Added a formatter to pretty print diagnostics

Package.swift

@@ -151,6 +151,10 @@ let package = Package(
       name: "SwiftBasicFormatTest",
       dependencies: ["SwiftBasicFormat"]
     ),
+    .testTarget(
+      name: "SwiftDiagnosticsTest",
+      dependencies: ["SwiftDiagnostics", "SwiftParser"]
+    ),
     .testTarget(
       name: "SwiftSyntaxTest",
       dependencies: ["SwiftSyntax", "_SwiftSyntaxTestSupport"]

Sources/SwiftDiagnostics/DiagnosticsFormatter.swift

@@ -0,0 +1,112 @@
+//===------------ DiagnosticsFormatter.swift - Formatter for diagnostics ----------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2022 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import SwiftSyntax
+
+public struct DiagnosticsFormatter {
+
+  /// A wrapper struct for a source line and its diagnostics
+  private struct AnnotatedSourceLine {
+    var diagnostics: [Diagnostic]
+    var sourceString: String
+  }
+
+  /// Number of lines which should be printed before and after the diagnostic message
+  static let contextSize = 2
+
+  /// Print given diagnostics for a given syntax tree on the command line
+  public static func annotatedSource(tree: SourceFileSyntax, diags: [Diagnostic]) -> String {
+    let slc = SourceLocationConverter(file: "", tree: tree)
+
+    // First, we need to put each line and its diagnostics together
+    var annotatedSourceLines = [AnnotatedSourceLine]()
+
+    for (sourceLineIndex, sourceLine) in slc.sourceLines.enumerated() {
+      let diagsForLine = diags.filter { diag in
+        return diag.location(converter: slc).line == (sourceLineIndex + 1)
+      }
+      annotatedSourceLines.append(AnnotatedSourceLine(diagnostics: diagsForLine, sourceString: sourceLine))
+    }
+
+    // Only lines with diagnostic messages should be printed, but including some context
+    let rangesToPrint = annotatedSourceLines.enumerated().compactMap { (lineIndex, sourceLine) -> Range<Int>? in
+      let lineNumber = lineIndex + 1
+      if !sourceLine.diagnostics.isEmpty {
+        return Range<Int>(uncheckedBounds: (lower: lineNumber - Self.contextSize, upper: lineNumber + Self.contextSize + 1))
+      }
+      return nil
+    }
+
+    var annotatedSource = ""
+
+    /// Keep track if a line missing char should be printed
+    var hasLineBeenSkipped = false
+
+    let maxNumberOfDigits = String(annotatedSourceLines.count).count
+
+    for (lineIndex, annotatedLine) in annotatedSourceLines.enumerated() {
+      let lineNumber = lineIndex + 1
+      guard rangesToPrint.contains(where: { range in
+        range.contains(lineNumber)
+      }) else {
+        hasLineBeenSkipped = true
+        continue
+      }
+
+      // line numbers should be right aligned
+      let lineNumberString = String(lineNumber)
+      let leadingSpaces = String(repeating: " ", count: maxNumberOfDigits - lineNumberString.count)
+      let linePrefix = "\(leadingSpaces)\(lineNumberString) │ "
+
+      // If necessary, print a line that indicates that there was lines skipped in the source code
+      if hasLineBeenSkipped && !annotatedSource.isEmpty {
+        let lineMissingInfoLine = String(repeating: " ", count: maxNumberOfDigits) + " ┆"
+        annotatedSource.append("\(lineMissingInfoLine)\n")
+      }
+      hasLineBeenSkipped = false
+
+      // print the source line
+      annotatedSource.append("\(linePrefix)\(annotatedLine.sourceString)")
+
+      // If the line did not end with \n (e.g. the last line), append it manually
+      if annotatedSource.last != "\n" {
+        annotatedSource.append("\n")
+      }
+
+      let columnsWithDiagnostics = Set(annotatedLine.diagnostics.map { $0.location(converter: slc).column ?? 0 })
+      let diagsPerColumn = Dictionary(grouping: annotatedLine.diagnostics) { diag in
+        diag.location(converter: slc).column ?? 0
+      }.sorted { lhs, rhs in
+        lhs.key > rhs.key
+      }
+
+      for (column, diags) in diagsPerColumn {
+        // compute the string that is shown before each message
+        var preMessage = String(repeating: " ", count: maxNumberOfDigits) + " ∣"
+        for c in 0..<column {
+          if columnsWithDiagnostics.contains(c) {
+            preMessage.append("│")
+          } else {
+            preMessage.append(" ")
+          }
+        }
+
+        for diag in diags.dropLast(1) {
+          annotatedSource.append("\(preMessage)├─ \(diag.message)\n")
+        }
+        annotatedSource.append("\(preMessage)╰─ \(diags.last!.message)\n")
+
+      }
+    }
+    return annotatedSource
+  }
+}

