ugh, variance...

src/twopass.rs

@@ -1,4 +1,4 @@
-use std::collections::HashMap;
+use std::{cell::RefCell, collections::HashMap};
 
 use crate::ast;
 
@@ -53,27 +53,62 @@ struct Call {
 	args: Vec<Expression>,
 }
 
-#[derive(Debug)]
-struct BuildContext<'a> {
-	parent: Option<&'a mut BuildContext<'a>>,
+#[derive(Debug, Clone, Default)]
+struct Scope {
 	upvalues: Vec<(ast::Ident, Ident)>,
 	scope: HashMap<ast::Ident, Ident>,
 }
 
+impl Scope {
+	fn new_toplevel() -> Self {
+		Self {
+			upvalues: vec![("_ENV".to_string(), Ident::Local(0))],
+			scope: [("_ENV".to_string(), Ident::Upvalue(0))]
+				.into_iter()
+				.collect(),
+		}
+	}
+}
+
+#[derive(Debug)]
+struct BuildContext<'a> {
+	parent: Option<&'a BuildContext<'a>>, // mustn't be &mut because of variance
+	scope: RefCell<Scope>,
+}
+
 impl BuildContext<'_> {
-	fn request(&mut self, name: &ast::Ident) -> Option<Ident> {
-		if let Some(ident) = self.scope.get(name) {
+	fn request(&self, name: &ast::Ident) -> Option<Ident> {
+		let mut scope = self.scope.borrow_mut();
+		if let Some(ident) = scope.scope.get(name) {
 			return Some(*ident);
 		}
-		if let Some(ident) = self.parent.as_mut()?.request(name) {
-			let index = self.upvalues.len();
-			self.upvalues.push((name.clone(), ident));
-			return Some(Ident::Upvalue(index));
+		if let Some(ident) = self.parent.as_ref()?.request(name) {
+			let index = scope.upvalues.len();
+			scope.upvalues.push((name.clone(), ident));
+			let ident = Ident::Upvalue(index);
+			scope.scope.insert(name.clone(), ident);
+			return Some(ident);
 		}
 		None
 	}
 }
 
-pub fn build(code: ast::Function) -> Function {
+fn build_in_scope(parent: &BuildContext<'_>, code: ast::Function) -> Function {
+	let scope = BuildContext {
+		parent: Some(parent),
+		scope: Default::default(),
+	};
+	if false {
+		build_in_scope(&scope, code.clone());
+		build_in_scope(&scope, code.clone());
+	}
 	todo!()
 }
+
+pub fn build(code: ast::Function) -> Function {
+	let mut root = BuildContext {
+		parent: None,
+		scope: RefCell::new(Scope::new_toplevel()),
+	};
+	build_in_scope(&mut root, code)
+}