add fallback rendering for block directives

Author: tayloraswift

Sources/MarkdownAST/BlockElements/MarkdownBlock.Directive.swift

@@ -1,3 +1,5 @@
+import MarkdownABI
+
 extension MarkdownBlock
 {
     public final
@@ -17,5 +19,29 @@ extension MarkdownBlock
             self.arguments = arguments
             super.init(elements)
         }
+
+        /// Emits a fallback description of the directive.
+        @inlinable public override
+        func emit(into binary:inout MarkdownBinaryEncoder)
+        {
+            binary[.pre]
+            {
+                $0[.code] = "<\(self.name)>"
+
+                if !self.arguments.isEmpty
+                {
+                    $0[.dl]
+                    {
+                        for (name, value):(String, String) in self.arguments
+                        {
+                            $0[.dt] = name
+                            $0[.dd] = value
+                        }
+                    }
+                }
+
+                super.emit(into: &$0)
+            }
+        }
     }
 }

Sources/MarkdownParsing/Parser/SwiftFlavoredMarkdownParser.swift

@@ -53,10 +53,10 @@ extension SwiftFlavoredMarkdownParser
 {
     private
     func block(
-        from markup:/* borrowing */ any _BlockMarkup,
+        from markup:borrowing any _BlockMarkup,
         in source:borrowing MarkdownSource) -> MarkdownBlock?
     {
-        switch /* copy */ markup
+        switch copy markup
         {
         case let block as _BlockQuote:
             return MarkdownBlock.Quote.init(block.blockChildren.compactMap

Sources/MarkdownSemantics/MarkdownInterpreter.swift

@@ -238,6 +238,11 @@ extension MarkdownInterpreter
                 case "Metadata":
                     metadata.update(with: block.elements)
 
+                case "Snippet":
+                    //  Don’t know how to handle these yet, so we just render them
+                    //  as code blocks.
+                    self.append(block)
+
                 case _:
                     //  Don’t know how to handle these yet, so we just render them
                     //  as code blocks.

Sources/SymbolGraphLinker/StaticLinker.swift

@@ -246,6 +246,13 @@ extension StaticLinker
     }
 }
 
+extension StaticLinker
+{
+    // struct Snippet
+    // {
+
+    // }
+}
 extension StaticLinker
 {
     public mutating