Better debugDescription for EncodingError and DecodingError

stdlib/public/core/Codable.swift

@@ -89,6 +89,58 @@ extension CodingKey {
   public var debugDescription: String {
     return description
   }
+
+  /// A simplified description: the int value, if present, in square brackets.
+  /// Otherwise, the string value by itself. Used when concatenating coding keys
+  /// to form a path when printing debug information.
+  /// - parameter isFirst: Whether this is the first key in a coding path, in
+  ///   which case we will omit the prepended '.' delimiter from string keys.
+  func errorPresentationDescription(isFirstInCodingPath isFirst: Bool = true) -> String {
+    if let intValue {
+      return "[\(intValue)]"
+    } else {
+      let delimiter = isFirst ? "" : "."
+      return "\(delimiter)\(stringValue.escapedForCodingKeyErrorPresentationDescription)"
+    }
+  }
+}
+
+private extension [any CodingKey] {
+  /// Concatenates the elements of an array of coding keys and joins them with
+  /// "/" separators to make them read like a path.
+  func errorPresentationDescription() -> String {
+    return (
+      self.prefix(1).map { $0.errorPresentationDescription(isFirstInCodingPath: true) }
+      + self.dropFirst(1).map { $0.errorPresentationDescription(isFirstInCodingPath: false) }
+    ).joined(separator: "")
+  }
+}
+
+extension String {
+  /// When printing coding paths, delimit string keys with a '.' (period). If
+  /// the key contains a period, escape it with backticks so that it can be
+  /// distinguished from the delimiter. Also escape backslashes and backticks
+  /// (but *not* periods) to avoid confusion with delimiters.
+  var escapedForCodingKeyErrorPresentationDescription: String {
+    let charactersThatNeedBackticks: Set<Character> = [".", "`", "\\"]
+    let charactersThatNeedEscaping: Set<Character> = ["`", "\\"]
+    assert(
+      charactersThatNeedEscaping.isSubset(of: charactersThatNeedBackticks),
+      "Only some characters in backticks will require further escaping to disambiguate them from the backticks"
+    )
+
+    var escaped = self
+    var needsBackticks = false
+    for (character, index) in zip(self, indices).reversed() {
+      if charactersThatNeedBackticks.contains(character) {
+        needsBackticks = true
+        if charactersThatNeedEscaping.contains(character) {
+          escaped.insert("\\", at: index)
+        }
+      }
+    }
+    return needsBackticks ? "`\(escaped)`" : self
+  }
 }
 
 //===----------------------------------------------------------------------===//
@@ -3724,6 +3776,80 @@ public enum DecodingError: Error {
   }
 }
 
+@available(SwiftStdlib 6.3, *)
+extension EncodingError: CustomDebugStringConvertible {
+  /// A textual representation of this encoding error, intended for debugging.
+  ///
+  /// - Important: The contents of the returned string are not guaranteed to
+  ///    remain stable: they may arbitrarily change in any Swift release.
+  @available(SwiftStdlib 6.3, *)
+  public var debugDescription: String {
+    let (message, context) = switch self {
+      case .invalidValue(let value, let context):
+        (
+          "EncodingError.invalidValue: \(String(reflecting: value)) (\(type(of: value)))",
+          context
+        )
+    }
+
+    var output = message
+
+    let contextDebugDescription = context.debugDescription
+
+    if !context.codingPath.isEmpty {
+      output.append(". Path: \(context.codingPath.errorPresentationDescription())")
+    }
+
+    if !contextDebugDescription.isEmpty {
+      output.append(". Debug description: \(context.debugDescription)")
+    }
+
+    if let underlyingError = context.underlyingError {
+      output.append(". Underlying error: \(underlyingError)")
+    }
+
+    return output
+  }
+}
+
+@available(SwiftStdlib 6.3, *)
+extension DecodingError: CustomDebugStringConvertible {
+  /// A textual representation of this decoding error, intended for debugging.
+  ///
+  /// - Important: The contents of the returned string are not guaranteed to
+  ///    remain stable: they may arbitrarily change in any Swift release.
+  @available(SwiftStdlib 6.3, *)
+  public var debugDescription: String {
+    let (message, context) = switch self {
+      case .typeMismatch(let expectedType, let context):
+        ("DecodingError.typeMismatch: expected value of type \(expectedType)", context)
+      case .valueNotFound(let expectedType, let context):
+        ("DecodingError.valueNotFound: Expected value of type \(expectedType) but found null instead", context)
+      case .keyNotFound(let expectedKey, let context):
+        ("DecodingError.keyNotFound: Key '\(expectedKey.errorPresentationDescription())' not found in keyed decoding container", context)
+      case .dataCorrupted(let context):
+        ("DecodingError.dataCorrupted: Data was corrupted", context)
+    }
+
+    var output = message
+
+    if !context.codingPath.isEmpty {
+      output.append(". Path: \(context.codingPath.errorPresentationDescription())")
+    }
+
+    let contextDebugDescription = context.debugDescription
+    if !contextDebugDescription.isEmpty {
+      output.append(". Debug description: \(contextDebugDescription)")
+    }
+
+    if let underlyingError = context.underlyingError {
+      output.append(". Underlying error: \(underlyingError)")
+    }
+
+    return output
+  }
+}
+
 // The following extensions allow for easier error construction.
 
