Add result builder convenience initializer for SubscriptDeclSyntax

Author: MoonMao42

Sources/SwiftSyntaxBuilder/SyntaxNodeWithBody.swift

@@ -371,6 +371,45 @@ extension SwitchExprSyntax {
   }
 }
 
+// MARK: - SubscriptDeclSyntax
+// SubscriptDeclSyntax is a special scenario as it don't have body or members
+// So we cannot conform to `HasTrailingCodeBlock` or `HasTrailingMemberDeclBlock`
+
+extension SubscriptDeclSyntax {
+  /// Construct a subscript with a single `get` accessor where `header` builds
+  /// the text before the opening `{` and `accessor` builds the accessor body.
+  ///
+  /// For example, to construct
+  ///
+  /// ```swift
+  /// subscript(_ index: Int) -> Element {
+  ///   return storage[index]
+  /// }
+  /// ```
+  ///
+  /// using this call
+  ///
+  /// ```swift
+  /// try SubscriptDeclSyntax("subscript(_ index: Int) -> Element") {
+  ///   StmtSyntax("return storage[index]")
+  /// }
+  /// ```
+  ///
+  /// Throws an error if `header` does not start a subscript declaration.
+  /// E.g. if calling `try SubscriptDeclSyntax("func foo") {}`
+  public init(
+    _ header: SyntaxNodeString,
+    @CodeBlockItemListBuilder accessor: () throws -> CodeBlockItemListSyntax
+  ) throws {
+    let decl = DeclSyntax("\(header) {}")
+    guard let castedDecl = decl.as(Self.self) else {
+      throw SyntaxStringInterpolationInvalidNodeTypeError(expectedType: Self.self, actualNode: decl)
+    }
+    self = castedDecl
+    self.accessorBlock = AccessorBlockSyntax(accessors: .getter(try accessor()))
+  }
+}
+
 // MARK: - VariableDeclSyntax
 // VariableDeclSyntax is a special scenario as it don't have body or members
 // So we cannot conform to `HasTrailingCodeBlock` or `HasTrailingMemberDeclBlock`

Tests/SwiftSyntaxBuilderTest/SubscriptTests.swift

@@ -0,0 +1,72 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2023 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import SwiftSyntax
+import SwiftSyntaxBuilder
+import XCTest
+
+final class SubscriptTests: XCTestCase {
+  func testSubscriptGetter() throws {
+    let buildable = try SubscriptDeclSyntax("subscript(_ index: Int) -> Element") {
+      StmtSyntax("return storage[index]")
+    }
+
+    assertBuildResult(
+      buildable,
+      """
+      subscript(_ index: Int) -> Element {
+          return storage[index]
+      }
+      """
+    )
+  }
+
+  func testSubscriptAccessorList() throws {
+    let buildable = try SubscriptDeclSyntax("subscript(_ index: Int) -> Element") {
+      AccessorDeclSyntax(accessorSpecifier: .keyword(.get)) {
+        StmtSyntax("return storage[index]")
+      }
+
+      AccessorDeclSyntax(accessorSpecifier: .keyword(.set)) {
+        ExprSyntax("storage[index] = newValue")
+      }
+    }
+
+    assertBuildResult(
+      buildable,
+      """
+      subscript(_ index: Int) -> Element {
+          get {
+              return storage[index]
+          }
+          set {
+              storage[index] = newValue
+          }
+      }
+      """
+    )
+  }
+
+  func testSubscriptProtocolRequirement() throws {
+    let buildable = DeclSyntax("subscript(_ index: Int) -> Element { get set }")
+
+    assertBuildResult(
+      buildable,
+      """
+      subscript(_ index: Int) -> Element {
+          get
+          set
+      }
+      """
+    )
+  }
+}