Replace the diagnostic handlers throughout SwiftIfConfig with [Diagnostic] returns

This provides a more consistent diagnostics story.

Author: DougGregor

Sources/SwiftIfConfig/ConfiguredRegionState.swift

@@ -24,29 +24,29 @@ public enum ConfiguredRegionState {
   /// The region is active and is part of the compiled program.
   case active
 
-  /// Evaluate the given `#if` condition using the given build configuration, throwing an error if there is
-  /// insufficient information to make a determination.
-  public init(
-    condition: some ExprSyntaxProtocol,
-    configuration: some BuildConfiguration,
-    diagnosticHandler: ((Diagnostic) -> Void)? = nil
-  ) throws {
+  /// Evaluate the given `#if` condition using the given build configuration
+  /// to determine its state and identify any problems encountered along the
+  /// way.
+  public static func evaluating(
+    _ condition: some ExprSyntaxProtocol,
+    in configuration: some BuildConfiguration
+  ) -> (state: ConfiguredRegionState, diagnostics: [Diagnostic]) {
     // Apply operator folding for !/&&/||.
-    let foldedCondition = try OperatorTable.logicalOperators.foldAll(condition) { error in
-      diagnosticHandler?(error.asDiagnostic)
-      throw error
+    var foldingDiagnostics: [Diagnostic] = []
+    let foldedCondition = OperatorTable.logicalOperators.foldAll(condition) { error in
+      foldingDiagnostics.append(contentsOf: error.asDiagnostics(at: condition))
     }.cast(ExprSyntax.self)
 
-    let (active, versioned) = try evaluateIfConfig(
+    let (active, versioned, evalDiagnostics) = evaluateIfConfig(
       condition: foldedCondition,
-      configuration: configuration,
-      diagnosticHandler: diagnosticHandler
+      configuration: configuration
     )
 
+    let diagnostics = foldingDiagnostics + evalDiagnostics
     switch (active, versioned) {
-    case (true, _): self = .active
-    case (false, false): self = .inactive
-    case (false, true): self = .unparsed
+    case (true, _): return (.active, diagnostics)
+    case (false, false): return (.inactive, diagnostics)
+    case (false, true): return (.unparsed, diagnostics)
     }
   }
 }

Sources/SwiftIfConfig/ConfiguredRegions.swift

@@ -80,11 +80,9 @@ fileprivate class ConfiguredRegionVisitor<Configuration: BuildConfiguration>: Sy
       }
 
       // For inactive clauses, distinguish between inactive and unparsed.
-      let isVersioned =
-        (try? clause.isVersioned(
-          configuration: configuration,
-          diagnosticHandler: nil
-        )) ?? true
+      let isVersioned = clause.isVersioned(
+        configuration: configuration
+      ).versioned
 
       // If this is within an active region, or this is an unparsed region,
       // record it.

Sources/SwiftIfConfig/IfConfigDecl+IfConfig.swift

@@ -44,13 +44,12 @@ extension IfConfigDeclSyntax {
       }
 
       // If this condition evaluates true, return this clause.
-      let isActive =
-        (try? evaluateIfConfig(
-          condition: condition,
-          configuration: configuration
-        ) { 
-          diagnostics.append($0)
-        })?.active ?? false
+      let (isActive, _, localDiagnostics) = evaluateIfConfig(
+        condition: condition,
+        configuration: configuration
+      )
+      diagnostics.append(contentsOf: localDiagnostics)
+
       if isActive {
         return (clause, diagnostics: diagnostics)
       }