	'Math.trunc': [NUMBER, Math.trunc],
	'Math.fround': [NUMBER, Math.fround],
	'Math.cbrt': [NUMBER, Math.cbrt],
	Number: [NUMBER, Number],
	'Number.isInteger': [NUMBER, Number.isInteger],
	'Number.isFinite': [NUMBER, Number.isFinite],
	'Number.isNaN': [NUMBER, Number.isNaN],
	'Number.isSafeInteger': [NUMBER, Number.isSafeInteger],
	'Number.parseFloat': [NUMBER, Number.parseFloat],
	'Number.parseInt': [NUMBER, Number.parseInt],
	String: [STRING, String],
	'String.fromCharCode': [STRING, String.fromCharCode],
	'String.fromCodePoint': [STRING, String.fromCodePoint]
};

/** @type {Record<string, any>} */
const global_constants = {
	'Math.PI': Math.PI,
	'Math.E': Math.E,
	'Math.LN10': Math.LN10,
	'Math.LN2': Math.LN2,
	'Math.LOG10E': Math.LOG10E,
	'Math.LOG2E': Math.LOG2E,
	'Math.SQRT2': Math.SQRT2,
	'Math.SQRT1_2': Math.SQRT1_2
};

export class Binding {
	/** @type {Scope} */
	scope;

	/** @type {Identifier} */
	node;

	/** @type {BindingKind} */
	kind;

	/** @type {DeclarationKind} */
	declaration_kind;

	/**
	 * What the value was initialized with.
	 * For destructured props such as `let { foo = 'bar' } = $props()` this is `'bar'` and not `$props()`
	 * @type {null | Expression | FunctionDeclaration | ClassDeclaration | ImportDeclaration | AST.EachBlock | AST.SnippetBlock}
	 */
	initial = null;

	/** @type {Array<{ node: Identifier; path: AST.SvelteNode[] }>} */
	references = [];

	/**
	 * For `legacy_reactive`: its reactive dependencies
	 * @type {Binding[]}
	 */
	legacy_dependencies = [];

	/**
	 * Legacy props: the `class` in `{ export klass as class}`. $props(): The `class` in { class: klass } = $props()
	 * @type {string | null}
	 */
	prop_alias = null;

	/**
	 * Additional metadata, varies per binding type
	 * @type {null | { inside_rest?: boolean }}
	 */
	metadata = null;

	mutated = false;
	reassigned = false;

	/**
	 *
	 * @param {Scope} scope
	 * @param {Identifier} node
	 * @param {BindingKind} kind
	 * @param {DeclarationKind} declaration_kind
	 * @param {Binding['initial']} initial
	 */
	constructor(scope, node, kind, declaration_kind, initial) {
		this.scope = scope;
		this.node = node;
		this.initial = initial;
		this.kind = kind;
		this.declaration_kind = declaration_kind;
	}

	get updated() {
		return this.mutated || this.reassigned;
	}

	/**
	 * @returns {this is Binding & { initial: ArrowFunctionExpression | FunctionDeclaration | FunctionExpression }}
	 */
	is_function() {
		if (this.updated) {
			// even if it's reassigned to another function,
			// we can't use it directly as e.g. an event handler
			return false;
		}

		const type = this.initial?.type;

		return (
			type === 'ArrowFunctionExpression' ||
			type === 'FunctionExpression' ||
			type === 'FunctionDeclaration'
		);
	}
}

class Evaluation {
	/** @type {Set<any>} */
	values;

	/**
	 * True if there is exactly one possible value
	 * @readonly
	 * @type {boolean}
	 */
	is_known = true;

	/**
	 * True if the value is known to not be null/undefined
	 * @readonly
	 * @type {boolean}
	 */
	is_defined = true;

	/**
	 * True if the value is known to be a string
	 * @readonly
	 * @type {boolean}
	 */
	is_string = true;

	/**
	 * True if the value is known to be a number
	 * @readonly
	 * @type {boolean}
	 */
	is_number = true;

	/**
	 * @readonly
	 * @type {any}
	 */
	value = undefined;

