Downgrade error about bad "_endian" platform condition to a warning

The compiler issues a warning, so do the same here.

Author: DougGregor

Sources/SwiftIfConfig/IfConfigError.swift

@@ -31,6 +31,7 @@ enum IfConfigError: Error, CustomStringConvertible {
   case ignoredTrailingComponents(version: VersionTuple, syntax: ExprSyntax)
   case integerLiteralCondition(syntax: ExprSyntax, replacement: Bool)
   case likelySimulatorPlatform(syntax: ExprSyntax)
+  case endiannessDoesNotMatch(syntax: ExprSyntax, argument: String)
 
   var description: String {
     switch self {
@@ -81,6 +82,9 @@ enum IfConfigError: Error, CustomStringConvertible {
     case .likelySimulatorPlatform:
       return
         "platform condition appears to be testing for simulator environment; use 'targetEnvironment(simulator)' instead"
+
+    case .endiannessDoesNotMatch:
+      return "unknown endianness for build configuration '_endian' (must be 'big' or 'little')"
     }
   }
 
@@ -100,7 +104,8 @@ enum IfConfigError: Error, CustomStringConvertible {
       .canImportTwoParameters(syntax: let syntax),
       .ignoredTrailingComponents(version: _, syntax: let syntax),
       .integerLiteralCondition(syntax: let syntax, replacement: _),
-      .likelySimulatorPlatform(syntax: let syntax):
+      .likelySimulatorPlatform(syntax: let syntax),
+      .endiannessDoesNotMatch(syntax: let syntax, argument: _):
       return Syntax(syntax)
 
     case .unsupportedVersionOperator(name: _, operator: let op):
@@ -118,7 +123,8 @@ extension IfConfigError: DiagnosticMessage {
 
   var severity: SwiftDiagnostics.DiagnosticSeverity {
     switch self {
-    case .compilerVersionSecondComponentNotWildcard, .ignoredTrailingComponents, .likelySimulatorPlatform:
+    case .compilerVersionSecondComponentNotWildcard, .ignoredTrailingComponents,
+      .likelySimulatorPlatform, .endiannessDoesNotMatch:
       return .warning
     default: return .error
     }

Sources/SwiftIfConfig/IfConfigEvaluation.swift

@@ -283,23 +283,36 @@ func evaluateIfConfig(
       )
 
     case ._endian:
-      // Ensure that we have a single argument that is a simple identifier,
-      // either "little" or "big".
+      // Ensure that we have a single argument that is a simple identifier.
       guard let argExpr = call.arguments.singleUnlabeledExpression,
-        let arg = argExpr.simpleIdentifierExpr,
-        let expectedEndianness = Endianness(rawValue: arg)
+        let arg = argExpr.simpleIdentifierExpr
       else {
         return recordError(
           .requiresUnlabeledArgument(
             name: fnName,
-            role: "endiannes ('big' or 'little')",
+            role: "endianness ('big' or 'little')",
             syntax: ExprSyntax(call)
           )
         )
       }
 
+      // The argument needs to be either "little" or "big". Otherwise, we assume
+      // it fails.
+      let isActive: Bool
+      if let expectedEndianness = Endianness(rawValue: arg) {
+        isActive = configuration.endianness == expectedEndianness
+      } else {
+        // Complain about unknown endianness
+        extraDiagnostics.append(
+          contentsOf: IfConfigError.endiannessDoesNotMatch(syntax: argExpr, argument: arg)
+            .asDiagnostics(at: argExpr)
+        )
+
+        isActive = false
+      }
+
       return (
-        active: configuration.endianness == expectedEndianness,
+        active: isActive,
         syntaxErrorsAllowed: fn.syntaxErrorsAllowed,
         diagnostics: extraDiagnostics
       )

Tests/SwiftIfConfigTest/EvaluateTests.swift

@@ -170,6 +170,19 @@ public class EvaluateTests: XCTestCase {
     assertIfConfig("_pointerBitWidth(_32)", .inactive)
     assertIfConfig("_hasAtomicBitWidth(_64)", .active)
     assertIfConfig("_hasAtomicBitWidth(_128)", .inactive)
+
+    assertIfConfig(
+      "_endian(mid)",
+      .inactive,
+      diagnostics: [
+        DiagnosticSpec(
+          message: "unknown endianness for build configuration '_endian' (must be 'big' or 'little')",
+          line: 1,
+          column: 9,
+          severity: .warning
+        )
+      ]
+    )
   }
 
   func testVersions() throws {