[Macros] generate diagnostics from thrown error

Sources/SwiftDiagnostics/Diagnostic.swift

@@ -71,3 +71,9 @@ public struct Diagnostic: CustomDebugStringConvertible {
     return "\(location): \(message)"
   }
 }
+
+public protocol DiagnosticsProviding: Error {
+  /// The diagnostics provided by this error.
+  /// At least one diagnostic should have `severity == .error`.
+  var diagnostics: [Diagnostic] { get }
+}

Sources/SwiftSyntaxMacros/MacroExpansionContext.swift

@@ -65,6 +65,51 @@ extension MacroExpansionContext {
   }
 }
 
+/// Diagnostic message used for thrown errors.
+private struct ThrownErrorDiagnostic: DiagnosticMessage {
+  let message: String
+
+  var severity: DiagnosticSeverity { .error }
+
+  var diagnosticID: MessageID {
+    .init(domain: "SwiftSyntaxMacros", id: "ThrownErrorDiagnostic")
+  }
+}
+
+extension MacroExpansionContext {
+  /// Add diagnostics from the error thrown during macro expansion.
+  public func addDiagnostics<S: SyntaxProtocol>(from error: Error, node: S) {
+    guard let diagnosticsProvider = error as? DiagnosticsProviding else {
+      diagnose(
+        Diagnostic(
+          node: Syntax(node),
+          message: ThrownErrorDiagnostic(message: String(describing: error))
+        )
+      )
+      return
+    }
+
+    let providedDiagnostics = diagnosticsProvider.diagnostics
+    for diagnostic in providedDiagnostics {
+      diagnose(diagnostic)
+    }
+
+    // handle possibility that none of the diagnostics was an error
+    if !providedDiagnostics.contains(
+      where: { $0.diagMessage.severity == .error }
+    ) {
+      diagnose(
+        Diagnostic(
+          node: Syntax(node),
+          message: ThrownErrorDiagnostic(
+            message: "macro expansion failed without generating an error"
+          )
+        )
+      )
+    }
+  }
+}
+
 /// Describe the position within a syntax node that can be used to compute
 /// source locations.
 public enum PositionInSyntaxNode {

Sources/SwiftSyntaxMacros/MacroSystem.swift

@@ -150,13 +150,7 @@ class MacroApplication<Context: MacroExpansionContext>: SyntaxRewriter {
             newItems.append(CodeBlockItemSyntax(item: .init(expandedExpr)))
           }
         } catch {
-          // Record the error
-          context.diagnose(
-            Diagnostic(
-              node: Syntax(node),
-              message: ThrownErrorDiagnostic(message: String(describing: error))
-            )
-          )
+          context.addDiagnostics(from: error, node: node)
         }
 
         continue
@@ -200,13 +194,7 @@ class MacroApplication<Context: MacroExpansionContext>: SyntaxRewriter {
             }
           )
         } catch {
-          // Record the error
-          context.diagnose(
-            Diagnostic(
-              node: Syntax(node),
-              message: ThrownErrorDiagnostic(message: String(describing: error))
-            )
-          )
+          context.addDiagnostics(from: error, node: node)
         }
 
         continue
@@ -376,13 +364,7 @@ extension MacroApplication {
         let newPeers = try peerMacro.expansion(of: attribute, providingPeersOf: decl, in: context)
         peers.append(contentsOf: newPeers)
       } catch {
-        // Record the error
-        context.diagnose(
-          Diagnostic(
-            node: Syntax(attribute),
-            message: ThrownErrorDiagnostic(message: String(describing: error))
-          )
-        )
+        context.addDiagnostics(from: error, node: attribute)
       }
     }
 
@@ -406,13 +388,7 @@ extension MacroApplication {
           )
         )
       } catch {
-        // Record the error
-        context.diagnose(
-          Diagnostic(
-            node: Syntax(attribute),
-            message: ThrownErrorDiagnostic(message: String(describing: error))
-          )
-        )
+        context.addDiagnostics(from: error, node: attribute)
       }
     }
 
@@ -473,13 +449,7 @@ extension MacroApplication {
           contentsOf: try _openExistential(typedDecl, do: expand)
         )
       } catch {
-        // Record the error
-        context.diagnose(
-          Diagnostic(
-            node: Syntax(attribute),
-            message: ThrownErrorDiagnostic(message: String(describing: error))
-          )
-        )
+        context.addDiagnostics(from: error, node: attribute)
       }
     }
 

Sources/SwiftSyntaxMacros/Syntax+MacroEvaluation.swift

@@ -13,17 +13,6 @@
 import SwiftDiagnostics
 import SwiftSyntax
 
-/// Diagnostic message used for thrown errors.
-struct ThrownErrorDiagnostic: DiagnosticMessage {
-  let message: String
-
-  var severity: DiagnosticSeverity { .error }
-
-  var diagnosticID: MessageID {
-    .init(domain: "SwiftSyntaxMacros", id: "ThrownErrorDiagnostic")
-  }
-}
-
 extension SyntaxProtocol {
   /// Detach the current node and inform the macro expansion context,
   /// if it needs to know.
@@ -51,14 +40,7 @@ extension MacroExpansionExprSyntax {
     do {
       return try exprMacro.expansion(of: detach(in: context), in: context)
     } catch {
-      // Record the error
-      context.diagnose(
-        Diagnostic(
-          node: Syntax(self),
-          message: ThrownErrorDiagnostic(message: String(describing: error))
-        )
-      )
-
+      context.addDiagnostics(from: error, node: self)
       return ExprSyntax(self)
     }
   }