	/**
	 *
	 * @param {Scope} scope
	 * @param {Expression} expression
	 * @param {Set<any>} values
	 */
	constructor(scope, expression, values) {
		this.values = values;

		switch (expression.type) {
			case 'Literal': {
				this.values.add(expression.value);
				break;
			}

			case 'Identifier': {
				const binding = scope.get(expression.name);

				if (binding) {
					if (
						binding.initial?.type === 'CallExpression' &&
						get_rune(binding.initial, scope) === '$props.id'
					) {
						this.values.add(STRING);
						break;
					}

					const is_prop =
						binding.kind === 'prop' ||
						binding.kind === 'rest_prop' ||
						binding.kind === 'bindable_prop';

					if (binding.initial?.type === 'EachBlock' && binding.initial.index === expression.name) {
						this.values.add(NUMBER);
						break;
					}

					if (!binding.updated && binding.initial !== null && !is_prop) {
						binding.scope.evaluate(/** @type {Expression} */ (binding.initial), this.values);
						break;
					}
				} else if (expression.name === 'undefined') {
					this.values.add(undefined);
					break;
				}

				// TODO glean what we can from reassignments
				// TODO one day, expose props and imports somehow

				this.values.add(UNKNOWN);
				break;
			}

			case 'BinaryExpression': {
				const a = scope.evaluate(/** @type {Expression} */ (expression.left)); // `left` cannot be `PrivateIdentifier` unless operator is `in`
				const b = scope.evaluate(expression.right);

				if (a.is_known && b.is_known) {
					this.values.add(binary[expression.operator](a.value, b.value));
					break;
				}

				switch (expression.operator) {
					case '!=':
					case '!==':
					case '<':
					case '<=':
					case '>':
					case '>=':
					case '==':
					case '===':
					case 'in':
					case 'instanceof':
						this.values.add(true);
						this.values.add(false);
						break;

					case '%':
					case '&':
					case '*':
					case '**':
					case '-':
					case '/':
					case '<<':
					case '>>':
					case '>>>':
					case '^':
					case '|':
						this.values.add(NUMBER);
						break;

					case '+':
						if (a.is_string || b.is_string) {
							this.values.add(STRING);
						} else if (a.is_number && b.is_number) {
							this.values.add(NUMBER);
						} else {
							this.values.add(STRING);
							this.values.add(NUMBER);
						}
						break;

					default:
						this.values.add(UNKNOWN);
				}
				break;
			}

			case 'ConditionalExpression': {
				const test = scope.evaluate(expression.test);
				const consequent = scope.evaluate(expression.consequent);
				const alternate = scope.evaluate(expression.alternate);

				if (test.is_known) {
					for (const value of (test.value ? consequent : alternate).values) {
						this.values.add(value);
					}
				} else {
					for (const value of consequent.values) {
						this.values.add(value);
					}

					for (const value of alternate.values) {
						this.values.add(value);
					}
				}
				break;
			}

			case 'LogicalExpression': {
				const a = scope.evaluate(expression.left);
				const b = scope.evaluate(expression.right);

				if (a.is_known) {
					if (b.is_known) {
						this.values.add(logical[expression.operator](a.value, b.value));
						break;
					}

					if (
						(expression.operator === '&&' && !a.value) ||
						(expression.operator === '||' && a.value) ||
						(expression.operator === '??' && a.value != null)
					) {
						this.values.add(a.value);
					} else {
						for (const value of b.values) {
							this.values.add(value);
						}
					}

					break;
				}

				for (const value of a.values) {
					this.values.add(value);
				}

				for (const value of b.values) {
					this.values.add(value);
				}
				break;
			}

			case 'UnaryExpression': {
				const argument = scope.evaluate(expression.argument);

				if (argument.is_known) {
					this.values.add(unary[expression.operator](argument.value));
					break;
				}

				switch (expression.operator) {
					case '!':
					case 'delete':
						this.values.add(false);
						this.values.add(true);
						break;

					case '+':
					case '-':
					case '~':
						this.values.add(NUMBER);
						break;

