fix: empty captured sequences produce empty struct instead of null

Author: zharinov

crates/plotnik-lib/src/analyze/type_check/infer.rs

@@ -22,7 +22,7 @@ use crate::analyze::visitor::{Visitor, walk_alt_expr, walk_def, walk_named_node,
 use crate::diagnostics::{DiagnosticKind, Diagnostics};
 use crate::parser::ast::{
     AltExpr, AltKind, AnonymousNode, CapturedExpr, Def, Expr, FieldExpr, NamedNode, QuantifiedExpr,
-    Ref, Root, SeqExpr,
+    Ref, Root, SeqExpr, is_truly_empty_scope,
 };
 use crate::parser::cst::SyntaxKind;
 use crate::query::source_map::SourceId;
@@ -458,8 +458,24 @@ impl<'a, 'd> InferenceVisitor<'a, 'd> {
     ) -> TypeId {
         match &inner_info.flow {
             TypeFlow::Void => {
-                let base_type = self.get_recursive_ref_type(inner).unwrap_or(TYPE_NODE);
-                self.annotation_to_alias(annotation, base_type)
+                // Truly empty sequences/alternations produce empty struct.
+                // E.g., `{ } @x` has type `{ x: {} }`.
+                // Non-empty sequences with void flow (e.g., suppressed captures)
+                // still produce Node for the capture.
+                if is_truly_empty_scope(inner) {
+                    let empty_struct = self.ctx.intern_struct(BTreeMap::new());
+                    match annotation {
+                        Some(AnnotationKind::String) => TYPE_STRING,
+                        Some(AnnotationKind::TypeName(name)) => {
+                            self.ctx.set_type_name(empty_struct, name);
+                            empty_struct
+                        }
+                        None => empty_struct,
+                    }
+                } else {
+                    let base_type = self.get_recursive_ref_type(inner).unwrap_or(TYPE_NODE);
+                    self.annotation_to_alias(annotation, base_type)
+                }
             }
             TypeFlow::Scalar(type_id) => {
                 // For array types with annotation, replace the element type

crates/plotnik-lib/src/compile/capture.rs

@@ -11,7 +11,7 @@ use crate::bytecode::ir::EffectIR;
 use crate::parser::ast::{self, Expr};
 
 use super::Compiler;
-use super::navigation::{inner_creates_scope, is_star_or_plus_quantifier};
+use super::navigation::{inner_creates_scope, is_star_or_plus_quantifier, is_truly_empty_scope};
 
 /// Capture effects to attach to the innermost match instruction.
 ///
@@ -42,21 +42,29 @@ impl Compiler<'_> {
         let is_array = is_star_or_plus_quantifier(inner);
 
         // Check if inner is a scope-creating expression (SeqExpr/AltExpr) that produces
-        // a structured type (Struct/Enum). Named nodes with bubble captures don't count -
-        // they still need Node because we're capturing the matched node, not the struct.
+        // a structured type (Struct/Enum) or truly empty struct. Named nodes with bubble
+        // captures don't count - they still need Node because we're capturing the matched
+        // node, not the struct.
         //
         // For FieldExpr, look through to the value. The parser treats `field: expr @cap` as
         // `(field: expr) @cap` so that quantifiers work on fields (e.g., `decorator: (x)*`
         // for repeating fields). This means captures wrap the FieldExpr, but the value
         // determines whether it produces a structured type. See `parse_expr_no_suffix`.
         let creates_structured_scope = inner.and_then(unwrap_field_value).is_some_and(|ei| {
-            inner_creates_scope(&ei)
-                && self
-                    .type_ctx
-                    .get_term_info(&ei)
-                    .and_then(|info| info.flow.type_id())
-                    .and_then(|id| self.type_ctx.get_type(id))
-                    .is_some_and(|shape| matches!(shape, TypeShape::Struct(_) | TypeShape::Enum(_)))
+            // Truly empty scopes (like `{ }`) produce empty struct
+            if is_truly_empty_scope(&ei) {
+                return true;
+            }
+            if !inner_creates_scope(&ei) {
+                return false;
+            }
+            let Some(info) = self.type_ctx.get_term_info(&ei) else {
+                return false;
+            };
+            info.flow
+                .type_id()
+                .and_then(|id| self.type_ctx.get_type(id))
+                .is_some_and(|shape| matches!(shape, TypeShape::Struct(_) | TypeShape::Enum(_)))
         });
 
         if !is_structured_ref && !creates_structured_scope && !is_array {

crates/plotnik-lib/src/compile/expressions.rs

@@ -18,7 +18,8 @@ use crate::parser::ast::{self, Expr};
 use super::Compiler;
 use super::capture::CaptureEffects;
 use super::navigation::{
-    check_trailing_anchor, inner_creates_scope, is_skippable_quantifier, is_star_or_plus_quantifier,
+    check_trailing_anchor, inner_creates_scope, is_skippable_quantifier,
+    is_star_or_plus_quantifier, is_truly_empty_scope,
 };
 
 impl Compiler<'_> {
@@ -429,7 +430,11 @@ impl Compiler<'_> {
         };
 
         // Struct scope: Obj → inner → EndObj+capture → exit
-        if inner_is_bubble {
+        // Also handle truly empty scopes (e.g., `{ } @x` produces empty struct)
+        let inner_is_truly_empty_scope = is_truly_empty_scope(&inner);
+        let needs_struct_scope = inner_is_bubble || inner_is_truly_empty_scope;
+
+        if needs_struct_scope {
             return if inner_creates_scope {
                 // Sequence/alternation: capture effects after EndObj (value is the struct)
                 self.compile_struct_scope(

crates/plotnik-lib/src/compile/navigation.rs

@@ -6,6 +6,9 @@
 use crate::bytecode::Nav;
 use crate::parser::ast::{Expr, SeqItem};
 
