Improve folding ranges if editor only supports line folding

If the folding range doesn't end at the end of the last line, exclude that line from the folding range since the end line gets folded away. This means if we reported `end.line`, we would eg. fold away the `}` that matches a `{`, which looks surprising.

If the folding range does end at the end of the line we are in cases that don't have a closing indicator (like comments), so we can fold the last line as well.

Author: ahoppen

Sources/SourceKitLSP/Swift/FoldingRange.swift

@@ -12,6 +12,7 @@
 
 import LSPLogging
 import LanguageServerProtocol
+import SKSupport
 import SwiftSyntax
 
 fileprivate final class FoldingRangeFinder: SyntaxAnyVisitor {
@@ -210,9 +211,21 @@ fileprivate final class FoldingRangeFinder: SyntaxAnyVisitor {
     let end = snapshot.positionOf(utf8Offset: end.utf8Offset)
     let range: FoldingRange
     if lineFoldingOnly {
+      // If the folding range doesn't end at the end of the last line, exclude that line from the folding range since
+      // the end line gets folded away. This means if we reported `end.line`, we would eg. fold away the `}` that
+      // matches a `{`, which looks surprising.
+      // If the folding range does end at the end of the line we are in cases that don't have a closing indicator (like
+      // comments), so we can fold the last line as well.
+      let endLine: Int
+      if snapshot.lineTable.isAtEndOfLine(end) {
+        endLine = end.line
+      } else {
+        endLine = end.line - 1
+      }
+
       // Since the client cannot fold less than a single line, if the
       // fold would span 1 line there's no point in reporting it.
-      guard end.line > start.line else {
+      guard endLine > start.line else {
         return .visitChildren
       }
 
@@ -221,7 +234,7 @@ fileprivate final class FoldingRangeFinder: SyntaxAnyVisitor {
       range = FoldingRange(
         startLine: start.line,
         startUTF16Index: nil,
-        endLine: end.line,
+        endLine: endLine,
         endUTF16Index: nil,
         kind: kind
       )
@@ -264,3 +277,14 @@ extension SwiftLanguageService {
     return ranges.sorted()
   }
 }
+
+fileprivate extension LineTable {
+  func isAtEndOfLine(_ position: Position) -> Bool {
+    guard position.line >= 0, position.line < self.count else {
+      return false
+    }
+    let line = self[position.line]
+    let suffixAfterPositionColumn = line[line.utf16.index(line.startIndex, offsetBy: position.utf16index)...]
+    return suffixAfterPositionColumn.allSatisfy(\.isNewline)
+  }
+}

Tests/SourceKitLSPTests/FoldingRangeTests.swift

@@ -96,17 +96,63 @@ final class FoldingRangeTests: XCTestCase {
     try await assertFoldingRanges(
       markedSource: """
         1️⃣func foo() {
-
-        2️⃣}
-        """
-      ,
+        2️⃣
+        }
+        """,
       expectedRanges: [
         FoldingRangeSpec(from: "1️⃣", to: "2️⃣")
       ],
       lineFoldingOnly: true
     )
   }
 
+  func testLineFoldingOfFunctionWithMultiLineParameters() async throws {
+    try await assertFoldingRanges(
+      markedSource: """
+        1️⃣func foo(
+        2️⃣  param: Int
+        3️⃣) {
+          print(param)
+        4️⃣
+        }
+        """,
+      expectedRanges: [
+        FoldingRangeSpec(from: "1️⃣", to: "2️⃣"),
+        FoldingRangeSpec(from: "3️⃣", to: "4️⃣"),
+      ],
+      lineFoldingOnly: true
+    )
+  }
+
+  func testLineFoldingOfComment() async throws {
+    try await assertFoldingRanges(
+      markedSource: """
+        1️⃣// abc
+        // def
+        2️⃣// ghi
+
+        """,
+      expectedRanges: [
+        FoldingRangeSpec(from: "1️⃣", to: "2️⃣", kind: .comment)
+      ],
+      lineFoldingOnly: true
+    )
+  }
+
+  func testLineFoldingOfCommentAtEndOfFile() async throws {
+    try await assertFoldingRanges(
+      markedSource: """
+        1️⃣// abc
+        // def
+        2️⃣// ghi
+        """,
+      expectedRanges: [
+        FoldingRangeSpec(from: "1️⃣", to: "2️⃣", kind: .comment)
+      ],
+      lineFoldingOnly: true
+    )
+  }
+
   func testLineFoldingDoesntReportSingleLine() async throws {
     try await assertFoldingRanges(
       markedSource: """
@@ -272,8 +318,8 @@ final class FoldingRangeTests: XCTestCase {
     try await assertFoldingRanges(
       markedSource: """
         let x = [1️⃣
-          1: "one", 
-          2: "two", 
+          1: "one",
+          2: "two",
           3: "three"
         2️⃣]
         """,