					case 'typeof':
						this.values.add(STRING);
						break;

					case 'void':
						this.values.add(undefined);
						break;

					default:
						this.values.add(UNKNOWN);
				}
				break;
			}

			case 'CallExpression': {
				const keypath = get_global_keypath(expression.callee, scope);

				if (keypath) {
					if (is_rune(keypath)) {
						const arg = /** @type {Expression | undefined} */ (expression.arguments[0]);

						switch (keypath) {
							case '$state':
							case '$state.raw':
							case '$derived':
								if (arg) {
									scope.evaluate(arg, this.values);
								} else {
									this.values.add(undefined);
								}
								break;

							case '$props.id':
								this.values.add(STRING);
								break;

							case '$effect.tracking':
								this.values.add(false);
								this.values.add(true);
								break;

							case '$derived.by':
								if (arg?.type === 'ArrowFunctionExpression' && arg.body.type !== 'BlockStatement') {
									scope.evaluate(arg.body, this.values);
									break;
								}

								this.values.add(UNKNOWN);
								break;

							default: {
								this.values.add(UNKNOWN);
							}
						}

						break;
					}

					if (
						Object.hasOwn(globals, keypath) &&
						expression.arguments.every((arg) => arg.type !== 'SpreadElement')
					) {
						const [type, fn] = globals[keypath];
						const values = expression.arguments.map((arg) => scope.evaluate(arg));

						if (fn && values.every((e) => e.is_known)) {
							this.values.add(fn(...values.map((e) => e.value)));
						} else {
							this.values.add(type);
						}

						break;
					}
				}

				this.values.add(UNKNOWN);
				break;
			}

			case 'TemplateLiteral': {
				let result = expression.quasis[0].value.cooked;

				for (let i = 0; i < expression.expressions.length; i += 1) {
					const e = scope.evaluate(expression.expressions[i]);

					if (e.is_known) {
						result += e.value + expression.quasis[i + 1].value.cooked;
					} else {
						this.values.add(STRING);
						break;
					}
				}

				this.values.add(result);
				break;
			}

			case 'MemberExpression': {
				const keypath = get_global_keypath(expression, scope);

				if (keypath && Object.hasOwn(global_constants, keypath)) {
					this.values.add(global_constants[keypath]);
					break;
				}

				this.values.add(UNKNOWN);
				break;
			}

			default: {
				this.values.add(UNKNOWN);
			}
		}

		for (const value of this.values) {
			this.value = value; // saves having special logic for `size === 1`

			if (value !== STRING && typeof value !== 'string') {
				this.is_string = false;
			}

			if (value !== NUMBER && typeof value !== 'number') {
				this.is_number = false;
			}

			if (value == null || value === UNKNOWN) {
				this.is_defined = false;
			}
		}

		if (this.values.size > 1 || typeof this.value === 'symbol') {
			this.is_known = false;
		}
	}
}

export class Scope {
	/** @type {ScopeRoot} */
	root;

	/**
	 * The immediate parent scope
	 * @type {Scope | null}
	 */
	parent;

	/**
	 * Whether or not `var` declarations are contained by this scope
	 * @type {boolean}
	 */
	#porous;

	/**
	 * A map of every identifier declared by this scope, and all the
	 * identifiers that reference it
	 * @type {Map<string, Binding>}
	 */
	declarations = new Map();

	/**
	 * A map of declarators to the bindings they declare
	 * @type {Map<VariableDeclarator | AST.LetDirective, Binding[]>}
	 */
	declarators = new Map();

	/**
	 * A set of all the names referenced with this scope
	 * — useful for generating unique names
	 * @type {Map<string, { node: Identifier; path: AST.SvelteNode[] }[]>}
	 */
	references = new Map();

	/**
	 * The scope depth allows us to determine if a state variable is referenced in its own scope,
	 * which is usually an error. Block statements do not increase this value
	 */
	function_depth = 0;

	/**
	 * If tracing of reactive dependencies is enabled for this scope
	 * @type {null | Expression}
	 */
	tracing = null;

