fix: Handle builtin out-parameter calls in RVM compiler

- Teach the hoister/destructuring planner to respect parent scope when building binding plans for extra arguments, so already-bound vars yield equality checks.
- Update the compiler’s function-call path to drop the trailing out-argument, run its binding plan after the call, and share call-target resolution logic.
- Add regression suites for builtin and user-defined out-parameter scenarios plus align the CLI example output when RVM returns undefined.

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

src/compiler/destructuring_planner/parameters.rs

@@ -37,12 +37,13 @@ pub fn create_loop_index_binding_plan<T: VariableBindingContext>(
 pub fn create_parameter_binding_plan<T: VariableBindingContext>(
     param_expr: &ExprRef,
     context: &T,
+    scoping: ScopingMode,
 ) -> Result<BindingPlan> {
     let mut newly_bound = BTreeSet::new();
     let destructuring_plan = create_destructuring_plan_with_tracking(
         param_expr,
         context,
-        ScopingMode::AllowShadowing,
+        scoping,
         &mut newly_bound,
     )
     .ok_or_else(|| BindingPlannerError::FailedToCreateDestructuringPlan {
@@ -50,7 +51,9 @@ pub fn create_parameter_binding_plan<T: VariableBindingContext>(
         span: param_expr.span().clone(),
     })?;
 
-    validate_pattern_bindings(param_expr, &newly_bound, context)?;
+    if scoping == ScopingMode::AllowShadowing {
+        validate_pattern_bindings(param_expr, &newly_bound, context)?;
+    }
 
     Ok(BindingPlan::Parameter {
         param_expr: param_expr.clone(),

src/compiler/hoist.rs

@@ -381,7 +381,9 @@ impl LoopHoister {
                     for param in args {
                         // Create binding plan for function parameter
                         match super::destructuring_planner::create_parameter_binding_plan(
-                            param, &context,
+                            param,
+                            &context,
+                            ScopingMode::AllowShadowing,
                         ) {
                             Ok(binding_plan) => {
                                 let expr_idx = param.as_ref().eidx();
@@ -708,7 +710,9 @@ impl LoopHoister {
                 // If the last parameter expression contains unbound vars, create a binding plan
                 if let Some(last_param) = params.last() {
                     match super::destructuring_planner::create_parameter_binding_plan(
-                        last_param, context,
+                        last_param,
+                        context,
+                        ScopingMode::RespectParent,
                     ) {
                         Ok(binding_plan) => {
                             let expr_idx = last_param.as_ref().eidx();

src/languages/rego/compiler/function_calls.rs

@@ -3,11 +3,24 @@
 
 use super::{Compiler, CompilerError, Register, Result};
 use crate::ast::ExprRef;
+use crate::builtins;
+use crate::compiler::destructuring_planner::plans::BindingPlan;
 use crate::lexer::Span;
 use crate::rvm::instructions::{BuiltinCallParams, FunctionCallParams};
 use crate::rvm::Instruction;
 use crate::utils::get_path_string;
-use alloc::vec::Vec;
+use alloc::{format, string::ToString, vec::Vec};
+
+enum CallTarget {
+    User {
+        rule_index: u16,
+        expected_args: Option<usize>,
+    },
+    Builtin {
+        builtin_index: u16,
+        expected_args: Option<usize>,
+    },
+}
 
 impl<'a> Compiler<'a> {
     pub(super) fn compile_function_call(
@@ -27,51 +40,138 @@ impl<'a> Compiler<'a> {
                 .map_err(|_| CompilerError::InvalidFunctionExpressionWithPackage.at(&span))?
         };
 
+        let mut out_param_plan: Option<(BindingPlan, Span)> = None;
+        let mut params_to_compile = params.len();
+
+        let call_target = self.determine_call_target(&original_fcn_path, &full_fcn_path, &span)?;
+
+        let expected_args = match &call_target {
+            CallTarget::User { expected_args, .. } => *expected_args,
+            CallTarget::Builtin { expected_args, .. } => *expected_args,
+        };
+
+        if let Some(expected) = expected_args {
+            if params.len() == expected + 1 {
+                if let Some(last_param) = params.last() {
+                    let plan = self.expect_binding_plan_for_expr(
+                        last_param,
+                        &format!("extra argument for function '{}'", original_fcn_path),
+                    )?;
+
+                    match plan {
+                        BindingPlan::Parameter { .. } => {
+                            out_param_plan = Some((plan, last_param.span().clone()));
+                            params_to_compile -= 1;
+                        }
+                        other => {
+                            return Err(CompilerError::UnexpectedBindingPlan {
+                                context: "function extra argument".to_string(),
+                                found: format!("{other:?}"),
+                            }
+                            .at(last_param.span()));
+                        }
+                    }
+                }
+            }
+        }
+
         let mut arg_regs = Vec::new();
-        for param in params.iter() {
+        for param in params.iter().take(params_to_compile) {
             let param_reg = self.compile_rego_expr_with_span(param, param.span(), false)?;
             arg_regs.push(param_reg);
         }
 
         let dest = self.alloc_register();
 
