Require variable declaration for pipe expressions

src/wasm-lib/kcl/src/executor.rs

@@ -2086,6 +2086,20 @@ const newVar = myVar + 1"#;
         );
     }
 
+    #[tokio::test(flavor = "multi_thread")]
+    async fn test_execute_top_level_pipe_without_variable() {
+        let ast = r#"startSketchOn('XY')
+  |> startProfileAt([0, 0], %)
+  |> lineTo([2, 2], %, $yo)
+"#;
+        let result = parse_execute(ast).await;
+        assert!(result.is_err());
+        assert_eq!(
+            result.unwrap_err().to_string(),
+            r#"syntax: KclErrorDetails { source_ranges: [SourceRange([0, 78])], message: "A top-level pipe expression must be assigned to a new variable declaration" }"#.to_owned()
+        );
+    }
+
     #[tokio::test(flavor = "multi_thread")]
     async fn test_execute_angled_line_that_intersects() {
         let ast_fn = |offset: &str| -> String {

src/wasm-lib/kcl/src/parser/parser_impl.rs

@@ -50,6 +50,37 @@ fn program(i: TokenSlice) -> PResult<Program> {
     // Once this is merged and stable, consider changing this as I think it's more accurate
     // without the -1.
     out.end -= 1;
+
+    // Prevent top-level pipe expressions without a variable declaration, giving
+    // a good error message that will help users fix their code.  This is a
+    // band-aid until we can use the artifact graph.
+    let source_ranges = out
+        .body
+        .iter()
+        .filter_map(|item| {
+            if let BodyItem::ExpressionStatement(ExpressionStatement {
+                expression: Value::PipeExpression(_),
+                start,
+                end,
+                ..
+            }) = item
+            {
+                Some(SourceRange([*start, *end]))
+            } else {
+                None
+            }
+        })
+        .collect::<Vec<_>>();
+    if !source_ranges.is_empty() {
+        return Err(ErrMode::Cut(
+            KclError::Syntax(KclErrorDetails {
+                source_ranges,
+                message: "A top-level pipe expression must be assigned to a new variable declaration".to_owned(),
+            })
+            .into(),
+        ));
+    }
+
     Ok(out)
 }