	/**
	 *
	 * @param {ScopeRoot} root
	 * @param {Scope | null} parent
	 * @param {boolean} porous
	 */
	constructor(root, parent, porous) {
		this.root = root;
		this.parent = parent;
		this.#porous = porous;
		this.function_depth = parent ? parent.function_depth + (porous ? 0 : 1) : 0;
	}

	/**
	 * @param {Identifier} node
	 * @param {Binding['kind']} kind
	 * @param {DeclarationKind} declaration_kind
	 * @param {null | Expression | FunctionDeclaration | ClassDeclaration | ImportDeclaration | AST.EachBlock | AST.SnippetBlock} initial
	 * @returns {Binding}
	 */
	declare(node, kind, declaration_kind, initial = null) {
		if (this.parent) {
			if (declaration_kind === 'var' && this.#porous) {
				return this.parent.declare(node, kind, declaration_kind);
			}

			if (declaration_kind === 'import') {
				return this.parent.declare(node, kind, declaration_kind, initial);
			}
		}

		if (this.declarations.has(node.name)) {
			const binding = this.declarations.get(node.name);
			if (binding && binding.declaration_kind !== 'var' && declaration_kind !== 'var') {
				// This also errors on function types, but that's arguably a good thing
				// declaring function twice is also caught by acorn in the parse phase
				e.declaration_duplicate(node, node.name);
			}
		}

		const binding = new Binding(this, node, kind, declaration_kind, initial);

		validate_identifier_name(binding, this.function_depth);

		this.declarations.set(node.name, binding);
		this.root.conflicts.add(node.name);
		return binding;
	}

	child(porous = false) {
		return new Scope(this.root, this, porous);
	}

	/**
	 * @param {string} preferred_name
	 * @returns {string}
	 */
	generate(preferred_name) {
		if (this.#porous) {
			return /** @type {Scope} */ (this.parent).generate(preferred_name);
		}

		preferred_name = preferred_name.replace(/[^a-zA-Z0-9_$]/g, '_').replace(/^[0-9]/, '_');
		let name = preferred_name;
		let n = 1;

		while (
			this.references.has(name) ||
			this.declarations.has(name) ||
			this.root.conflicts.has(name) ||
			is_reserved(name)
		) {
			name = `${preferred_name}_${n++}`;
		}

		this.references.set(name, []);
		this.root.conflicts.add(name);
		return name;
	}

	/**
	 * @param {string} name
	 * @returns {Binding | null}
	 */
	get(name) {
		return this.declarations.get(name) ?? this.parent?.get(name) ?? null;
	}

	/**
	 * @param {VariableDeclarator | AST.LetDirective} node
	 * @returns {Binding[]}
	 */
	get_bindings(node) {
		const bindings = this.declarators.get(node);
		if (!bindings) {
			throw new Error('No binding found for declarator');
		}
		return bindings;
	}

	/**
	 * @param {string} name
	 * @returns {Scope | null}
	 */
	owner(name) {
		return this.declarations.has(name) ? this : this.parent && this.parent.owner(name);
	}

	/**
	 * @param {Identifier} node
	 * @param {AST.SvelteNode[]} path
	 */
	reference(node, path) {
		path = [...path]; // ensure that mutations to path afterwards don't affect this reference
		let references = this.references.get(node.name);

		if (!references) this.references.set(node.name, (references = []));

		references.push({ node, path });

		const binding = this.declarations.get(node.name);
		if (binding) {
			binding.references.push({ node, path });
		} else if (this.parent) {
			this.parent.reference(node, path);
		} else {
			// no binding was found, and this is the top level scope,
			// which means this is a global
			this.root.conflicts.add(node.name);
		}
	}

	/**
	 * Does partial evaluation to find an exact value or at least the rough type of the expression.
	 * Only call this once scope has been fully generated in a first pass,
	 * else this evaluates on incomplete data and may yield wrong results.
	 * @param {Expression} expression
	 * @param {Set<any>} [values]
	 */
	evaluate(expression, values = new Set()) {
		return new Evaluation(this, expression, values);
	}
}