+// Re-export from parser for compile module consumers
+pub use crate::parser::is_truly_empty_scope;
+
 /// Check if an expression is anonymous (string literal or wildcard).
 pub fn expr_is_anonymous(expr: Option<&Expr>) -> bool {
     matches!(expr, Some(Expr::AnonymousNode(_)))

crates/plotnik-lib/src/engine/engine_tests.rs

@@ -547,3 +547,23 @@ fn wildcard_named_skips_anonymous() {
 fn wildcard_bare_matches_anonymous() {
     snap!("Q = (program (return_statement _ @x))", "return 42");
 }
+
+/// Empty captured sequence should produce empty struct, not null or panic.
+/// Regression test for: type inference treating `{ }` as Node instead of empty struct.
+#[test]
+fn regression_empty_captured_sequence() {
+    snap!(
+        "Q = (program (expression_statement (identifier) @id { } @empty))",
+        "x"
+    );
+}
+
+/// Optional empty captured sequence should produce empty struct when matched.
+/// Regression test for: `{ }? @maybe` producing null instead of `{}`.
+#[test]
+fn regression_optional_empty_captured_sequence() {
+    snap!(
+        "Q = (program (expression_statement (identifier) @id { }? @maybe))",
+        "x"
+    );
+}

crates/plotnik-lib/src/engine/materializer.rs

@@ -145,18 +145,21 @@ impl<'t> Materializer<'t> for ValueMaterializer<'_> {
                 }
                 RuntimeEffect::EndObj => {
                     if let Some(Builder::Object(fields)) = stack.pop() {
-                        // Preserve pending if this object is empty and pending has a value.
-                        // This allows the preamble's Obj/EndObj to work correctly for
-                        // entrypoints that return non-struct values (enums, scalars).
-                        // For void-returning queries (no pending, empty object), return null.
                         if !fields.is_empty() {
+                            // Non-empty object: always produce the object value
                             pending = Some(Value::Object(fields));
                         } else if pending.is_none() {
-                            // Empty object with no pending = void result → null
-                            // (This is the preamble's empty wrapper for void-returning queries)
-                            pending = None;
+                            // Empty object with no pending value:
+                            // - If nested (stack.len() > 1): produce empty object {}
+                            //   This handles captured empty sequences like `{ } @x`
+                            //   Note: stack always has at least the result_builder, so we check > 1
+                            // - If at root (stack.len() <= 1): void result → null
+                            if stack.len() > 1 {
+                                pending = Some(Value::Object(vec![]));
+                            }
+                            // else: pending stays None (void result)
                         }
-                        // else: non-empty pending, keep it (passthrough for enums, etc.)
+                        // else: pending has a value, keep it (passthrough for enums, suppressive, etc.)
                     }
                 }
                 RuntimeEffect::Enum(idx) => {

crates/plotnik-lib/src/engine/snapshots/plotnik_lib__engine__engine_tests__regression_empty_captured_sequence.snap

@@ -0,0 +1,18 @@
+---
+source: crates/plotnik-lib/src/engine/engine_tests.rs
+---
+Q = (program (expression_statement (identifier) @id { } @empty))
+---
+x
+---
+{
+  "id": {
+    "kind": "identifier",
+    "text": "x",
+    "span": [
+      0,
+      1
+    ]
+  },
+  "empty": {}
+}

crates/plotnik-lib/src/engine/snapshots/plotnik_lib__engine__engine_tests__regression_optional_empty_captured_sequence.snap

@@ -0,0 +1,18 @@
+---
+source: crates/plotnik-lib/src/engine/engine_tests.rs
+---
+Q = (program (expression_statement (identifier) @id { }? @maybe))
+---
+x
+---
+{
+  "id": {
+    "kind": "identifier",
+    "text": "x",
+    "span": [
+      0,
+      1
+    ]
+  },
+  "maybe": {}
+}

crates/plotnik-lib/src/parser/ast.rs

@@ -469,3 +469,14 @@ impl NegatedField {
             .find(|t| t.kind() == SyntaxKind::Id)
     }
 }
+
+/// Checks if expression is a truly empty scope (sequence/alternation with no children).
+/// Used to distinguish `{ } @x` (empty struct) from `{(expr) @_} @x` (Node capture).
+pub fn is_truly_empty_scope(inner: &Expr) -> bool {
+    match inner {
+        Expr::SeqExpr(seq) => seq.children().next().is_none(),
+        Expr::AltExpr(alt) => alt.branches().next().is_none(),
+        Expr::QuantifiedExpr(q) => q.inner().is_some_and(|i| is_truly_empty_scope(&i)),
+        _ => false,
+    }
+}

crates/plotnik-lib/src/parser/mod.rs

@@ -42,7 +42,8 @@ pub use cst::{SyntaxKind, SyntaxNode, SyntaxToken};
 
 pub use ast::{
     AltExpr, AltKind, Anchor, AnonymousNode, Branch, CapturedExpr, Def, Expr, FieldExpr, NamedNode,
-    NegatedField, QuantifiedExpr, Ref, Root, SeqExpr, SeqItem, Type, token_src,
+    NegatedField, QuantifiedExpr, Ref, Root, SeqExpr, SeqItem, Type, is_truly_empty_scope,
+    token_src,
 };
 
 pub use core::{ParseResult, Parser};