feat(tvix/compiler): implement -> (implication) operator

Similar to `||`, but inverting the left-hand side.

In other words, `a -> b` is essentially rewritten as `!a || b`.

Change-Id: I8a62da65ff070b389e46048d047a54279060a97b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6152
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/compiler.rs

@@ -149,7 +149,7 @@ impl Compiler {
         match op.operator().unwrap() {
             BinOpKind::And => return self.compile_and(op),
             BinOpKind::Or => return self.compile_or(op),
-            BinOpKind::Implication => todo!(),
+            BinOpKind::Implication => return self.compile_implication(op),
 
             _ => {}
         };
@@ -374,6 +374,26 @@ impl Compiler {
         Ok(())
     }
 
+    fn compile_implication(&mut self, node: rnix::types::BinOp) -> EvalResult<()> {
+        debug_assert!(
+            matches!(node.operator(), Some(BinOpKind::Implication)),
+            "compile_implication called with wrong operator kind: {:?}",
+            node.operator(),
+        );
+
+        // Leave left-hand side value on the stack and invert it.
+        self.compile(node.lhs().unwrap())?;
+        self.chunk.add_op(OpCode::OpInvert);
+
+        // Exactly as `||` (because `a -> b` = `!a || b`).
+        let end_idx = self.chunk.add_op(OpCode::OpJumpIfTrue(0));
+        self.chunk.add_op(OpCode::OpPop);
+        self.compile(node.rhs().unwrap())?;
+        self.patch_jump(end_idx);
+
+        Ok(())
+    }
+
     fn patch_jump(&mut self, idx: CodeIdx) {
         let offset = self.chunk.code.len() - 1 - idx.0;