-        if self.is_user_defined_function(&full_fcn_path) {
-            let rule_index = self.get_or_assign_rule_index(&full_fcn_path)?;
-            let mut args_array = [0u8; 8];
-            let num_args = arg_regs.len().min(8) as u8;
-            for (i, &reg) in arg_regs.iter().take(8).enumerate() {
-                args_array[i] = reg;
-            }
+        match call_target {
+            CallTarget::User { rule_index, .. } => {
+                let mut args_array = [0u8; 8];
+                let num_args = arg_regs.len().min(8) as u8;
+                for (i, &reg) in arg_regs.iter().take(8).enumerate() {
+                    args_array[i] = reg;
+                }
 
-            let params_index = self.program.add_function_call_params(FunctionCallParams {
-                func_rule_index: rule_index,
-                dest,
-                num_args,
-                args: args_array,
-            });
-            self.emit_instruction(Instruction::FunctionCall { params_index }, &span);
-        } else if self.is_builtin(&original_fcn_path) {
-            let builtin_index = self.get_builtin_index(&original_fcn_path)?;
-            let mut args_array = [0u8; 8];
-            let num_args = arg_regs.len().min(8) as u8;
-            for (i, &reg) in arg_regs.iter().take(8).enumerate() {
-                args_array[i] = reg;
+                let params_index = self.program.add_function_call_params(FunctionCallParams {
+                    func_rule_index: rule_index,
+                    dest,
+                    num_args,
+                    args: args_array,
+                });
+                self.emit_instruction(Instruction::FunctionCall { params_index }, &span);
             }
+            CallTarget::Builtin { builtin_index, .. } => {
+                let mut args_array = [0u8; 8];
+                let num_args = arg_regs.len().min(8) as u8;
+                for (i, &reg) in arg_regs.iter().take(8).enumerate() {
+                    args_array[i] = reg;
+                }
 
-            let params_index = self.program.add_builtin_call_params(BuiltinCallParams {
-                dest,
-                builtin_index,
-                num_args,
-                args: args_array,
-            });
-            self.emit_instruction(Instruction::BuiltinCall { params_index }, &span);
-        } else {
-            return Err(CompilerError::UnknownFunction {
-                name: original_fcn_path,
+                let params_index = self.program.add_builtin_call_params(BuiltinCallParams {
+                    dest,
+                    builtin_index,
+                    num_args,
+                    args: args_array,
+                });
+                self.emit_instruction(Instruction::BuiltinCall { params_index }, &span);
             }
-            .at(&span));
+        }
+
+        if let Some((plan, plan_span)) = &out_param_plan {
+            self.apply_binding_plan(plan, dest, plan_span)
+                .map_err(|err| CompilerError::from(err).at(plan_span))?;
+            self.emit_instruction(Instruction::LoadBool { dest, value: true }, &span);
         }
 
         Ok(dest)
     }
+
+    fn lookup_builtin_arity(&self, name: &str) -> Option<usize> {
+        if name == "print" {
+            Some(2)
+        } else {
+            builtins::BUILTINS
+                .get(name)
+                .map(|(_, arity)| *arity as usize)
+        }
+    }
+}
+
+impl<'a> Compiler<'a> {
+    fn determine_call_target(
+        &mut self,
+        original_fcn_path: &str,
+        full_fcn_path: &str,
+        span: &Span,
+    ) -> Result<CallTarget> {
+        if self.is_user_defined_function(full_fcn_path) {
+            let rule_index = self.get_or_assign_rule_index(full_fcn_path)?;
+            let expected_args = self
+                .policy
+                .inner
+                .functions
+                .get(full_fcn_path)
+                .map(|(_, arity, _)| *arity as usize)
+                .or_else(|| {
+                    self.rule_function_param_count
+                        .get(rule_index as usize)
+                        .and_then(|count| *count)
+                });
+            Ok(CallTarget::User {
+                rule_index,
+                expected_args,
+            })
+        } else if self.is_builtin(original_fcn_path) {
+            let builtin_index = self.get_builtin_index(original_fcn_path)?;
+            let expected_args = self.lookup_builtin_arity(original_fcn_path);
+            Ok(CallTarget::Builtin {
+                builtin_index,
+                expected_args,
+            })
+        } else {
+            Err(CompilerError::UnknownFunction {
+                name: original_fcn_path.to_string(),
+            }
+            .at(span))
+        }
+    }
 }

