enable build-params evaluation of for-expressions in .bicepparam

src/Bicep.Cli.IntegrationTests/BuildParamsCommandTests.cs

@@ -327,6 +327,99 @@ param vnetConfigs array
             paramsObject.Should().HaveValueAtPath("parameters.sharedGroupName.value", "rg-search-foo-nonprod");
         }
 
+        [TestMethod]
+        public async Task Build_params_for_expression_variable_should_succeed()
+        {
+            var outputPath = FileHelper.GetUniqueTestOutputPath(TestContext);
+
+            _ = FileHelper.SaveResultFile(
+                TestContext,
+                "main.bicep",
+                """
+                type FleetConfig = {
+                    namePrefix: string
+                    sku: string
+                    capacity: int
+                    clusteringPolicy: string
+                }
+
+                param testMatrix FleetConfig[]
+                """,
+                outputPath);
+
+            var paramsPath = FileHelper.SaveResultFile(
+                TestContext,
+                "main.bicepparam",
+                """
+                using './main.bicep'
+
+                var matrix = [
+                    {
+                        namePrefix: 'e10impactx4'
+                        sku: 'Enterprise_E10'
+                        capacity: 4
+                    }
+                    {
+                        namePrefix: 'e10impact'
+                        sku: 'Enterprise_E10'
+                        capacity: 2
+                    }
+                ]
+
+                var type1 = [for item in matrix: {
+                    namePrefix: item.namePrefix
+                    sku: item.sku
+                    capacity: item.capacity
+                    clusteringPolicy: 'EnterpriseCluster'
+                }]
+
+                var type2 = [for item in matrix: {
+                    namePrefix: '${item.namePrefix}-ent'
+                    sku: item.sku
+                    capacity: item.capacity
+                    clusteringPolicy: 'OSSCluster'
+                }]
+
+                param testMatrix = concat(type1, type2)
+                """,
+                outputPath);
+
+            var result = await Bicep(CreateDefaultSettings(), "build-params", paramsPath, "--stdout");
+
+            result.Should().Succeed();
+
+            var parametersStdout = result.Stdout.FromJson<BuildParamsStdout>();
+            var paramsObject = parametersStdout.parametersJson.FromJson<JToken>();
+            paramsObject.Should().HaveValueAtPath("parameters.testMatrix.value", JToken.Parse("""
+                [
+                    {
+                        "namePrefix": "e10impactx4",
+                        "sku": "Enterprise_E10",
+                        "capacity": 4,
+                        "clusteringPolicy": "EnterpriseCluster"
+                    },
+                    {
+                        "namePrefix": "e10impact",
+                        "sku": "Enterprise_E10",
+                        "capacity": 2,
+                        "clusteringPolicy": "EnterpriseCluster"
+                    },
+                    {
+                        "namePrefix": "e10impactx4-ent",
+                        "sku": "Enterprise_E10",
+                        "capacity": 4,
+                        "clusteringPolicy": "OSSCluster"
+                    },
+                    {
+                        "namePrefix": "e10impact-ent",
+                        "sku": "Enterprise_E10",
+                        "capacity": 2,
+                        "clusteringPolicy": "OSSCluster"
+                    }
+                ]
+            """));
+        }
+
         [TestMethod]
         public async Task Build_params_extends_variable_uses_base_params_not_overridden()
         {

src/Bicep.Core.UnitTests/Emit/ParameterAssignmentEvaluatorTests.cs

@@ -0,0 +1,32 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+using Bicep.Core.UnitTests.Assertions;
+using Bicep.Core.UnitTests.Utils;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Newtonsoft.Json.Linq;
+
+namespace Bicep.Core.UnitTests.Emit;
+
+[TestClass]
+public class ParameterAssignmentEvaluatorTests
+{
+    [TestMethod]
+    public void BuildParams_ForExpressionVariable_EvaluatesToValue()
+    {
+        var services = new ServiceBuilder().WithEmptyAzResources();
+
+        var result = CompilationHelper.CompileParams(
+            services,
+            ("main.bicep", "param p int[]"),
+            ("parameters.bicepparam", """
+            using 'main.bicep'
+
+            var x = [for item in [1, 2]: item * 2]
+            param p = x
+            """));
+
+        result.Should().NotHaveAnyDiagnostics();
+        result.Parameters.Should().HaveValueAtPath("parameters.p.value", JToken.Parse("[2, 4]"));
+    }
+}

src/Bicep.Core/Emit/ParameterAssignmentEvaluator.cs

@@ -25,6 +25,24 @@ namespace Bicep.Core.Emit;
 