/** @type {Record<BinaryOperator, (left: any, right: any) => any>} */
const binary = {
	'!=': (left, right) => left != right,
	'!==': (left, right) => left !== right,
	'<': (left, right) => left < right,
	'<=': (left, right) => left <= right,
	'>': (left, right) => left > right,
	'>=': (left, right) => left >= right,
	'==': (left, right) => left == right,
	'===': (left, right) => left === right,
	in: (left, right) => left in right,
	instanceof: (left, right) => left instanceof right,
	'%': (left, right) => left % right,
	'&': (left, right) => left & right,
	'*': (left, right) => left * right,
	'**': (left, right) => left ** right,
	'+': (left, right) => left + right,
	'-': (left, right) => left - right,
	'/': (left, right) => left / right,
	'<<': (left, right) => left << right,
	'>>': (left, right) => left >> right,
	'>>>': (left, right) => left >>> right,
	'^': (left, right) => left ^ right,
	'|': (left, right) => left | right
};

/** @type {Record<UnaryOperator, (argument: any) => any>} */
const unary = {
	'-': (argument) => -argument,
	'+': (argument) => +argument,
	'!': (argument) => !argument,
	'~': (argument) => ~argument,
	typeof: (argument) => typeof argument,
	void: () => undefined,
	delete: () => true
};

/** @type {Record<LogicalOperator, (left: any, right: any) => any>} */
const logical = {
	'||': (left, right) => left || right,
	'&&': (left, right) => left && right,
	'??': (left, right) => left ?? right
};

export class ScopeRoot {
	/** @type {Set<string>} */
	conflicts = new Set();

	/**
	 * @param {string} preferred_name
	 */
	unique(preferred_name) {
		preferred_name = preferred_name.replace(/[^a-zA-Z0-9_$]/g, '_');
		let final_name = preferred_name;
		let n = 1;

		while (this.conflicts.has(final_name)) {
			final_name = `${preferred_name}_${n++}`;
		}

		this.conflicts.add(final_name);
		const id = b.id(final_name);
		return id;
	}
}

/**
 * @param {AST.SvelteNode} ast
 * @param {ScopeRoot} root
 * @param {boolean} allow_reactive_declarations
 * @param {Scope | null} parent
 */
export function create_scopes(ast, root, allow_reactive_declarations, parent) {
	/** @typedef {{ scope: Scope }} State */

	/**
	 * A map of node->associated scope. A node appearing in this map does not necessarily mean that it created a scope
	 * @type {Map<AST.SvelteNode, Scope>}
	 */
	const scopes = new Map();
	const scope = new Scope(root, parent, false);
	scopes.set(ast, scope);

	/** @type {State} */
	const state = { scope };

	/** @type {[Scope, { node: Identifier; path: AST.SvelteNode[] }][]} */
	const references = [];

	/** @type {[Scope, Pattern | MemberExpression][]} */
	const updates = [];

	/**
	 * An array of reactive declarations, i.e. the `a` in `$: a = b * 2`
	 * @type {Identifier[]}
	 */
	const possible_implicit_declarations = [];

	/**
	 * @param {Scope} scope
	 * @param {Pattern[]} params
	 */
	function add_params(scope, params) {
		for (const param of params) {
			for (const node of extract_identifiers(param)) {
				scope.declare(node, 'normal', param.type === 'RestElement' ? 'rest_param' : 'param');
			}
		}
	}

	/**
	 * @type {Visitor<Node, State, AST.SvelteNode>}
	 */
	const create_block_scope = (node, { state, next }) => {
		const scope = state.scope.child(true);
		scopes.set(node, scope);

		next({ scope });
	};

	/**
	 * @type {Visitor<AST.ElementLike, State, AST.SvelteNode>}
	 */
	const SvelteFragment = (node, { state, next }) => {
		const scope = state.scope.child();
		scopes.set(node, scope);
		next({ scope });
	};