Sources/SwiftSyntax/SourceLocation.swift

@@ -114,6 +114,8 @@ public struct SourceRange: Hashable, Codable, CustomDebugStringConvertible {
 /// part of the same tree that was used to initialize this class.
 public final class SourceLocationConverter {
   let file: String
+  /// The source of the file, modelled as data so it can contain invalid UTF-8.
+  let source: [UInt8]
   /// Array of lines and the position at the start of the line.
   let lines: [AbsolutePosition]
   /// Position at end of file.
@@ -125,6 +127,7 @@ public final class SourceLocationConverter {
   public init<SyntaxType: SyntaxProtocol>(file: String, tree: SyntaxType) {
     assert(tree.parent == nil, "SourceLocationConverter must be passed the root of the syntax tree")
     self.file = file
+    self.source = tree.syntaxTextBytes
     (self.lines, endOfFile) = computeLines(tree: Syntax(tree))
     assert(tree.byteSize == endOfFile.utf8Offset)
   }
@@ -134,9 +137,35 @@ public final class SourceLocationConverter {
   ///   - source: The source code to convert positions to line/columns for.
   public init(file: String, source: String) {
     self.file = file
+    self.source = Array(source.utf8)
     (self.lines, endOfFile) = computeLines(source)
     assert(source.utf8.count == endOfFile.utf8Offset)
   }
+
+  /// Execute the body with an array that contains each source line.
+  func withSourceLines<T>(_ body: ([SyntaxText]) throws -> T) rethrows -> T {
+    return try source.withUnsafeBufferPointer { (sourcePointer: UnsafeBufferPointer<UInt8>) in
+      var result: [SyntaxText] = []
+      var previousLoc = AbsolutePosition.startOfFile
+      assert(lines.first == AbsolutePosition.startOfFile)
+      for lineStartLoc in lines.dropFirst() + [endOfFile] {
+        result.append(SyntaxText(
+          baseAddress: sourcePointer.baseAddress?.advanced(by: previousLoc.utf8Offset),
+          count: lineStartLoc.utf8Offset - previousLoc.utf8Offset
+        ))
+        previousLoc = lineStartLoc
+      }
+      return try body(result)
+    }
+  }
+
+  /// Return the source lines of the file as `String`s.
+  /// Because `String` cannot model invalid UTF-8, the concatenation of these source lines might not be source-accurate in case there are Unicode errors in the source file, but for most practical purposes, this should not pose an issue.
+  public var sourceLines: [String] {
+    return withSourceLines { syntaxTextLines in
+      return syntaxTextLines.map { String(syntaxText: $0) }
+    }
+  }
 
   /// Convert a `AbsolutePosition` to a `SourceLocation`. If the position is
   /// exceeding the file length then the `SourceLocation` for the end of file

Sources/swift-parser-test/swift-parser-test.swift

@@ -163,18 +163,17 @@ class PrintDiags: ParsableCommand {
         languageVersion: swiftVersion,
         enableBareSlashRegexLiteral: enableBareSlashRegex
       )
+
       var diags = ParseDiagnosticsGenerator.diagnostics(for: tree)
-
+      print(DiagnosticsFormatter.annotatedSource(tree: tree, diags: diags))
+
       if foldSequences {
         diags += foldAllSequences(tree).1
       }
 
       if diags.isEmpty {
         print("No diagnostics produced")
       }
-      for diag in diags {
-        print("\(diag.debugDescription)")
-      }
     }
   }
 }

Tests/SwiftDiagnosticsTest/DiagnosticsFormatterTests.swift

@@ -0,0 +1,81 @@
+//===------------------ DiagnosticsFormatterTests.swift -------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2022 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+import XCTest
+import SwiftDiagnostics
+import SwiftParser
+
+final class DiagnosticsFormatterTests: XCTestCase {
+
+  func annotate(source: String) throws -> String {
+    let tree = try Parser.parse(source: source)
+    let diags = ParseDiagnosticsGenerator.diagnostics(for: tree)
+    return DiagnosticsFormatter.annotatedSource(tree: tree, diags: diags)
+  }
+
+  func testSingleDiagnostic() throws {
+    let source = """
+    var foo = bar +
+    """
+    let expectedOutput = """
+    1 │ var foo = bar +
+      ∣                ╰─ expected expression in variable
+
+    """
+    XCTAssertEqual(expectedOutput, try annotate(source: source))
+  }
+
+  func testMultipleDiagnosticsInOneLine() throws {
+    let source = """
+    foo.[].[].[]
+    """
+    let expectedOutput = """
+    1 │ foo.[].[].[]
+      ∣     │  │  ╰─ expected identifier in member access
+      ∣     │  ╰─ expected identifier in member access
+      ∣     ╰─ expected identifier in member access
+
+    """
+    XCTAssertEqual(expectedOutput, try annotate(source: source))
+  }
+
+  func testLineSkipping() throws {
+    let source = """
+    var i = 1
+    i = 2
+    i = foo(
+    i = 4
+    i = 5
+    i = 6
+    i = 7
+    i = 8
+    i = 9
+    i = 10
+    i = bar(
+    """
+    let expectedOutput = """
+     2 │ i = 2
+     3 │ i = foo(
+     4 │ i = 4
+       ∣      ╰─ expected ')' to end function call
+     5 │ i = 5
+     6 │ i = 6
+       ┆
+     9 │ i = 9
+    10 │ i = 10
+    11 │ i = bar(
+       ∣         ├─ expected value in function call
+       ∣         ╰─ expected ')' to end function call
+
+    """
+    XCTAssertEqual(expectedOutput, try annotate(source: source))
+  }
+}