fix(tvix/eval): open/close additional scope around with

This is required to correctly clean up the `with` values.

At the moment, the attrset from which identifiers are being taken is
always pushed on the stack. This means that it must also be removed
again, otherwise in an expression like

  with { a = 15; }; a

The final stack is `[ { a = 15; } 15 ]` *after the last operation*,
which means that the attrset is still on there as garbage.

This has little practical impact right now because it is always
shadowed by the fact that the actual expression value is at the right
location, but becomes relevant when accounting for upvalue captures.

Change-Id: I69e9745bfaa4d6bbcb60ee71f4dc3f8d8695d16a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6303
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/compiler/mod.rs

@@ -885,6 +885,7 @@ impl Compiler {
     // pop/remove the indices of attribute sets that are implicitly in
     // scope through `with` on the "with-stack".
     fn compile_with(&mut self, node: ast::With) {
+        self.begin_scope();
         // TODO: Detect if the namespace is just an identifier, and
         // resolve that directly (thus avoiding duplication on the
         // stack).
@@ -900,6 +901,7 @@ impl Compiler {
 
         self.chunk().push_op(OpCode::OpPopWith);
         self.scope_mut().with_stack.pop();
+        self.end_scope();
     }
 
     fn compile_lambda(&mut self, node: ast::Lambda) {