[Lint] Adjust capitalization rule to properly handle underscored names

Author: xedin

Sources/SwiftFormatRules/TypeNamesShouldBeCapitalized.swift

@@ -53,15 +53,20 @@ public final class TypeNamesShouldBeCapitalized : SyntaxLintRule {
   }
 
   private func diagnoseNameConventionMismatch<T: DeclSyntaxProtocol>(_ type: T, name: TokenSyntax) {
-    if let firstChar = name.text.first, !firstChar.isUppercase {
+    let leadingUnderscores = name.text.prefix { $0 == "_" }
+    if let firstChar = name.text[leadingUnderscores.endIndex...].first,
+       firstChar.uppercased() != String(firstChar) {
       diagnose(.capitalizeTypeName(name: name.text), on: type, severity: .convention)
     }
   }
 }
 
 extension Finding.Message {
   public static func capitalizeTypeName(name: String) -> Finding.Message {
-    let capitalized = name.prefix(1).uppercased() + name.dropFirst()
+    var capitalized = name
+    let leadingUnderscores = capitalized.prefix { $0 == "_" }
+    let charAt = leadingUnderscores.endIndex
+    capitalized.replaceSubrange(charAt...charAt, with: capitalized[charAt].uppercased())
     return "type names should be capitalized: \(name) -> \(capitalized)"
   }
 }

Tests/SwiftFormatRulesTests/TypeNamesShouldBeCapitalizedTests.swift

@@ -71,4 +71,58 @@ final class TypeNamesShouldBeCapitalizedTests: LintOrFormatRuleTestCase {
     XCTAssertDiagnosed(.capitalizeTypeName(name: "data"))
     XCTAssertNotDiagnosed(.capitalizeTypeName(name: "Value"))
   }
+
+  func testThatUnderscoredNamesAreDiagnosed() {
+    let input =
+      """
+      protocol _p {
+        associatedtype _value
+        associatedtype __Value
+      }
+
+      protocol ___Q {
+      }
+
+      struct _data {
+        typealias _x = Int
+      }
+
+      struct _Data {}
+
+      actor _internalActor {}
+
+      enum __e {
+      }
+
+      enum _OtherE {
+      }
+
+      func test() {
+        class _myClass {}
+        do {
+          class _MyClass {}
+        }
+      }
+
+      distributed actor __greeter {}
+      distributed actor __InternalGreeter {}
+      """
+
+    performLint(TypeNamesShouldBeCapitalized.self, input: input)
+
+    XCTAssertDiagnosed(.capitalizeTypeName(name: "_p"))
+    XCTAssertNotDiagnosed(.capitalizeTypeName(name: "___Q"))
+    XCTAssertDiagnosed(.capitalizeTypeName(name: "_value"))
+    XCTAssertNotDiagnosed(.capitalizeTypeName(name: "__Value"))
+    XCTAssertDiagnosed(.capitalizeTypeName(name: "_data"))
+    XCTAssertNotDiagnosed(.capitalizeTypeName(name: "_Data"))
+    XCTAssertDiagnosed(.capitalizeTypeName(name: "_x"))
+    XCTAssertDiagnosed(.capitalizeTypeName(name: "_internalActor"))
+    XCTAssertDiagnosed(.capitalizeTypeName(name: "__e"))
+    XCTAssertNotDiagnosed(.capitalizeTypeName(name: "__OtherE"))
+    XCTAssertDiagnosed(.capitalizeTypeName(name: "_myClass"))
+    XCTAssertNotDiagnosed(.capitalizeTypeName(name: "_MyClass"))
+    XCTAssertDiagnosed(.capitalizeTypeName(name: "__greeter"))
+    XCTAssertNotDiagnosed(.capitalizeTypeName(name: "__InternalGreeter"))
+  }
 }