	/**
	 * @type {Visitor<AST.Component | AST.SvelteComponent | AST.SvelteSelf, State, AST.SvelteNode>}
	 */
	const Component = (node, context) => {
		node.metadata.scopes = {
			default: context.state.scope.child()
		};

		if (node.type === 'SvelteComponent') {
			context.visit(node.expression);
		}

		const default_state = determine_slot(node)
			? context.state
			: { scope: node.metadata.scopes.default };

		for (const attribute of node.attributes) {
			if (attribute.type === 'LetDirective') {
				context.visit(attribute, default_state);
			} else {
				context.visit(attribute);
			}
		}

		for (const child of node.fragment.nodes) {
			let state = default_state;

			const slot_name = determine_slot(child);

			if (slot_name !== null) {
				node.metadata.scopes[slot_name] = context.state.scope.child();

				state = {
					scope: node.metadata.scopes[slot_name]
				};
			}

			context.visit(child, state);
		}
	};

	/**
	 * @type {Visitor<AST.AnimateDirective | AST.TransitionDirective | AST.UseDirective, State, AST.SvelteNode>}
	 */
	const SvelteDirective = (node, { state, path, visit }) => {
		state.scope.reference(b.id(node.name.split('.')[0]), path);

		if (node.expression) {
			visit(node.expression);
		}
	};

	walk(ast, state, {
		// references
		Identifier(node, { path, state }) {
			const parent = path.at(-1);
			if (
				parent &&
				is_reference(node, /** @type {Node} */ (parent)) &&
				// TSTypeAnnotation, TSInterfaceDeclaration etc - these are normally already filtered out,
				// but for the migration they aren't, so we need to filter them out here
				// TODO -> once migration script is gone we can remove this check
				!parent.type.startsWith('TS')
			) {
				references.push([state.scope, { node, path: path.slice() }]);
			}
		},
		LabeledStatement(node, { path, next }) {
			if (path.length > 1 || !allow_reactive_declarations) return next();
			if (node.label.name !== '$') return next();

			// create a scope for the $: block
			const scope = state.scope.child();
			scopes.set(node, scope);

			if (
				node.body.type === 'ExpressionStatement' &&
				node.body.expression.type === 'AssignmentExpression'
			) {
				for (const id of extract_identifiers(node.body.expression.left)) {
					if (!id.name.startsWith('$')) {
						possible_implicit_declarations.push(id);
					}
				}
			}

			next({ scope });
		},

		SvelteFragment,
		SlotElement: SvelteFragment,
		SvelteElement: SvelteFragment,
		RegularElement: SvelteFragment,

		LetDirective(node, context) {
			const scope = context.state.scope;

			/** @type {Binding[]} */
			const bindings = [];
			scope.declarators.set(node, bindings);

			if (node.expression) {
				for (const id of extract_identifiers_from_destructuring(node.expression)) {
					const binding = scope.declare(id, 'template', 'const');
					scope.reference(id, [context.path[context.path.length - 1], node]);
					bindings.push(binding);
				}
			} else {
				/** @type {Identifier} */
				const id = {
					name: node.name,
					type: 'Identifier',
					start: node.start,
					end: node.end
				};
				const binding = scope.declare(id, 'template', 'const');
				scope.reference(id, [context.path[context.path.length - 1], node]);
				bindings.push(binding);
			}
		},

		Component: (node, context) => {
			context.state.scope.reference(b.id(node.name), context.path);
			Component(node, context);
		},
		SvelteSelf: Component,
		SvelteComponent: Component,

		// updates
		AssignmentExpression(node, { state, next }) {
			updates.push([state.scope, node.left]);
			next();
		},

		UpdateExpression(node, { state, next }) {
			updates.push([state.scope, /** @type {Identifier | MemberExpression} */ (node.argument)]);
			next();
		},

		ImportDeclaration(node, { state }) {
			for (const specifier of node.specifiers) {
				state.scope.declare(specifier.local, 'normal', 'import', node);
			}
		},