 internal struct _GenericIndexKey: CodingKey, Sendable {

test/Macros/macro_plugin_error.swift

@@ -37,7 +37,7 @@ func test() {
   // FIXME: -module-abi-name ABI name is leaking.
 
   let _: String = #fooMacro(1)
-  // expected-error @-1 {{typeMismatch(_CompilerSwiftCompilerPluginMessageHandling.PluginToHostMessage}}
+  // expected-error @-1 {{typeMismatch}}
   let _: String = #fooMacro(2)
   // expected-error @-1 {{failed to receive result from plugin (from macro 'fooMacro')}}
   let _: String = #fooMacro(3)

test/abi/macOS/arm64/stdlib.swift

@@ -1148,3 +1148,13 @@ Added: __swift_debug_metadataAllocatorPageSize
 Added: __swift_retainRelease_slowpath_mask_v1
 Added: _swift_release_preservemost
 Added: _swift_retain_preservemost
+
+// SE-0489 Better debugDescription for EncodingError and DecodingError
+Added: _$ss13DecodingErrorO16debugDescriptionSSvg
+Added: _$ss13DecodingErrorO16debugDescriptionSSvpMV
+Added: _$ss13DecodingErrorOs28CustomDebugStringConvertiblesMc
+Added: _$ss13DecodingErrorOs28CustomDebugStringConvertiblesWP
+Added: _$ss13EncodingErrorO16debugDescriptionSSvg
+Added: _$ss13EncodingErrorO16debugDescriptionSSvpMV
+Added: _$ss13EncodingErrorOs28CustomDebugStringConvertiblesMc
+Added: _$ss13EncodingErrorOs28CustomDebugStringConvertiblesWP

test/abi/macOS/x86_64/stdlib.swift

@@ -1143,3 +1143,13 @@ Removed: _$sSS10_wordIndex6beforeSS0B0VAD_tF
 // Internal info exposed for swift-inspect.
 Added: __swift_debug_allocationPoolSize
 Added: __swift_debug_metadataAllocatorPageSize
+
+// SE-0489 Better debugDescription for EncodingError and DecodingError
+Added: _$ss13DecodingErrorO16debugDescriptionSSvg
+Added: _$ss13DecodingErrorO16debugDescriptionSSvpMV
+Added: _$ss13DecodingErrorOs28CustomDebugStringConvertiblesMc
+Added: _$ss13DecodingErrorOs28CustomDebugStringConvertiblesWP
+Added: _$ss13EncodingErrorO16debugDescriptionSSvg
+Added: _$ss13EncodingErrorO16debugDescriptionSSvpMV
+Added: _$ss13EncodingErrorOs28CustomDebugStringConvertiblesMc
+Added: _$ss13EncodingErrorOs28CustomDebugStringConvertiblesWP

test/api-digester/stability-stdlib-abi-without-asserts.test

@@ -879,4 +879,8 @@ Func _float64ToStringImpl(_:_:_:_:) is a new API without '@available'
 Func _int64ToStringImpl(_:_:_:_:_:) is a new API without '@available'
 Func _uint64ToStringImpl(_:_:_:_:_:) is a new API without '@available'
 
+// New conformances from SE-0489: Better debugDescription for EncodingError and DecodingError
+Enum DecodingError has added a conformance to an existing protocol CustomDebugStringConvertible
+Enum EncodingError has added a conformance to an existing protocol CustomDebugStringConvertible
+
 // *** DO NOT DISABLE OR XFAIL THIS TEST. *** (See comment above.)