Improve output when validation fails (#744)

With backtrace support built into command-line Swift, calling `fatalError`
for configuration/validation errors isn't very effective, since the error
messages get hidden by the backtrace. This switches to simply printing
the validation message to stderr and exiting with a failing error code,
which possibly should have always been the behavior.

This also revamps the file structure for validators.

Author: natecook1000

Sources/ArgumentParser/CMakeLists.txt

@@ -21,7 +21,6 @@ add_library(ArgumentParser
   "Parsable Types/EnumerableFlag.swift"
   "Parsable Types/ExpressibleByArgument.swift"
   "Parsable Types/ParsableArguments.swift"
-  "Parsable Types/ParsableArgumentsValidation.swift"
   "Parsable Types/ParsableCommand.swift"
 
   Parsing/ArgumentDecoder.swift
@@ -47,7 +46,13 @@ add_library(ArgumentParser
   Utilities/Platform.swift
   Utilities/SequenceExtensions.swift
   Utilities/StringExtensions.swift
-  Utilities/Tree.swift)
+  Utilities/Tree.swift
+  
+  Validators/CodingKeyValidator.swift
+  Validators/NonsenseFlagsValidator.swift
+  Validators/ParsableArgumentsValidation.swift
+  Validators/PositionalArgumentsValidator.swift
+  Validators/UniqueNamesValidator.swift)
 # NOTE: workaround for CMake not setting up include flags yet
 set_target_properties(ArgumentParser PROPERTIES
   INTERFACE_INCLUDE_DIRECTORIES ${CMAKE_Swift_MODULE_DIRECTORY})

Sources/ArgumentParser/Parsable Properties/Argument.swift

@@ -82,7 +82,7 @@ public struct Argument<Value>:
       case .value(let v):
         return v
       case .definition:
-        fatalError(directlyInitializedError)
+        configurationFailure(directlyInitializedError)
       }
     }
     set {

Sources/ArgumentParser/Parsable Properties/Flag.swift

@@ -107,7 +107,7 @@ public struct Flag<Value>: Decodable, ParsedWrapper {
       case .value(let v):
         return v
       case .definition:
-        fatalError(directlyInitializedError)
+        configurationFailure(directlyInitializedError)
       }
     }
     set {

Sources/ArgumentParser/Parsable Properties/Option.swift

@@ -87,7 +87,7 @@ public struct Option<Value>: Decodable, ParsedWrapper {
       case .value(let v):
         return v
       case .definition:
-        fatalError(directlyInitializedError)
+        configurationFailure(directlyInitializedError)
       }
     }
     set {

Sources/ArgumentParser/Parsable Properties/OptionGroup.swift

@@ -101,7 +101,7 @@ public struct OptionGroup<Value: ParsableArguments>: Decodable, ParsedWrapper {
       case .value(let v):
         return v
       case .definition:
-        fatalError(directlyInitializedError)
+        configurationFailure(directlyInitializedError)
       }
     }
     set {

Sources/ArgumentParser/Parsable Types/ParsableArguments.swift

@@ -290,7 +290,7 @@ extension ArgumentSet {
     do {
       try type._validate(parent: parent)
     } catch {
-      assertionFailure("\(error)")
+      configurationFailure("\(error)")
     }
     #endif
 
@@ -321,11 +321,23 @@ extension ArgumentSet {
   }
 }
 
+/// Prints the given message to standard error and exits with a failure code.
+///
+/// - Parameter message: The message to print to standard error. `message`
+///   should be pre-wrapped, if desired.
+func configurationFailure(_ message: String) -> Never {
+  var errorOut = Platform.standardError
+  print("\n", to: &errorOut)
+  print(String(repeating: "-", count: 70), to: &errorOut)
+  print(message, to: &errorOut)
+  print(String(repeating: "-", count: 70), to: &errorOut)
+  print("\n", to: &errorOut)
+  Platform.exit(Platform.exitCodeFailure)
+}
+
 /// The fatal error message to display when someone accesses a
 /// `ParsableArguments` type after initializing it directly.
 internal let directlyInitializedError = """
-
-  --------------------------------------------------------------------
   Can't read a value from a parsable argument definition.
 
   This error indicates that a property declared with an `@Argument`,
@@ -335,6 +347,4 @@ internal let directlyInitializedError = """
   To get a valid value, either call one of the static parsing methods
   (`parse`, `parseAsRoot`, or `main`) or define an initializer that
   initializes _every_ property of your parsable type.
-  --------------------------------------------------------------------
-
   """