Correctly use UTF-8 indices in highlight calculations

Author: DougGregor

Sources/SwiftDiagnostics/DiagnosticsFormatter.swift

@@ -151,18 +151,19 @@ public struct DiagnosticsFormatter {
     }.mergingOverlappingRanges()
 
     // Map the column ranges into index ranges within the source string itself.
-    let sourceString = annotatedLine.sourceString
+    let sourceStringUTF8 = annotatedLine.sourceString.utf8
     let highlightIndexRanges: [Range<String.Index>] = highlightRanges.map { highlightRange in
-      let startIndex = sourceString.index(sourceString.startIndex, offsetBy: highlightRange.lowerBound - 1)
-      let endIndex = sourceString.index(startIndex, offsetBy: highlightRange.count)
+      let startIndex = sourceStringUTF8.index(sourceStringUTF8.startIndex, offsetBy: highlightRange.lowerBound - 1)
+      let endIndex = sourceStringUTF8.index(startIndex, offsetBy: highlightRange.count)
       return startIndex..<endIndex
     }
 
     // Form the annotated string by copying in text from the original source,
     // highlighting the column ranges.
     var resultSourceString: String = ""
-    var sourceIndex = sourceString.startIndex
     let annotation = ANSIAnnotation.sourceHighlight
+    let sourceString = annotatedLine.sourceString
+    var sourceIndex = sourceString.startIndex
     for highlightRange in highlightIndexRanges {
       // Text before the highlight range
       resultSourceString += sourceString[sourceIndex..<highlightRange.lowerBound]

Tests/SwiftDiagnosticsTest/DiagnosticsFormatterTests.swift

@@ -123,11 +123,11 @@ final class DiagnosticsFormatterTests: XCTestCase {
 
   func testColoringWithHighlights() {
     let source = """
-      for (i = 0; i != 10; i += 1) { }
+      for (i = 🐮; i != 👩‍👩‍👦‍👦; i += 1) { }
       """
 
     let expectedOutput = """
-      \u{001B}[0;36m1 │\u{001B}[0;0m for \u{001B}[4;39m(i\u{001B}[0;0m \u{001B}[4;39m= 0; i != 10; i += 1)\u{001B}[0;0m { }
+      \u{001B}[0;36m1 │\u{001B}[0;0m for \u{001B}[4;39m(i\u{001B}[0;0m \u{001B}[4;39m= 🐮; i != 👩‍👩‍👦‍👦; i += 1)\u{001B}[0;0m { }
         \u{001B}[0;36m∣\u{001B}[0;0m │      ╰─ \u{001B}[1;31merror: expected ')' to end tuple pattern\u{001B}[0;0m
         \u{001B}[0;36m∣\u{001B}[0;0m ╰─ \u{001B}[1;31merror: C-style for statement has been removed in Swift 3\u{001B}[0;0m