refactor(tvix/eval): introduce UpvalueCarrier trait

This trait abstracts over the commonalities of upvalue handling
between closures and thunks.

It allows the VM to simplify the code used for setting up upvalues,
without duplicating between the two different types.

Note that this does not yet refactor the VM code to optimally make use
of this.

Change-Id: If8de5181f26ae1fa00d554f1ae6ea473ee4b6070
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6347
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/lib.rs

@@ -4,6 +4,7 @@ mod compiler;
 mod errors;
 mod eval;
 mod opcode;
+mod upvalues;
 mod value;
 mod vm;
 mod warnings;

tvix/eval/src/upvalues.rs

@@ -0,0 +1,39 @@
+//! This module encapsulates some logic for upvalue handling, which is
+//! relevant to both thunks (delayed computations for lazy-evaluation)
+//! as well as closures (lambdas that capture variables from the
+//! surrounding scope).
+
+use std::cell::{Ref, RefMut};
+
+use crate::{opcode::UpvalueIdx, Value};
+
+/// `UpvalueCarrier` is implemented by all types that carry upvalues.
+pub trait UpvalueCarrier {
+    fn upvalue_count(&self) -> usize;
+
+    /// Read-only accessor for the stored upvalues.
+    fn upvalues(&self) -> Ref<'_, [Value]>;
+
+    /// Mutable accessor for stored upvalues.
+    fn upvalues_mut(&self) -> RefMut<'_, Vec<Value>>;
+
+    /// Read an upvalue at the given index.
+    fn upvalue(&self, idx: UpvalueIdx) -> Ref<'_, Value> {
+        Ref::map(self.upvalues(), |v| &v[idx.0])
+    }
+
+    /// Push an upvalue at the end of the upvalue list.
+    fn push_upvalue(&self, value: Value) {
+        self.upvalues_mut().push(value);
+    }
+
+    /// Resolve deferred upvalues from the provided stack slice,
+    /// mutating them in the internal upvalue slots.
+    fn resolve_deferred_upvalues(&self, stack: &[Value]) {
+        for upvalue in self.upvalues_mut().iter_mut() {
+            if let Value::DeferredUpvalue(idx) = upvalue {
+                *upvalue = stack[idx.0].clone();
+            }
+        }
+    }
+}

tvix/eval/src/value/function.rs

@@ -1,10 +1,10 @@
 //! This module implements the runtime representation of functions.
 use std::{
-    cell::{Ref, RefCell},
+    cell::{Ref, RefCell, RefMut},
     rc::Rc,
 };
 
-use crate::{chunk::Chunk, opcode::UpvalueIdx, Value};
+use crate::{chunk::Chunk, upvalues::UpvalueCarrier, Value};
 
 #[derive(Clone, Debug)]
 pub struct Lambda {
@@ -49,26 +49,21 @@ impl Closure {
         Ref::map(self.0.borrow(), |c| &c.lambda.chunk)
     }
 
-    pub fn upvalue(&self, idx: UpvalueIdx) -> Ref<'_, Value> {
-        Ref::map(self.0.borrow(), |c| &c.upvalues[idx.0])
+    pub fn lambda(&self) -> Rc<Lambda> {
+        self.0.borrow().lambda.clone()
     }
+}
 
-    pub fn upvalue_count(&self) -> usize {
+impl UpvalueCarrier for Closure {
+    fn upvalue_count(&self) -> usize {
         self.0.borrow().lambda.upvalue_count
     }
 
-    pub fn push_upvalue(&self, value: Value) {
-        self.0.borrow_mut().upvalues.push(value)
+    fn upvalues(&self) -> Ref<'_, [Value]> {
+        Ref::map(self.0.borrow(), |c| c.upvalues.as_slice())
     }
 
-    /// Resolve the deferred upvalues in the closure from a slice of
-    /// the current stack, using the indices stored in the deferred
-    /// values.
-    pub fn resolve_deferred_upvalues(&self, stack: &[Value]) {
-        for upvalue in self.0.borrow_mut().upvalues.iter_mut() {
-            if let Value::DeferredUpvalue(idx) = upvalue {
-                *upvalue = stack[idx.0].clone();
-            }
-        }
+    fn upvalues_mut(&self) -> RefMut<'_, Vec<Value>> {
+        RefMut::map(self.0.borrow_mut(), |c| &mut c.upvalues)
     }
 }

tvix/eval/src/value/thunk.rs

@@ -18,9 +18,12 @@
 //! object, but when forcing a thunk, the runtime *must* mutate the
 //! memoisable slot.
 
-use std::{cell::RefCell, rc::Rc};
+use std::{
+    cell::{Ref, RefCell, RefMut},
+    rc::Rc,
+};
 
-use crate::Value;
+use crate::{upvalues::UpvalueCarrier, Value};
 
 use super::Lambda;
 
@@ -53,3 +56,27 @@ impl Thunk {
         })))
     }
 }
+
+impl UpvalueCarrier for Thunk {
+    fn upvalue_count(&self) -> usize {
+        if let ThunkRepr::Suspended { lambda, .. } = &*self.0.borrow() {
+            return lambda.upvalue_count;
+        }
+
+        panic!("upvalues() on non-suspended thunk");
+    }
+
+    fn upvalues(&self) -> Ref<'_, [Value]> {
+        Ref::map(self.0.borrow(), |thunk| match thunk {
+            ThunkRepr::Suspended { upvalues, .. } => upvalues.as_slice(),
+            _ => panic!("upvalues() on non-suspended thunk"),
+        })
+    }
+
+    fn upvalues_mut(&self) -> RefMut<'_, Vec<Value>> {
+        RefMut::map(self.0.borrow_mut(), |thunk| match thunk {
+            ThunkRepr::Suspended { upvalues, .. } => upvalues,
+            _ => panic!("upvalues() on non-suspended thunk"),
+        })
+    }
+}

tvix/eval/src/vm.rs

@@ -7,6 +7,7 @@ use crate::{
     chunk::Chunk,
     errors::{Error, ErrorKind, EvalResult},
     opcode::{ConstantIdx, Count, JumpOffset, OpCode, StackIdx},
+    upvalues::UpvalueCarrier,
     value::{Closure, Lambda, NixAttrs, NixList, Value},
 };