[SwiftSyntax] Add replace method to SyntaxCollections

test/SwiftSyntax/SyntaxCollections.swift

@@ -0,0 +1,112 @@
+// RUN: %target-run-simple-swift
+// REQUIRES: executable_test
+// REQUIRES: OS=macosx
+// REQUIRES: objc_interop
+
+import Foundation
+import StdlibUnittest
+import SwiftSyntax
+
+func integerLiteralElement(_ int: Int) -> ArrayElementSyntax {
+    let literal = SyntaxFactory.makeIntegerLiteral("\(int)")
+    return SyntaxFactory.makeArrayElement(
+        expression: SyntaxFactory.makeIntegerLiteralExpr(digits: literal),
+        trailingComma: nil)
+}
+
+var SyntaxCollectionsAPI = TestSuite("SyntaxCollectionsAPI")
+
+SyntaxCollectionsAPI.test("AppendingElement") {
+    let arrayElementList = SyntaxFactory.makeArrayElementList([
+        integerLiteralElement(0)
+    ])
+
+    let newArrayElementList = arrayElementList.appending(integerLiteralElement(1))
+
+    expectEqual(newArrayElementList.count, 2)
+    expectNotNil(newArrayElementList.child(at: 1))
+    expectEqual("\(newArrayElementList.child(at: 1)!)", "1")
+}
+
+SyntaxCollectionsAPI.test("InsertingElement") {
+    let arrayElementList = SyntaxFactory.makeArrayElementList([
+        integerLiteralElement(1)
+    ])
+
+    var newArrayElementList = arrayElementList.inserting(integerLiteralElement(0), at: 0)
+
+    expectEqual(newArrayElementList.count, 2)
+    expectNotNil(newArrayElementList.child(at: 0))
+    expectEqual("\(newArrayElementList.child(at: 0)!)", "0")
+
+    newArrayElementList = newArrayElementList.inserting(integerLiteralElement(2), at: 2)
+
+    expectEqual(newArrayElementList.count, 3)
+    expectNotNil(newArrayElementList.child(at: 2))
+    expectEqual("\(newArrayElementList.child(at: 2)!)", "2")
+}
+
+SyntaxCollectionsAPI.test("PrependingElement") {
+    let arrayElementList = SyntaxFactory.makeArrayElementList([
+        integerLiteralElement(1)
+    ])
+
+    let newArrayElementList = arrayElementList.prepending(integerLiteralElement(0))
+
+    expectEqual(newArrayElementList.count, 2)
+    expectNotNil(newArrayElementList.child(at: 0))
+    expectEqual("\(newArrayElementList.child(at: 0)!)", "0")
+}
+
+SyntaxCollectionsAPI.test("RemovingFirstElement") {
+    let arrayElementList = SyntaxFactory.makeArrayElementList([
+        integerLiteralElement(0),
+        integerLiteralElement(1)
+    ])
+
+    let newArrayElementList = arrayElementList.removingFirst()
+
+    expectEqual(newArrayElementList.count, 1)
+    expectNotNil(newArrayElementList.child(at: 0))
+    expectEqual("\(newArrayElementList.child(at: 0)!)", "1")
+}
+
+SyntaxCollectionsAPI.test("RemovingLastElement") {
+    let arrayElementList = SyntaxFactory.makeArrayElementList([
+        integerLiteralElement(0),
+        integerLiteralElement(1)
+    ])
+
+    let newArrayElementList = arrayElementList.removingLast()
+
+    expectEqual(newArrayElementList.count, 1)
+    expectNotNil(newArrayElementList.child(at: 0))
+    expectEqual("\(newArrayElementList.child(at: 0)!)", "0")
+}
+
+SyntaxCollectionsAPI.test("RemovingElement") {
+    let arrayElementList = SyntaxFactory.makeArrayElementList([
+        integerLiteralElement(0)
+    ])
+
+    let newArrayElementList = arrayElementList.removing(childAt: 0)
+
+    expectEqual(newArrayElementList.count, 0)
+    expectNil(newArrayElementList.child(at: 0))
+}
+
+SyntaxCollectionsAPI.test("ReplacingElement") {
+    let arrayElementList = SyntaxFactory.makeArrayElementList([
+        integerLiteralElement(0),
+        integerLiteralElement(1),
+        integerLiteralElement(2)
+    ])
+
+    let newArrayElementList = arrayElementList.replacing(childAt: 2,
+                                                         with: integerLiteralElement(3))
+
+    expectNotNil(newArrayElementList.child(at: 2))
+    expectEqual("\(newArrayElementList.child(at: 2)!)", "3")
+}
+
+runAllTests()

tools/SwiftSyntax/SyntaxCollections.swift.gyb

@@ -93,6 +93,24 @@ public struct ${node.name}: _SyntaxBase {
     return replacingLayout(newLayout)
   }
 
+  /// Creates a new `${node.name}` by replacing the syntax element
+  /// at the provided index.
+  ///
+  /// - Parameters:
+  ///   - index: The index at which to replace the element in the collection.
+  ///   - syntax: The element to replace with.
+  ///
+  /// - Returns: A new `${node.name}` with the new element at the provided index.
+  public func replacing(childAt index: Int,
+                        with syntax: ${node.collection_element_type}) -> ${node.name} {
+    var newLayout = data.raw.layout
+    /// Make sure the index is a valid index for replacing
+    precondition((newLayout.startIndex..<newLayout.endIndex).contains(index),
+                 "replacing node at invalid index \(index)")
+    newLayout[index] = syntax.raw
+    return replacingLayout(newLayout)
+  }
+
   /// Creates a new `${node.name}` by removing the syntax element at the
   /// provided index.
   ///