feat(check): warn on @caption applied to a loop (MDL042)

@caption on a loop is silently dropped — Mendix for-loops have no caption
(Microflows$LoopedActivity has no Caption property; Studio Pro auto-labels them
from the iterator). Previously mxcli accepted the annotation and discarded it,
so users had no signal it was ignored.

Emit an MDL042 warning at check time pointing users to @annotation (the
supported way to attach a note to a loop). @annotation and plain loops are
unaffected.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ako

mdl/executor/validate_microflow.go

@@ -166,6 +166,17 @@ func (v *microflowValidator) walkBody(body []ast.MicroflowStatement) {
 			// RETRIEVE populates a list variable — remove from empty tracking
 			delete(v.emptyListVars, stmt.Variable)
 		case *ast.LoopStmt:
+			// Check: @caption on a loop is silently dropped — Mendix for-loops
+			// have no caption (Microflows$LoopedActivity has no Caption
+			// property; Studio Pro auto-labels them from the iterator). The
+			// supported way to label a loop is an annotation note.
+			if stmt.Annotations != nil && stmt.Annotations.Caption != "" {
+				v.addViolation("MDL042", linter.SeverityWarning,
+					"@caption on a loop has no effect — Mendix loops have no caption "+
+						"(the loop activity has no Caption property, so it is dropped). "+
+						"Use @annotation to attach a note to the loop instead.",
+					"Replace @caption with @annotation to label the loop")
+			}
 			// Check: nested loop anti-pattern
 			if v.loopDepth > 0 {
 				v.addViolation("MDL001", linter.SeverityWarning,

mdl/executor/validate_microflow_loop_caption_test.go

@@ -0,0 +1,54 @@
+// SPDX-License-Identifier: Apache-2.0
+
+package executor
+
+import (
+	"testing"
+
+	"github.com/mendixlabs/mxcli/mdl/ast"
+)
+
+func mfWithLoopAnnotations(ann *ast.ActivityAnnotations) *ast.CreateMicroflowStmt {
+	return &ast.CreateMicroflowStmt{
+		Name: ast.QualifiedName{Module: "Sample", Name: "MF"},
+		Body: []ast.MicroflowStatement{
+			&ast.LoopStmt{
+				LoopVariable: "Item",
+				ListVariable: "Items",
+				Annotations:  ann,
+				Body:         []ast.MicroflowStatement{},
+			},
+		},
+	}
+}
+
+func loopHasMDL042(stmt *ast.CreateMicroflowStmt) bool {
+	for _, v := range ValidateMicroflow(stmt) {
+		if v.RuleID == "MDL042" {
+			return true
+		}
+	}
+	return false
+}
+
+// TestValidateMicroflow_CaptionOnLoopWarns guards MDL042: @caption on a loop is
+// silently dropped (Mendix loops have no Caption property), so check must warn.
+func TestValidateMicroflow_CaptionOnLoopWarns(t *testing.T) {
+	if !loopHasMDL042(mfWithLoopAnnotations(&ast.ActivityAnnotations{Caption: "Process things"})) {
+		t.Error("expected MDL042 warning for @caption on a loop")
+	}
+}
+
+// @annotation (the supported way to label a loop) must NOT warn.
+func TestValidateMicroflow_AnnotationOnLoopNoWarn(t *testing.T) {
+	if loopHasMDL042(mfWithLoopAnnotations(&ast.ActivityAnnotations{AnnotationText: "Process things"})) {
+		t.Error("MDL042 must not fire for @annotation on a loop")
+	}
+}
+
+// A loop with no annotations must not warn.
+func TestValidateMicroflow_PlainLoopNoWarn(t *testing.T) {
+	if loopHasMDL042(mfWithLoopAnnotations(nil)) {
+		t.Error("MDL042 must not fire for a plain loop")
+	}
+}