Add option to customize number of spaces leading // comments

Add an option `spacesBeforeLineComments` that allows customization of
number of spaces between the first non-trivial token and a line comment
with the `//` prefix. Defaults to 2, the historical default value.

Author: dduan

Documentation/Configuration.md

@@ -31,6 +31,9 @@ top-level keys and values:
     lines that are allowed to be present in a source file. Any number larger
     than this will be collapsed down to the maximum.
 
+*   `spacesBeforeEndOfLineComments` _(number)_: The number of spaces between 
+    the last token on a non-empty line and a line comment starting with `//`.
+
 *   `respectsExistingLineBreaks` _(boolean)_: Indicates whether or not existing
     line breaks in the source code should be honored (if they are valid
     according to the style guidelines being enforced). If this settings is

Sources/SwiftFormat/API/Configuration+Default.swift

@@ -26,6 +26,7 @@ extension Configuration {
     self.lineLength = 100
     self.tabWidth = 8
     self.indentation = .spaces(2)
+    self.spacesBeforeEndOfLineComments = 2
     self.respectsExistingLineBreaks = true
     self.lineBreakBeforeControlFlowKeywords = false
     self.lineBreakBeforeEachArgument = false

Sources/SwiftFormat/API/Configuration.swift

@@ -28,6 +28,7 @@ public struct Configuration: Codable, Equatable {
     case version
     case maximumBlankLines
     case lineLength
+    case spacesBeforeEndOfLineComments
     case tabWidth
     case indentation
     case respectsExistingLineBreaks
@@ -66,6 +67,9 @@ public struct Configuration: Codable, Equatable {
   /// The maximum length of a line of source code, after which the formatter will break lines.
   public var lineLength: Int
 
+  // Number of spaces that preceeds line comments.
+  public var spacesBeforeEndOfLineComments: Int
+
   /// The width of the horizontal tab in spaces.
   ///
   /// This value is used when converting indentation types (for example, from tabs into spaces).
@@ -225,6 +229,9 @@ public struct Configuration: Codable, Equatable {
     self.lineLength =
       try container.decodeIfPresent(Int.self, forKey: .lineLength)
       ?? defaults.lineLength
+    self.spacesBeforeEndOfLineComments =
+      try container.decodeIfPresent(Int.self, forKey: .spacesBeforeEndOfLineComments)
+      ?? defaults.spacesBeforeEndOfLineComments
     self.tabWidth =
       try container.decodeIfPresent(Int.self, forKey: .tabWidth)
       ?? defaults.tabWidth
@@ -288,6 +295,7 @@ public struct Configuration: Codable, Equatable {
     try container.encode(version, forKey: .version)
     try container.encode(maximumBlankLines, forKey: .maximumBlankLines)
     try container.encode(lineLength, forKey: .lineLength)
+    try container.encode(spacesBeforeEndOfLineComments, forKey: .spacesBeforeEndOfLineComments)
     try container.encode(tabWidth, forKey: .tabWidth)
     try container.encode(indentation, forKey: .indentation)
     try container.encode(respectsExistingLineBreaks, forKey: .respectsExistingLineBreaks)

Sources/SwiftFormat/PrettyPrint/TokenStreamCreator.swift

@@ -3171,7 +3171,7 @@ fileprivate final class TokenStreamCreator: SyntaxVisitor {
       return (
         true,
         [
-          .space(size: 2, flexible: true),
+          .space(size: config.spacesBeforeEndOfLineComments, flexible: true),
           .comment(Comment(kind: .line, text: text), wasEndOfLine: true),
           // There must be a break with a soft newline after the comment, but it's impossible to
           // know which kind of break must be used. Adding this newline is deferred until the

Tests/SwiftFormatTests/PrettyPrint/CommentTests.swift

@@ -1,4 +1,5 @@
 import _SwiftFormatTestSupport
+import SwiftFormat
 
 final class CommentTests: PrettyPrintTestCase {
   func testDocumentationComments() {
@@ -199,6 +200,152 @@ final class CommentTests: PrettyPrintTestCase {
     assertPrettyPrintEqual(input: input, expected: expected, linelength: 45)
   }
 
