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diff --git a/tvix/eval/src/upvalues.rs b/tvix/eval/src/upvalues.rs
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+//! 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).
+//!
+//! The upvalues of a scope are whatever data are needed at runtime
+//! in order to resolve each free variable in the scope to a value.
+//! "Upvalue" is a term taken from Lua.
+
+use std::ops::Index;
+
+use crate::{opcode::UpvalueIdx, Value};
+
+/// Structure for carrying upvalues of an UpvalueCarrier.  The
+/// implementation of this struct encapsulates the logic for
+/// capturing and accessing upvalues.
+///
+/// Nix's `with` cannot be used to shadow an enclosing binding --
+/// like Rust's `use xyz::*` construct, but unlike Javascript's
+/// `with (xyz)`.  This means that Nix has two kinds of identifiers,
+/// which can be distinguished at compile time:
+///
+/// - Static identifiers, which are bound in some enclosing scope by
+///   `let`, `name:` or `{name}:`
+/// - Dynamic identifiers, which are not bound in any enclosing
+///   scope
+#[derive(Clone, Debug)]
+pub struct Upvalues {
+    /// The upvalues of static identifiers.  Each static identifier
+    /// is assigned an integer identifier at compile time, which is
+    /// an index into this Vec.
+    static_upvalues: Vec<Value>,
+
+    /// The upvalues of dynamic identifiers, if any exist.  This
+    /// consists of the value passed to each enclosing `with val;`,
+    /// from outermost to innermost.
+    with_stack: Option<Vec<Value>>,
+}
+
+impl Upvalues {
+    pub fn with_capacity(count: usize) -> Self {
+        Upvalues {
+            static_upvalues: Vec::with_capacity(count),
+            with_stack: None,
+        }
+    }
+
+    /// Push an upvalue at the end of the upvalue list.
+    pub fn push(&mut self, value: Value) {
+        self.static_upvalues.push(value);
+    }
+
+    /// Set the captured with stack.
+    pub fn set_with_stack(&mut self, with_stack: Vec<Value>) {
+        self.with_stack = Some(with_stack);
+    }
+
+    pub fn with_stack(&self) -> Option<&Vec<Value>> {
+        self.with_stack.as_ref()
+    }
+
+    pub fn with_stack_len(&self) -> usize {
+        match &self.with_stack {
+            None => 0,
+            Some(stack) => stack.len(),
+        }
+    }
+
+    /// Resolve deferred upvalues from the provided stack slice,
+    /// mutating them in the internal upvalue slots.
+    pub fn resolve_deferred_upvalues(&mut self, stack: &[Value]) {
+        for upvalue in self.static_upvalues.iter_mut() {
+            if let Value::DeferredUpvalue(update_from_idx) = upvalue {
+                *upvalue = stack[update_from_idx.0].clone();
+            }
+        }
+    }
+}
+
+impl Index<UpvalueIdx> for Upvalues {
+    type Output = Value;
+
+    fn index(&self, index: UpvalueIdx) -> &Self::Output {
+        &self.static_upvalues[index.0]
+    }
+}