feat(tvix/eval): implement scope poisoning for true/false/null

These tokens are optionally parsed as identifiers by Nix, which means
that within any scopes that resolve them the compiler needs to track
whether they have been overridden to know whether to emit the literal
instructions or resolve a variable.

This is implemented by a new concept of "scope poisoning", where the
compiler's scope structure tracks whether or not any builtin
identifiers have been overridden.

Change-Id: I3ab711146e229f843f6e1f0343385382ee0aecb6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6227
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/compiler.rs

@@ -9,8 +9,8 @@
 //!
 //! However, at the time that the AST is passed to the compiler we
 //! have verified that `rnix` considers the code to be correct, so all
-//! variants are filed. In cases where the invariant is guaranteed by
+//! variants are fulfilled. In cases where the invariant is guaranteed
-//! the code in this module, `debug_assert!` has been used to catch
+//! by the code in this module, `debug_assert!` has been used to catch
 //! mistakes early during development.
 
 use path_clean::PathClean;
@@ -69,6 +69,16 @@ struct Scope {
     // Stack indices of attribute sets currently in scope through
     // `with`.
     with_stack: Vec<With>,
+
+    // Certain symbols are considered to be "poisoning" the scope when
+    // defined. This is because users are allowed to override symbols
+    // like 'true' or 'null'.
+    //
+    // To support this efficiently, the depth at which a poisoning
+    // occured is tracked here.
+    poisoned_true: usize,
+    poisoned_false: usize,
+    poisoned_null: usize,
 }
 
 struct Compiler {
@@ -343,9 +353,9 @@ impl Compiler {
             // optimised information about any "weird" stuff that's
             // happened to the scope (such as overrides of these
             // literals, or builtins).
-            "true" => self.chunk.push_op(OpCode::OpTrue),
+            "true" if self.scope.poisoned_true == 0 => self.chunk.push_op(OpCode::OpTrue),
-            "false" => self.chunk.push_op(OpCode::OpFalse),
+            "false" if self.scope.poisoned_false == 0 => self.chunk.push_op(OpCode::OpFalse),
-            "null" => self.chunk.push_op(OpCode::OpNull),
+            "null" if self.scope.poisoned_null == 0 => self.chunk.push_op(OpCode::OpNull),
 
             name => {
                 // Note: `with` and some other special scoping
@@ -817,6 +827,19 @@ impl Compiler {
     fn end_scope(&mut self) {
         let mut scope = &mut self.scope;
         debug_assert!(scope.scope_depth != 0, "can not end top scope");
+
+        // If this scope poisoned any builtins or special identifiers,
+        // they need to be reset.
+        if scope.poisoned_true == scope.scope_depth {
+            scope.poisoned_true = 0;
+        }
+        if scope.poisoned_false == scope.scope_depth {
+            scope.poisoned_false = 0;
+        }
+        if scope.poisoned_null == scope.scope_depth {
+            scope.poisoned_null = 0;
+        }
+
         scope.scope_depth -= 1;
 
         // When ending a scope, all corresponding locals need to be
@@ -846,6 +869,23 @@ impl Compiler {
     }
 
     fn push_local<S: Into<String>>(&mut self, name: S) {
+        // Set up scope poisoning if required.
+        let name = name.into();
+        match name.as_str() {
+            "true" if self.scope.poisoned_true == 0 => {
+                self.scope.poisoned_true = self.scope.scope_depth
+            }
+
+            "false" if self.scope.poisoned_false == 0 => {
+                self.scope.poisoned_false = self.scope.scope_depth
+            }
+
+            "null" if self.scope.poisoned_null == 0 => {
+                self.scope.poisoned_null = self.scope.scope_depth
+            }
+
+            _ => {}
+        };
         self.scope.locals.push(Local {
             name: name.into(),
             depth: self.scope.scope_depth,

tvix/eval/src/tests/tvix_tests/eval-okay-poisoned-scopes.exp

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

tvix/eval/src/tests/tvix_tests/eval-okay-poisoned-scopes.nix

@@ -0,0 +1,6 @@
+let
+  true = 1;
+  false = 2;
+  null = 3;
+in
+[ true false null ]

tvix/eval/src/tests/tvix_tests/eval-okay-unpoison-scope.exp

@@ -0,0 +1 @@
+[ true false null 1 2 3 ]

tvix/eval/src/tests/tvix_tests/eval-okay-unpoison-scope.nix

@@ -0,0 +1,7 @@
+let
+  poisoned = let
+    true = 1;
+    false = 2;
+    null = 3;
+  in [ true false null ];
+in [ true false null ] ++ poisoned