Implementation of IdentifiersMustBeASCII (swiftlang#62)

Author: atamez31

tools/swift-format/Sources/Rules/IdentifiersMustBeASCII.swift

@@ -8,5 +8,18 @@ import SwiftSyntax
 ///
 /// - SeeAlso: https://google.github.io/swift#identifiers
 public final class IdentifiersMustBeASCII: SyntaxLintRule {
+  public override func visit(_ node: IdentifierPatternSyntax) {
+    let identifier = node.identifier.text
+    let invalidCharacters = identifier.unicodeScalars.filter { !$0.isASCII }.map { $0.description }
+    
+    if !invalidCharacters.isEmpty {
+      diagnose(.nonASCIICharsNotAllowed(invalidCharacters, identifier), on: node)
+    }
+  }
+}
 
+extension Diagnostic.Message {
+  static func nonASCIICharsNotAllowed(_ invalidCharacters: [String], _ identifierName: String) -> Diagnostic.Message {
+    return .init(.warning, "The identifier '\(identifierName)' contains the following non-ASCII characters: \(invalidCharacters.joined(separator: ", "))")
+  }
 }

tools/swift-format/Tests/SwiftFormatTests/IdentifiersMustBeASCIITests.swift

@@ -0,0 +1,26 @@
+import Foundation
+import SwiftSyntax
+import XCTest
+
+@testable import Rules
+
+public class IdentifiersMustBeASCIITests: DiagnosingTestCase {
+  public func testInvalidIdentifiers() {
+    let input =
+    """
+      let Te$t = 1
+      var fo😎o = 2
+      let Δx = newX - previousX
+      var 🤩😆 = 20
+      """
+    performLint(IdentifiersMustBeASCII.self, input: input)
+    XCTAssertDiagnosed(.nonASCIICharsNotAllowed(["😎"],"fo😎o"))
+    XCTAssertDiagnosed(.nonASCIICharsNotAllowed(["🤩", "😆"], "🤩😆"))
+  }
+  
+  #if !os(macOS)
+  static let allTests = [
+    IdentifiersMustBeASCIITests.testInvalidIdentifiers,
+    ]
+  #endif
+}