Customize "unknown infix operator" diagnostic to make it clearer

Only && and || are allowed as infix operators within an #if condition, so
note that in the diagnostic (just like the compiler does).

Author: DougGregor

Sources/SwiftIfConfig/IfConfigError.swift

@@ -34,6 +34,7 @@ enum IfConfigError: Error, CustomStringConvertible {
   case endiannessDoesNotMatch(syntax: ExprSyntax, argument: String)
   case macabiIsMacCatalyst(syntax: ExprSyntax)
   case expectedModuleName(syntax: ExprSyntax)
+  case badInfixOperator(syntax: ExprSyntax)
 
   var description: String {
     switch self {
@@ -93,6 +94,9 @@ enum IfConfigError: Error, CustomStringConvertible {
 
     case .expectedModuleName:
       return "expected module name"
+
+    case .badInfixOperator:
+      return "expected '&&' or '||' expression"
     }
   }
 
@@ -115,7 +119,8 @@ enum IfConfigError: Error, CustomStringConvertible {
       .likelySimulatorPlatform(syntax: let syntax),
       .endiannessDoesNotMatch(syntax: let syntax, argument: _),
       .macabiIsMacCatalyst(syntax: let syntax),
-      .expectedModuleName(syntax: let syntax):
+      .expectedModuleName(syntax: let syntax),
+      .badInfixOperator(syntax: let syntax):
       return Syntax(syntax)
 
     case .unsupportedVersionOperator(name: _, operator: let op):

Sources/SwiftIfConfig/IfConfigEvaluation.swift

@@ -589,6 +589,18 @@ extension IfConfigClauseSyntax {
   ) -> (folded: ExprSyntax, diagnostics: [Diagnostic]) {
     var foldingDiagnostics: [Diagnostic] = []
     let foldedCondition = OperatorTable.logicalOperators.foldAll(condition) { error in
+      // Replace the "unknown infix operator" diagnostic with a custom one
+      // that mentions that only '&&' and '||' are allowed.
+      if case .missingOperator(_, referencedFrom: let syntax) = error,
+        let binOp = syntax.parent?.as(BinaryOperatorExprSyntax.self)
+      {
+
+        foldingDiagnostics.append(
+          IfConfigError.badInfixOperator(syntax: ExprSyntax(binOp)).asDiagnostic
+        )
+        return
+      }
+
       foldingDiagnostics.append(contentsOf: error.asDiagnostics(at: condition))
     }.cast(ExprSyntax.self)
     return (folded: foldedCondition, diagnostics: foldingDiagnostics)

Tests/SwiftIfConfigTest/EvaluateTests.swift

@@ -148,6 +148,24 @@ public class EvaluateTests: XCTestCase {
         )
       ]
     )
+
+    assertIfConfig(
+      "A == B",
+      .unparsed,
+      configuration: buildConfig,
+      diagnostics: [
+        DiagnosticSpec(
+          message: "expected '&&' or '||' expression",
+          line: 1,
+          column: 3
+        ),
+        DiagnosticSpec(
+          message: "invalid conditional compilation expression",
+          line: 1,
+          column: 1
+        ),
+      ]
+    )
   }
 
   func testFeatures() throws {