Add simple test coverage

Author: rauhul

Package.swift

@@ -208,6 +208,11 @@ let package = Package(
       dependencies: ["_SwiftSyntaxTestSupport", "SwiftDiagnostics", "SwiftParser", "SwiftSyntaxMacros", "SwiftSyntaxMacroExpansion"]
     ),
 
+    .testTarget(
+      name: "SwiftSyntaxMacrosTestSupportTests",
+      dependencies: ["SwiftDiagnostics", "SwiftSyntax", "SwiftSyntaxMacros", "SwiftSyntaxMacrosTestSupport"]
+    ),
+
     // MARK: SwiftParser
 
     .target(

Sources/SwiftSyntaxMacrosTestSupport/Assertions.swift

@@ -128,7 +128,7 @@ public struct DiagnosticSpec {
   /// The expected severity of the diagnostic
   public let severity: DiagnosticSeverity
 
-  /// If not `nil`, the text the diagnostic is expected to highlight
+  /// If not `nil`, the text fragments the diagnostic is expected to highlight
   public let highlights: [String]?
 
   /// The notes that are expected to be attached to the diagnostic
@@ -237,7 +237,7 @@ func assertDiagnostic(
       XCTFail(
         """
         Expected \(highlights.count) highlights but received \(diag.highlights.count):
-        \(diag.highlights.map(\.debugDescription).joined(separator: "\n"))
+        \(diag.highlights.map(\.trimmedDescription).joined(separator: "\n"))
         """,
         file: spec.originatorFile,
         line: spec.originatorLine

Tests/SwiftSyntaxMacrosTestSupportTests/AssertionsTests.swift

@@ -0,0 +1,106 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 SwiftDiagnostics
+import SwiftSyntax
+import SwiftSyntaxMacros
+import SwiftSyntaxMacrosTestSupport
+import XCTest
+
+final class AssertionsTests: XCTestCase {
+  struct OnlyStruct: DiagnosticMessage {
+    var message = "'@NoStruct' cannot be applied to struct types"
+    var diagnosticID = MessageID(domain: "\(AssertionsTests.self)", id: "\(OnlyStruct.self)")
+    var severity = DiagnosticSeverity.error
+  }
+
+  struct NoStruct: MemberMacro {
+    static func expansion(
+      of node: AttributeSyntax,
+      providingMembersOf declaration: some DeclGroupSyntax,
+      conformingTo protocols: [TypeSyntax],
+      in context: some MacroExpansionContext
+    ) throws -> [DeclSyntax] {
+      if let structDecl = declaration.as(StructDeclSyntax.self) {
+        context.diagnose(
+          .init(
+            node: structDecl.structKeyword,
+            message: OnlyStruct()
+          )
+        )
+      }
+      return []
+    }
+  }
+
+  struct NoStructMultiHighlight: MemberMacro {
+    static func expansion(
+      of node: AttributeSyntax,
+      providingMembersOf declaration: some DeclGroupSyntax,
+      conformingTo protocols: [TypeSyntax],
+      in context: some MacroExpansionContext
+    ) throws -> [DeclSyntax] {
+      if let structDecl = declaration.as(StructDeclSyntax.self) {
+        context.diagnose(
+          .init(
+            node: structDecl.structKeyword,
+            message: OnlyStruct(),
+            highlights: [Syntax(structDecl.structKeyword), Syntax(structDecl.name)]
+          )
+        )
+      }
+      return []
+    }
+  }
+
+  func testAssertMacroExpansionIgnoresHighlightMatchingIfNil() {
+    assertMacroExpansion(
+      "@NoStruct struct S { }",
+      expandedSource: "struct S { }",
+      diagnostics: [
+        .init(message: OnlyStruct().message, line: 1, column: 11)
+      ],
+      macros: ["NoStruct": NoStruct.self]
+    )
+  }
+
+  func testAssertMacroExpansionMatchesSingleStringHighlight() {
+    assertMacroExpansion(
+      "@NoStruct struct S { }",
+      expandedSource: "struct S { }",
+      diagnostics: [
+        .init(message: OnlyStruct().message, line: 1, column: 11, highlight: "struct")
+      ],
+      macros: ["NoStruct": NoStruct.self]
+    )
+  }
+
+  func testAssertMacroExpansionMatchesArrayHighlight() {
+    assertMacroExpansion(
+      "@NoStruct struct S { }",
+      expandedSource: "struct S { }",
+      diagnostics: [
+        .init(message: OnlyStruct().message, line: 1, column: 11, highlight: "struct")
+      ],
+      macros: ["NoStruct": NoStruct.self]
+    )
+
+    assertMacroExpansion(
+      "@NoStruct struct S { }",
+      expandedSource: "struct S { }",
+      diagnostics: [
+        .init(message: OnlyStruct().message, line: 1, column: 11, highlights: ["struct", "S"])
+      ],
+      macros: ["NoStruct": NoStructMultiHighlight.self]
+    )
+  }
+}