Better diagnostic message

Author: wickwirew

Sources/Compiler/DiagnosticReporter.swift

@@ -13,12 +13,46 @@ public struct StdoutDiagnosticReporter: DiagnosticReporter {
     public init() {}
 
     public func report(diagnostic: Diagnostic, source: String, fileName: String) {
-        let source = source[diagnostic.location.range]
+        let range = diagnostic.location.range
+        let start = startOfLine(index: range.lowerBound, source: source)
+        // Note: This uses `lowerBound` as well to make sure we only get one line
+        let end = endOfLine(index: range.lowerBound, source: source)
+        let source = source[start ..< end]
+        let distanceToStart = source.distance(from: start, to: range.lowerBound)
+        let indent = String(repeating: " ", count: distanceToStart)
+        let underline = String(repeating: "^", count: source.distance(from: range.lowerBound, to: range.upperBound))
 
         print("""
+        Error in \(fileName)
+        
         \(source)
+        \(indent)\(underline)
         
         \(diagnostic.message)
         """)
     }
+    
+    private func startOfLine(
+        index: String.Index,
+        source: String
+    ) -> Substring.Index {
+        var index = index
+        while index > source.startIndex {
+            let nextIndex = source.index(before: index)
+            guard !source[nextIndex].isNewline else { break }
+            index = nextIndex
+        }
+        return index
+    }
+    
+    private func endOfLine(
+        index: String.Index,
+        source: String
+    ) -> Substring.Index {
+        var index = index
+        while index < source.endIndex, !source[index].isNewline {
+            index = source.index(after: index)
+        }
+        return index
+    }
 }