tests/rvm/rego/cases/builtins_out_params.yaml

@@ -0,0 +1,83 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+
+# Builtin Out-Parameter Test Suite
+# Verifies the compiler handles builtin return-argument syntax consistently
+# across variable bindings, equality checks, scheduler ordering, and literals.
+
+cases:
+  - note: builtin_out_param_simple_binding
+    data: {}
+    modules:
+      - |
+        package test
+
+        rule1 if {
+            floor(1.001, x)
+            x == 1
+        }
+    query: data.test.rule1
+    want_result: true
+
+  - note: builtin_out_param_scheduler_reordering
+    data: {}
+    modules:
+      - |
+        package test
+
+        rule2 if {
+            x == 1
+            floor(1.001, x)
+        }
+    query: data.test.rule2
+    want_result: true
+
+  - note: builtin_out_param_existing_binding_equality
+    data: {}
+    modules:
+      - |
+        package test
+
+        rule3 if {
+            x := 1
+            floor(1.001, x)
+        }
+    query: data.test.rule3
+    want_result: true
+
+  - note: builtin_out_param_existing_binding_mismatch
+    data: {}
+    modules:
+      - |
+        package test
+
+        rule31 if {
+            x := 2
+            floor(1.001, x)
+        }
+    query: data.test.rule31
+    want_result: "#undefined"
+
+  - note: builtin_out_param_literal_success
+    data: {}
+    modules:
+      - |
+        package test
+
+        rule4 if {
+            floor(1.001, 1)
+        }
+    query: data.test.rule4
+    want_result: true
+
+  - note: builtin_out_param_literal_failure
+    data: {}
+    modules:
+      - |
+        package test
+
+        rule5 if {
+            floor(1.001, 2)
+        }
+    query: data.test.rule5
+    want_result: "#undefined"

tests/rvm/rego/cases/user_fcn_out_params.yaml

@@ -0,0 +1,107 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+
+# User-Defined Out-Parameter Test Suite
+# Mirrors builtin coverage but routes through the my_floor helper rule
+# to ensure planner handling stays correct for user-defined function rules.
+
+cases:
+  - note: user_fcn_out_param_simple_binding
+    data: {}
+    modules:
+      - |
+        package test
+
+        my_floor(x) := y if {
+            floor(x, y)
+        }
+
+        rule1 if {
+            my_floor(1.001, x)
+            x == 1
+        }
+    query: data.test.rule1
+    want_result: true
+
+  - note: user_fcn_out_param_scheduler_reordering
+    data: {}
+    modules:
+      - |
+        package test
+
+        my_floor(x) := y if {
+            floor(x, y)
+        }
+
+        rule2 if {
+            x == 1
+            my_floor(1.001, x)
+        }
+    query: data.test.rule2
+    want_result: true
+
+  - note: user_fcn_out_param_existing_binding_equality
+    data: {}
+    modules:
+      - |
+        package test
+
+        my_floor(x) := y if {
+            floor(x, y)
+        }
+
+        rule3 if {
+            x := 1
+            my_floor(1.001, x)
+        }
+    query: data.test.rule3
+    want_result: true
+
+  - note: user_fcn_out_param_existing_binding_mismatch
+    data: {}
+    modules:
+      - |
+        package test
+
+        my_floor(x) := y if {
+            floor(x, y)
+        }
+
+        rule31 if {
+            x := 2
+            my_floor(1.001, x)
+        }
+    query: data.test.rule31
+    want_result: "#undefined"
+
+  - note: user_fcn_out_param_literal_success
+    data: {}
+    modules:
+      - |
+        package test
+
+        my_floor(x) := y if {
+            floor(x, y)
+        }
+
+        rule4 if {
+            my_floor(1.001, 1)
+        }
+    query: data.test.rule4
+    want_result: true
+
+  - note: user_fcn_out_param_literal_failure
+    data: {}
+    modules:
+      - |
+        package test
+
+        my_floor(x) := y if {
+            floor(x, y)
+        }
+
+        rule5 if {
+            my_floor(1.001, 2)
+        }
+    query: data.test.rule5
+    want_result: "#undefined"