Make Identifier initializer failable

When trying to create an Identifier from a non-identifier token, the
initializer should fail, returning nil

Additionally the identifier property of the TokenSyntax should also return nil

Author: adammcarter

Sources/SwiftSyntax/Identifier.swift

@@ -15,8 +15,10 @@ public struct Identifier: Equatable, Hashable, Sendable {
   /// The sanitized `text` of a token.
   public let name: String
 
-  public init(_ token: TokenSyntax) {
-    let text = token.text
+  public init?(_ token: TokenSyntax) {
+    guard case .identifier(let text) = token.tokenKind else {
+      return nil
+    }
 
     self.name =
       if text.contains("`") {

Sources/SwiftSyntax/TokenSyntax.swift

@@ -154,7 +154,7 @@ public struct TokenSyntax: SyntaxProtocol, SyntaxHashable {
   }
 
   /// An identifier created from `self`.
-  public var identifier: Identifier {
+  public var identifier: Identifier? {
     Identifier(self)
   }
 

Tests/SwiftSyntaxTest/IdentifierTests.swift

@@ -16,17 +16,23 @@ import XCTest
 class IdentifierTests: XCTestCase {
   public func testInit() {
     let basicToken = TokenSyntax(stringLiteral: "sometoken")
-    XCTAssertEqual(Identifier(basicToken).name, "sometoken")
+    XCTAssertEqual(Identifier(basicToken)?.name, "sometoken")
 
     let backtickedToken = TokenSyntax(stringLiteral: "`backtickedtoken`")
-    XCTAssertEqual(Identifier(backtickedToken).name, "backtickedtoken")
+    XCTAssertEqual(Identifier(backtickedToken)?.name, "backtickedtoken")
 
     let multiBacktickedToken = TokenSyntax(stringLiteral: "```backtickedtoken```")
-    XCTAssertEqual(Identifier(multiBacktickedToken).name, "backtickedtoken")
+    XCTAssertEqual(Identifier(multiBacktickedToken)?.name, "backtickedtoken")
+
+    let nonIdentifierToken = DeclSyntax("let a = 1").firstToken(viewMode: .all)!
+    XCTAssertNil(Identifier(nonIdentifierToken))
   }
 
   public func testTokenSyntaxIdentifier() {
     let tokenSyntax = TokenSyntax(stringLiteral: "sometoken")
     XCTAssertEqual(tokenSyntax.identifier, Identifier(tokenSyntax))
+
+    let nonIdentifierToken = DeclSyntax("let a = 1").firstToken(viewMode: .all)!
+    XCTAssertNil(nonIdentifierToken.identifier)
   }
 }