feat(tvix/eval): implement assert operator

This implements `assert`, which evaluates an expression and aborts
evaluation if the value is not `true`.

At this point we should introduce eval-failed-* tests; probably
asserting against some representation of the error enum?

Change-Id: If54c8f616d89b829c1860a4835dde60a2cd70d7a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6230
Reviewed-by: grfn <grfn@gws.fyi>
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/compiler.rs

@@ -170,6 +170,11 @@ impl Compiler {
                 self.compile_with(node)
             }
 
+            rnix::SyntaxKind::NODE_ASSERT => {
+                let node = rnix::types::Assert::cast(node).unwrap();
+                self.compile_assert(node)
+            }
+
             kind => panic!("visiting unsupported node: {:?}", kind),
         }
     }
@@ -795,6 +800,17 @@ impl Compiler {
         self.compile(node.body().unwrap())
     }
 
+    fn compile_assert(&mut self, node: rnix::types::Assert) -> EvalResult<()> {
+        // Compile the assertion condition to leave its value on the stack.
+        self.compile(node.condition().unwrap())?;
+        self.chunk.push_op(OpCode::OpAssert);
+
+        // The runtime will abort evaluation at this point if the
+        // assertion failed, if not the body simply continues on like
+        // normal.
+        self.compile(node.body().unwrap())
+    }
+
     // Emit the literal string value of an identifier. Required for
     // several operations related to attribute sets, where identifiers
     // are used as string keys.

tvix/eval/src/errors.rs

@@ -33,6 +33,8 @@ pub enum Error {
     UnknownDynamicVariable(String),
 
     ParseErrors(Vec<rnix::parser::ParseError>),
+
+    AssertionFailed,
 }
 
 impl Display for Error {

tvix/eval/src/opcode.rs

@@ -72,4 +72,7 @@ pub enum OpCode {
 
     // Close scopes while leaving their expression value around.
     OpCloseScope(usize), // number of locals to pop
+
+    // Asserts stack top is a boolean, and true.
+    OpAssert,
 }

tvix/eval/src/vm.rs

@@ -304,6 +304,12 @@ impl VM {
 
                     return Err(Error::UnknownDynamicVariable(ident.to_string()));
                 }
+
+                OpCode::OpAssert => {
+                    if !self.pop().as_bool()? {
+                        return Err(Error::AssertionFailed);
+                    }
+                }
             }
 
             #[cfg(feature = "disassembler")]