		FunctionExpression(node, { state, next }) {
			const scope = state.scope.child();
			scopes.set(node, scope);

			if (node.id) scope.declare(node.id, 'normal', 'function');

			add_params(scope, node.params);
			next({ scope });
		},

		FunctionDeclaration(node, { state, next }) {
			if (node.id) state.scope.declare(node.id, 'normal', 'function', node);

			const scope = state.scope.child();
			scopes.set(node, scope);

			add_params(scope, node.params);
			next({ scope });
		},

		ArrowFunctionExpression(node, { state, next }) {
			const scope = state.scope.child();
			scopes.set(node, scope);

			add_params(scope, node.params);
			next({ scope });
		},

		ForStatement: create_block_scope,
		ForInStatement: create_block_scope,
		ForOfStatement: create_block_scope,
		SwitchStatement: create_block_scope,
		BlockStatement(node, context) {
			const parent = context.path.at(-1);
			if (
				parent?.type === 'FunctionDeclaration' ||
				parent?.type === 'FunctionExpression' ||
				parent?.type === 'ArrowFunctionExpression'
			) {
				// We already created a new scope for the function
				context.next();
			} else {
				create_block_scope(node, context);
			}
		},

		ClassDeclaration(node, { state, next }) {
			if (node.id) state.scope.declare(node.id, 'normal', 'let', node);
			next();
		},

		VariableDeclaration(node, { state, path, next }) {
			const is_parent_const_tag = path.at(-1)?.type === 'ConstTag';
			for (const declarator of node.declarations) {
				/** @type {Binding[]} */
				const bindings = [];

				state.scope.declarators.set(declarator, bindings);

				for (const id of extract_identifiers(declarator.id)) {
					const binding = state.scope.declare(
						id,
						is_parent_const_tag ? 'template' : 'normal',
						node.kind,
						declarator.init
					);
					bindings.push(binding);
				}
			}

			next();
		},

		CatchClause(node, { state, next }) {
			if (node.param) {
				const scope = state.scope.child(true);
				scopes.set(node, scope);

				for (const id of extract_identifiers(node.param)) {
					scope.declare(id, 'normal', 'let');
				}

				next({ scope });
			} else {
				next();
			}
		},

		EachBlock(node, { state, visit }) {
			visit(node.expression);

			// context and children are a new scope
			const scope = state.scope.child();
			scopes.set(node, scope);

			if (node.context) {
				// declarations
				for (const id of extract_identifiers(node.context)) {
					const binding = scope.declare(id, 'each', 'const');

					let inside_rest = false;
					let is_rest_id = false;
					walk(node.context, null, {
						Identifier(node) {
							if (inside_rest && node === id) {
								is_rest_id = true;
							}
						},
						RestElement(_, { next }) {
							const prev = inside_rest;
							inside_rest = true;
							next();
							inside_rest = prev;
						}
					});

					binding.metadata = { inside_rest: is_rest_id };
				}

				// Visit to pick up references from default initializers
				visit(node.context, { scope });
			}

			if (node.index) {
				const is_keyed =
					node.key &&
					(node.key.type !== 'Identifier' || !node.index || node.key.name !== node.index);
				scope.declare(b.id(node.index), is_keyed ? 'template' : 'normal', 'const', node);
			}
			if (node.key) visit(node.key, { scope });

			// children
			for (const child of node.body.nodes) {
				visit(child, { scope });
			}
			if (node.fallback) visit(node.fallback, { scope });

			node.metadata = {
				expression: create_expression_metadata(),
				keyed: false,
				contains_group_binding: false,
				index: scope.root.unique('$$index'),
				declarations: scope.declarations,
				is_controlled: false
			};
		},

		AwaitBlock(node, context) {
			context.visit(node.expression);

			if (node.pending) {
				context.visit(node.pending);
			}

			if (node.then) {
				context.visit(node.then);
				if (node.value) {
					const then_scope = /** @type {Scope} */ (scopes.get(node.then));
					const value_scope = context.state.scope.child();
					scopes.set(node.value, value_scope);
					context.visit(node.value, { scope: value_scope });
					for (const id of extract_identifiers(node.value)) {
						then_scope.declare(id, 'template', 'const');
						value_scope.declare(id, 'normal', 'const');
					}
				}
			}

			if (node.catch) {
				context.visit(node.catch);
				if (node.error) {
					const catch_scope = /** @type {Scope} */ (scopes.get(node.catch));
					const error_scope = context.state.scope.child();
					scopes.set(node.error, error_scope);
					context.visit(node.error, { scope: error_scope });
					for (const id of extract_identifiers(node.error)) {
						catch_scope.declare(id, 'template', 'const');
						error_scope.declare(id, 'normal', 'const');
					}
				}
			}
		},

