Remove trailing spaces from comments.

This removes trailing spaces at the end of each line in all types of comments. While Xcode supports automatically deleting these spaces, not all users enable it or even use Xcode for all development.

Author: dylansturg

Sources/SwiftFormatPrettyPrint/Comment.swift

@@ -14,6 +14,24 @@ import Foundation
 import SwiftFormatConfiguration
 import SwiftSyntax
 
+extension StringProtocol {
+  /// Trims whitespace from the end of a string, returning a new string with no trailing whitespace.
+  ///
+  /// If the string is only whitespace, an empty string is returned.
+  ///
+  /// - Returns: The string with trailing whitespace removed.
+  func trimmingTrailingWhitespace() -> String {
+    if isEmpty { return String() }
+    let scalars = unicodeScalars
+    var idx = scalars.index(before: scalars.endIndex)
+    while scalars[idx].properties.isWhitespace {
+      if idx == scalars.startIndex { return String() }
+      idx = scalars.index(before: idx)
+    }
+    return String(String.UnicodeScalarView(scalars[...idx]))
+  }
+}
+
 struct Comment {
   enum Kind {
     case line, docLine, block, docBlock
@@ -47,17 +65,23 @@ struct Comment {
 
     switch kind {
     case .line, .docLine:
-      self.text = [text]
+      self.text = [text.trimmingTrailingWhitespace()]
       self.text[0].removeFirst(kind.prefixLength)
       self.length = self.text.reduce(0, { $0 + $1.count + kind.prefixLength + 1 })
 
     case .block, .docBlock:
       var fulltext: String = text
       fulltext.removeFirst(kind.prefixLength)
       fulltext.removeLast(2)
-      self.text = fulltext.split(separator: "\n", omittingEmptySubsequences: false).map {
-        String($0)
+      let lines = fulltext.split(separator: "\n", omittingEmptySubsequences: false)
+
+      // The last line in a block style comment contains the "*/" pattern to end the comment. The
+      // trailing space(s) need to be kept in that line to have space between text and "*/".
+      var trimmedLines = lines.dropLast().map({ $0.trimmingTrailingWhitespace() })
+      if let lastLine = lines.last {
+        trimmedLines.append(String(lastLine))
       }
+      self.text = trimmedLines
       self.length = self.text.reduce(0, { $0 + $1.count }) + kind.prefixLength + 3
     }
   }

Sources/SwiftFormatPrettyPrint/TokenStreamCreator.swift

@@ -2730,10 +2730,15 @@ fileprivate final class TokenStreamCreator: SyntaxVisitor {
       case .blockComment(let text):
         if index > 0 || isStartOfFile {
           appendToken(.comment(Comment(kind: .block, text: text), wasEndOfLine: false))
-          // We place a size-0 break after the comment to allow a discretionary newline after the
-          // comment if the user places one here but the comment is otherwise adjacent to a text
-          // token.
-          appendToken(.break(.same, size: 0))
+          // There is always a break after the comment to allow a discretionary newline after it.
+          var breakSize = 0
+          if index + 1 < trivia.endIndex {
+            let nextPiece = trivia[index + 1]
+            // The original number of spaces is intentionally discarded, but 1 space is allowed in
+            // case the comment is followed by another token instead of a newline.
+            if case .spaces = nextPiece { breakSize = 1 }
+          }
+          appendToken(.break(.same, size: breakSize))
           isStartOfFile = false
         }
         requiresNextNewline = false

Tests/SwiftFormatPrettyPrintTests/CommentTests.swift

@@ -639,5 +639,47 @@ final class CommentTests: PrettyPrintTestCase {
     assertPrettyPrintEqual(input: input, expected: expected, linelength: 100)
   }
 
+  func testTrailingSpacesInComments() {
+    // Xcode has a commonly enabled setting to delete trailing spaces, which also applies to
+    // multi-line strings. The trailing spaces are intentionally written using a unicode escape
+    // sequence to ensure they aren't deleted.
+    let input = """
+      /// This is a trailing space documentation comment.\u{0020}\u{0020}\u{0020}\u{0020}\u{0020}
+      ///        This is a leading space documentation comment.
+      func foo() {
+        //       leading spaces are fine
+        // trailing spaces should go\u{0020}\u{0020}\u{0020}\u{0020}\u{0020}
+        let out = "123"
+      }
 
+      /**
+       *    This is a leading space doc block comment
+       * This is a trailing space doc block comment\u{0020}\u{0020}\u{0020}\u{0020}\u{0020}
+       */
+      func foo() {
+        /*    block comment    */ let out = "123"
+      }
+      """
+
+    let expected = """
+      /// This is a trailing space documentation comment.
+      ///        This is a leading space documentation comment.
+      func foo() {
+        //       leading spaces are fine
+        // trailing spaces should go
+        let out = "123"
+      }
+
+      /**
+       *    This is a leading space doc block comment
+       * This is a trailing space doc block comment
+       */
+      func foo() {
+        /*    block comment    */ let out = "123"
+      }
+
+      """
+
+    assertPrettyPrintEqual(input: input, expected: expected, linelength: 60)
+  }
 }