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-rw-r--r--tvix/eval/src/compiler/mod.rs96
1 files changed, 51 insertions, 45 deletions
diff --git a/tvix/eval/src/compiler/mod.rs b/tvix/eval/src/compiler/mod.rs
index 2985c7e90e0b..34a15330c227 100644
--- a/tvix/eval/src/compiler/mod.rs
+++ b/tvix/eval/src/compiler/mod.rs
@@ -19,10 +19,9 @@ mod scope;
 use codemap::Span;
 use rnix::ast::{self, AstToken};
 use smol_str::SmolStr;
-use std::cell::RefCell;
 use std::collections::HashMap;
 use std::path::{Path, PathBuf};
-use std::rc::Rc;
+use std::rc::{Rc, Weak};
 use std::sync::Arc;
 
 use crate::chunk::Chunk;
@@ -42,6 +41,10 @@ pub struct CompilationOutput {
     pub lambda: Rc<Lambda>,
     pub warnings: Vec<EvalWarning>,
     pub errors: Vec<Error>,
+
+    // This field must outlive the rc::Weak reference which breaks
+    // the builtins -> import -> builtins reference cycle.
+    pub globals: Rc<GlobalsMap>,
 }
 
 /// Represents the lambda currently being compiled.
@@ -69,11 +72,19 @@ impl LambdaCtx {
     }
 }
 
-/// Alias for the map of globally available functions that should
-/// implicitly be resolvable in the global scope.
-type GlobalsMap = HashMap<&'static str, Rc<dyn Fn(&mut Compiler, Span)>>;
+/// The map of globally available functions that should implicitly
+/// be resolvable in the global scope.
+pub type GlobalsMap = HashMap<&'static str, Rc<dyn Fn(&mut Compiler, Span)>>;
+
+/// Functions with this type are used to construct a
+/// self-referential `builtins` object; it takes a weak reference to
+/// its own result, similar to how nixpkgs' overlays work.
+/// Rc::new_cyclic() is what "ties the knot".  The heap allocation
+/// (Box) and vtable (dyn) do not impair runtime or compile-time
+/// performance; they exist only during compiler startup.
+pub type GlobalsMapFunc = Box<dyn FnOnce(&Weak<GlobalsMap>) -> GlobalsMap>;
 
-struct Compiler<'observer> {
+pub struct Compiler<'observer> {
     contexts: Vec<LambdaCtx>,
     warnings: Vec<EvalWarning>,
     errors: Vec<Error>,
@@ -85,7 +96,7 @@ struct Compiler<'observer> {
     /// 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,
+    globals: Rc<GlobalsMap>,
 
     /// File reference in the codemap contains all known source code
     /// and is used to track the spans from which instructions where
@@ -108,7 +119,7 @@ impl<'observer> Compiler<'observer> {
     pub(crate) fn new(
         location: Option<PathBuf>,
         file: Arc<codemap::File>,
-        globals: Rc<RefCell<HashMap<&'static str, Value>>>,
+        globals: Rc<GlobalsMap>,
         observer: &'observer mut dyn CompilerObserver,
     ) -> EvalResult<Self> {
         let mut root_dir = match location {
@@ -136,8 +147,6 @@ impl<'observer> Compiler<'observer> {
             root_dir.pop();
         }
 
-        let globals = globals.borrow();
-
         #[cfg(not(target_arch = "wasm32"))]
         debug_assert!(root_dir.is_absolute());
 
@@ -145,7 +154,7 @@ impl<'observer> Compiler<'observer> {
             root_dir,
             file,
             observer,
-            globals: prepare_globals(&globals),
+            globals,
             contexts: vec![LambdaCtx::new()],
             warnings: vec![],
             errors: vec![],
@@ -186,7 +195,7 @@ impl Compiler<'_> {
 
     /// Emit a single constant to the current bytecode chunk and track
     /// the source span from which it was compiled.
-    fn emit_constant<T: ToSpan>(&mut self, value: Value, node: &T) {
+    pub(super) fn emit_constant<T: ToSpan>(&mut self, value: Value, node: &T) {
         let idx = self.chunk().push_constant(value);
         self.push_op(OpCode::OpConstant(idx), node);
     }
@@ -1174,54 +1183,50 @@ fn optimise_tail_call(chunk: &mut Chunk) {
 
 /// 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.
+/// that are always part of the language.  This also "ties the knot"
+/// required in order for import to have a reference cycle back to
+/// the globals.
 ///
 /// 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, span| {
-            compiler.push_op(OpCode::OpTrue, &span);
-        }),
-    );
-
-    globals.insert(
-        "false",
-        Rc::new(|compiler, span| {
-            compiler.push_op(OpCode::OpFalse, &span);
-        }),
-    );
-
-    globals.insert(
-        "null",
-        Rc::new(|compiler, span| {
-            compiler.push_op(OpCode::OpNull, &span);
-        }),
-    );
-
-    for (ident, value) in additional.iter() {
-        let value: Value = value.clone();
+pub fn prepare_globals(additional: GlobalsMapFunc) -> Rc<GlobalsMap> {
+    Rc::new_cyclic(Box::new(|weak: &Weak<GlobalsMap>| {
+        let mut globals = additional(weak);
+
         globals.insert(
-            ident,
-            Rc::new(move |compiler, span| compiler.emit_constant(value.clone(), &span)),
+            "true",
+            Rc::new(|compiler, span| {
+                compiler.push_op(OpCode::OpTrue, &span);
+            }),
+        );
+
+        globals.insert(
+            "false",
+            Rc::new(|compiler, span| {
+                compiler.push_op(OpCode::OpFalse, &span);
+            }),
+        );
+
+        globals.insert(
+            "null",
+            Rc::new(|compiler, span| {
+                compiler.push_op(OpCode::OpNull, &span);
+            }),
         );
-    }
 
-    globals
+        globals
+    }))
 }
 
 pub fn compile(
     expr: &ast::Expr,
     location: Option<PathBuf>,
     file: Arc<codemap::File>,
-    globals: Rc<RefCell<HashMap<&'static str, Value>>>,
+    globals: Rc<GlobalsMap>,
     observer: &mut dyn CompilerObserver,
 ) -> EvalResult<CompilationOutput> {
-    let mut c = Compiler::new(location, file, globals, observer)?;
+    let mut c = Compiler::new(location, file, globals.clone(), observer)?;
 
     let root_span = c.span_for(expr);
     let root_slot = c.scope_mut().declare_phantom(root_span, false);
@@ -1240,5 +1245,6 @@ pub fn compile(
         lambda,
         warnings: c.warnings,
         errors: c.errors,
+        globals: globals,
     })
 }