		SnippetBlock(node, context) {
			const state = context.state;
			let scope = state.scope;

			scope.declare(node.expression, 'normal', 'function', node);

			const child_scope = state.scope.child();
			scopes.set(node, child_scope);

			for (const param of node.parameters) {
				for (const id of extract_identifiers(param)) {
					child_scope.declare(id, 'snippet', 'let');
				}
			}

			context.next({ scope: child_scope });
		},

		Fragment: (node, context) => {
			const scope = context.state.scope.child(node.metadata.transparent);
			scopes.set(node, scope);
			context.next({ scope });
		},

		BindDirective(node, context) {
			updates.push([
				context.state.scope,
				/** @type {Identifier | MemberExpression} */ (node.expression)
			]);
			context.next();
		},

		TransitionDirective: SvelteDirective,
		AnimateDirective: SvelteDirective,
		UseDirective: SvelteDirective,
		// using it's own function instead of `SvelteDirective` because
		// StyleDirective doesn't have expressions and are generally already
		// handled by `Identifier`. This is the special case for the shorthand
		// eg <button style:height /> where the variable has the same name of
		// the css property
		StyleDirective(node, { path, state, next }) {
			if (node.value === true) {
				state.scope.reference(b.id(node.name), path.concat(node));
			}
			next();
		}

		// TODO others
	});

	for (const id of possible_implicit_declarations) {
		const binding = scope.get(id.name);
		if (binding) continue; // TODO can also be legacy_reactive if declared outside of reactive statement

		scope.declare(id, 'legacy_reactive', 'let');
	}

	// we do this after the fact, so that we don't need to worry
	// about encountering references before their declarations
	for (const [scope, { node, path }] of references) {
		scope.reference(node, path);
	}

	for (const [scope, node] of updates) {
		for (const expression of unwrap_pattern(node)) {
			const left = object(expression);
			const binding = left && scope.get(left.name);

			if (binding !== null && left !== binding.node) {
				if (left === expression) {
					binding.reassigned = true;
				} else {
					binding.mutated = true;
				}
			}
		}
	}

	return {
		scope,
		scopes
	};
}

/**
 * @template {{ scope: Scope, scopes: Map<AST.SvelteNode, Scope> }} State
 * @param {AST.SvelteNode} node
 * @param {Context<AST.SvelteNode, State>} context
 */
export function set_scope(node, { next, state }) {
	const scope = state.scopes.get(node);
	next(scope !== undefined && scope !== state.scope ? { ...state, scope } : state);
}

/**
 * Returns the name of the rune if the given expression is a `CallExpression` using a rune.
 * @param {Node | null | undefined} node
 * @param {Scope} scope
 */
export function get_rune(node, scope) {
	if (!node) return null;
	if (node.type !== 'CallExpression') return null;

	const keypath = get_global_keypath(node.callee, scope);

	if (!keypath || !is_rune(keypath)) return null;
	return keypath;
}

/**
 * Returns the name of the rune if the given expression is a `CallExpression` using a rune.
 * @param {Expression | Super} node
 * @param {Scope} scope
 */
function get_global_keypath(node, scope) {
	let n = node;

	let joined = '';

	while (n.type === 'MemberExpression') {
		if (n.computed) return null;
		if (n.property.type !== 'Identifier') return null;
		joined = '.' + n.property.name + joined;
		n = n.object;
	}

	if (n.type === 'CallExpression' && n.callee.type === 'Identifier') {
		joined = '()' + joined;
		n = n.callee;
	}

	if (n.type !== 'Identifier') return null;

	const binding = scope.get(n.name);
	if (binding !== null) return null; // rune name, but references a variable or store

	return n.name + joined;
}