+  func testLineCommentsWithCustomLeadingSpaces() {
+    let pairs: [(String, String)] = [
+        (
+            """
+            // Line Comment0
+
+            // Line Comment1
+            // Line Comment2
+            let a = 123
+            let b = "456"  // End of line comment
+            let c = "More content"
+
+            """,
+            """
+            // Line Comment0
+
+            // Line Comment1
+            // Line Comment2
+            let a = 123
+            let b = "456"   // End of line comment
+            let c = "More content"
+
+            """
+        ),
+        (
+          """
+          // Comment 3
+          // Comment 4
+
+          let reallyLongVariableName = 123 // This comment should not wrap
+          // and should not combine with this comment
+
+          func MyFun() {
+            // just a comment
+          }
+          """,
+          """
+          // Comment 3
+          // Comment 4
+
+          let reallyLongVariableName = 123   // This comment should not wrap
+          // and should not combine with this comment
+
+          func MyFun() {
+            // just a comment
+          }
+
+          """
+        ),
+        (
+          """
+          func MyFun() {
+            // Comment 1
+            // Comment 2
+            let a = 123
+
+            let b = 456  // Comment 3
+          }
+
+          func MyFun() {
+            let c = 789 // Comment 4
+            // Comment 5
+          }
+          """,
+          """
+          func MyFun() {
+            // Comment 1
+            // Comment 2
+            let a = 123
+
+            let b = 456   // Comment 3
+          }
+
+          func MyFun() {
+            let c = 789   // Comment 4
+            // Comment 5
+          }
+
+          """
+        ),
+        (
+          """
+          let a = myfun(123 // Cmt 7
+          )
+          let a = myfun(var1: 123 // Cmt 7
+          )
+
+          guard condition else { return // Cmt 6
+          }
+
+          switch myvar {
+          case .one, .two, // three
+               .four:
+            dostuff()
+          default: ()
+          }
+
+          """,
+          """
+          let a = myfun(
+            123   // Cmt 7
+          )
+          let a = myfun(
+            var1: 123   // Cmt 7
+          )
+
+          guard condition else {
+            return   // Cmt 6
+          }
+
+          switch myvar {
+          case .one, .two,   // three
+            .four:
+            dostuff()
+          default: ()
+          }
+
+          """
+        ),
+        (
+          """
+          let a = 123 +  // comment
+            b + c
+
+          let d = 123
+          // Trailing Comment
+          """,
+          """
+          let a =
+            123   // comment
+            + b + c
+
+          let d = 123
+          // Trailing Comment
+
+          """
+        ),
+    ]
+
+    var config = Configuration.forTesting
+    config.spacesBeforeEndOfLineComments = 3
+    for (input, expected) in pairs {
+      assertPrettyPrintEqual(input: input, expected: expected, linelength: 45, configuration: config)
+    }
+  }
+
   func testContainerLineComments() {
     let input =
       """
@@ -274,6 +421,82 @@ final class CommentTests: PrettyPrintTestCase {
     assertPrettyPrintEqual(input: input, expected: expected, linelength: 80)
   }
 
+  func testContainerLineCommentsWithCustomLeadingSpaces() {
+    let input =
+      """
+      // Array comment
+      let a = [456, // small comment
+        789]
+
+      // Dictionary comment
+      let b = ["abc": 456, // small comment
+        "def": 789]
+
+      // Trailing comment
+      let c = [123, 456  // small comment
+      ]
+
+      // Multiline comment
+      let d = [123,
+        // comment line 1
+        // comment line 2
+        456
+      ]
+
+      /* Array comment */
+      let a = [456, /* small comment */
+        789]
+
+      /* Dictionary comment */
+      let b = ["abc": 456, /* small comment */
+        "def": 789]
+      """
+
+    let expected =
+      """
+      // Array comment
+      let a = [
+        456, // small comment
+        789,
+      ]
+
+      // Dictionary comment
+      let b = [
+        "abc": 456, // small comment
+        "def": 789,
+      ]
+
+      // Trailing comment
+      let c = [
+        123, 456, // small comment
+      ]
+
+      // Multiline comment
+      let d = [
+        123,
+        // comment line 1
+        // comment line 2
+        456,
+      ]
+
+      /* Array comment */
+      let a = [
+        456, /* small comment */
+        789,
+      ]
+
+      /* Dictionary comment */
+      let b = [
+        "abc": 456, /* small comment */
+        "def": 789,
+      ]
+
+      """
+    var config = Configuration.forTesting
+    config.spacesBeforeEndOfLineComments = 1
+    assertPrettyPrintEqual(input: input, expected: expected, linelength: 80, configuration: config)
+  }
+
   func testDocumentationBlockComments() {
     let input =
       """