Improve SVD2SwiftPlugin errors

Updates the SVD2SwiftPlugin to format JSON decoding errors into human
readable strings.

Plugins/SVD2SwiftPlugin/SVD2SwiftPlugin.swift

@@ -30,8 +30,15 @@ struct SVD2SwiftPlugin: BuildToolPlugin {
     // Load the list of peripherals to generate from the config file.
     let pluginConfigURL = URL(fileURLWithPath: pluginConfigFile.string)
     let pluginConfigData = try Data(contentsOf: pluginConfigURL)
-    let pluginConfig = try JSONDecoder()
-      .decode(SVD2SwiftPluginConfiguration.self, from: pluginConfigData)
+    let pluginConfig: SVD2SwiftPluginConfiguration
+    do {
+      pluginConfig = try JSONDecoder()
+        .decode(SVD2SwiftPluginConfiguration.self, from: pluginConfigData)
+    } catch let error as DecodingError {
+      throw SVD2SwiftPluginConfigurationDecodingError(
+        url: pluginConfigURL,
+        error: error)
+    }
     guard !pluginConfig.peripherals.isEmpty else {
       throw SVD2SwiftPluginError.missingPeripherals(target, pluginConfigFile)
     }

Plugins/SVD2SwiftPlugin/SVD2SwiftPluginConfigurationDecodingError.swift

@@ -0,0 +1,61 @@
+//===----------------------------------------------------------*- swift -*-===//
+//
+// This source file is part of the Swift MMIO open source project
+//
+// Copyright (c) 2024 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
+//
+//===----------------------------------------------------------------------===//
+
+import Foundation
+
+struct SVD2SwiftPluginConfigurationDecodingError {
+  var description: String
+}
+
+extension SVD2SwiftPluginConfigurationDecodingError: CustomStringConvertible {
+  init(url: URL, error: DecodingError) {
+    func format(_ codingPath: [any CodingKey]) -> String{
+      if codingPath.isEmpty {
+        return "<root>"
+      } else {
+        var path = ""
+        for key in codingPath {
+          if !path.isEmpty {
+            path.append(".")
+          }
+          if let index = key.intValue {
+            path.append("[")
+            path.append("\(index)")
+            path.append("]")
+          } else {
+            path.append(key.stringValue)
+          }
+        }
+        return path
+      }
+    }
+
+    let errorFragment = switch error {
+    case let .typeMismatch(type, context):
+      "Expected type \"\(type)\" at path \"\(format(context.codingPath))\"."
+    case let .valueNotFound(_, context):
+      "Expected value at path \"\(format(context.codingPath))\"."
+    case let .keyNotFound(key, context):
+      "Expected key at path \"\(format(context.codingPath + [key]))\"."
+    case let .dataCorrupted(context):
+      "Data corrupted at path \"\(format(context.codingPath))\"."
+    default:
+      "Unknown decoding error: \(error)"
+    }
+    self.description = """
+      \(url.path):1:1: \
+      Failed to read \(FileKind.svd2swift): \
+      \(errorFragment)
+      """
+  }
+}
+
+extension SVD2SwiftPluginConfigurationDecodingError: Error {}

Tests/SVD2SwiftPluginTests/svd2swift.json

@@ -3,7 +3,7 @@
     "TIMER0",
     "TIMER1",
   ],
-  "indentation-width": 2,
+  "indentation-width": 1,
   "namespace-under-device": true,
   "instance-member-peripherals": false,
   "device-name": "Banana"