fix(tvix/eval): correctly resolve dynamic upvalues one scope up

This does not yet correctly resolve them if they are more than one
scope up, however.

Change-Id: I6687073c60aee0282f2b6ffc98b34c1e96a60f20
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6319
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/compiler/mod.rs

@@ -992,12 +992,6 @@ impl Compiler {
             LocalPosition::Unknown => { /* continue below */ }
         };
 
-        // Determine whether the upvalue is a dynamic variable in the
-        // enclosing context.
-        if self.contexts[ctx_idx - 1].scope.has_with() {
-            return Some(self.add_upvalue(ctx_idx, Upvalue::Dynamic(SmolStr::new(name))));
-        }
-
         // If the upvalue comes from even further up, we need to
         // recurse to make sure that the upvalues are created at each
         // level.
@@ -1005,6 +999,13 @@ impl Compiler {
             return Some(self.add_upvalue(ctx_idx, Upvalue::Upvalue(idx)));
         }
 
+        // If the resolution of a statically known upvalue failed,
+        // attempt to resolve a dynamic one (i.e. search for enclosing
+        // `with` blocks and make that resolution dynamic).
+        if self.contexts[ctx_idx - 1].scope.has_with() {
+            return Some(self.add_upvalue(ctx_idx, Upvalue::Dynamic(SmolStr::new(name))));
+        }
+
         None
     }
 

tvix/eval/src/tests/tvix_tests/eval-okay-closure-with-shadowing.exp

@@ -0,0 +1 @@
+1

tvix/eval/src/tests/tvix_tests/eval-okay-closure-with-shadowing.nix

@@ -0,0 +1,14 @@
+# If a closure closes over a variable that is statically known *and*
+# available dynamically through `with`, the statically known one must
+# have precedence.
+
+let
+  # introduce statically known `a` (this should be the result)
+  a = 1;
+in
+
+# introduce some closure depth to force both kinds of upvalue
+# resolution, and introduce a dynamically known `a` within the
+# closures
+let f = b: with { a = 2; }; c: a + b + c;
+in f 0 0

tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-with.exp

@@ -0,0 +1 @@
+[ 1 2 3 4 ]

tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-with.nix

@@ -0,0 +1,6 @@
+with { a = 1; b = 1; };
+with { b = 2; c = 2; };
+with { c = 3; d = 3; };
+with { d = 4; };
+
+[ a b c d ]