 public class ParameterAssignmentEvaluator
 {
+    private sealed class ForLoopIndexRewriter : ExpressionRewriteVisitor
+    {
+        private readonly long index;
+
+        private ForLoopIndexRewriter(long index)
+        {
+            this.index = index;
+        }
+
+        public static Expression Rewrite(Expression expression, long index)
+        {
+            return new ForLoopIndexRewriter(index).Replace(expression);
+        }
+
+        public override Expression ReplaceCopyIndexExpression(CopyIndexExpression expression) => new IntegerLiteralExpression(expression.SourceSyntax, index);
+
+        public override Expression ReplaceForLoopExpression(ForLoopExpression expression) => expression;
+    }
     private class ParameterAssignmentEvaluationContext : IEvaluationContext
     {
         private readonly TemplateExpressionEvaluationHelper evaluationHelper;
@@ -419,7 +437,7 @@ public Result EvaluateParameter(ParameterAssignmentSymbol parameter)
 
                 try
                 {
-                    return Result.For(parameterConverter.ConvertExpression(intermediate).EvaluateExpression(context));
+                    return Result.For(EvaluateExpression(parameterConverter, intermediate, context));
                 }
                 catch (Exception ex)
                 {
@@ -478,7 +496,7 @@ public ImmutableDictionary<string, Result> EvaluateExtensionConfigAssignment(Ext
                     {
                         try
                         {
-                            propertyResult = Result.For(converter.ConvertExpression(intermediate).EvaluateExpression(context));
+                            propertyResult = Result.For(EvaluateExpression(converter, intermediate, context));
                         }
                         catch (Exception ex)
                         {
@@ -542,7 +560,7 @@ private Result EvaluateVariable(VariableSymbol variable)
                     var variableConverter = GetConverterForVariable(variable);
                     var intermediate = variableConverter.ConvertToIntermediateExpression(variable.DeclaringVariable.Value);
 
-                    return Result.For(variableConverter.ConvertExpression(intermediate).EvaluateExpression(context));
+                    return Result.For(EvaluateExpression(variableConverter, intermediate, context));
                 }
                 catch (Exception ex)
                 {
@@ -560,7 +578,7 @@ private Result EvaluateSynthesizeVariableExpression(string name, Expression expr
                 {
                     var evalContext = GetExpressionEvaluationContextForModel(model);
                     var exprConverter = converterCache.GetOrAdd(model, m => new ExpressionConverter(new EmitterContext(m)));
-                    return Result.For(exprConverter.ConvertExpression(expression).EvaluateExpression(evalContext));
+                    return Result.For(EvaluateExpression(exprConverter, expression, evalContext));
                 }
                 catch (Exception e)
                 {
@@ -790,7 +808,7 @@ public ExpressionEvaluationResult EvaluateExpression(SyntaxBase expressionSyntax
         {
             var context = GetExpressionEvaluationContext();
             var intermediate = converter.ConvertToIntermediateExpression(expressionSyntax);
-            var result = converter.ConvertExpression(intermediate).EvaluateExpression(context);
+            var result = EvaluateExpression(converter, intermediate, context);
             return ExpressionEvaluationResult.For(result);
         }
         catch (Exception ex)
@@ -800,6 +818,33 @@ public ExpressionEvaluationResult EvaluateExpression(SyntaxBase expressionSyntax
         }
     }
 
+    private JToken EvaluateExpression(ExpressionConverter expressionConverter, Expression expression, ParameterAssignmentEvaluationContext context)
+    {
+        if (expression is ForLoopExpression forLoop)
+        {
+            return EvaluateForLoopExpression(expressionConverter, forLoop, context);
+        }
+
+        return expressionConverter.ConvertExpression(expression).EvaluateExpression(context);
+    }
+
+    private JToken EvaluateForLoopExpression(ExpressionConverter expressionConverter, ForLoopExpression forLoop, ParameterAssignmentEvaluationContext context)
+    {
+        var source = EvaluateExpression(expressionConverter, forLoop.Expression, context);
+        if (source is not JArray sourceArray)
+        {
+            throw new InvalidOperationException("For-expression source must be an array.");
+        }
+
+        var results = new JArray();
+        for (var i = 0; i < sourceArray.Count; i++)
+        {
+            results.Add(EvaluateExpression(expressionConverter, ForLoopIndexRewriter.Rewrite(forLoop.Body, i), context));
+        }
+
+        return results;
+    }
+
     /// <summary>
     /// Rewrites the external input function calls to use the externalInputs function with the index of the external input.
     /// e.g. externalInput('sys.cli', 'foo') becomes externalInputs('0')