output: render top level doc comments

those at the beginning of each file, prefixed with "//!".

src/analyze.zig

@@ -15,6 +15,9 @@ pub const Query = union(enum) {
 pub fn search(alloc: std.mem.Allocator, ais: *output.Ais, source: [:0]const u8, query: ?Query) !void {
     var tree = try std.zig.parse(alloc, source);
     defer tree.deinit(alloc);
+
+    try output.renderTopLevelDocComments(ais, tree);
+
     var insert_newline = false;
     for (tree.rootDecls()) |decl| {
         if (!isPublic(tree, decl)) {

src/output.zig

@@ -21,6 +21,15 @@ pub const indent_delta = 4;
 /// Ais is writer type-independent AutoIndentingStream used by all renderXxx funcs.
 pub const Ais = AutoIndentingStream(TypeErasedWriter.Writer);
 
+// outputs all the line comments at the beginning of the tree.
+pub fn renderTopLevelDocComments(ais: *Ais, tree: Ast) !void {
+    const comment_end_loc = tree.tokens.items(.start)[0];
+    _ = try renderComments(ais, tree, 0, comment_end_loc);
+    if (tree.tokens.items(.tag)[0] == .container_doc_comment) {
+        try renderContainerDocComments(ais, tree, 0);
+    }
+}
+
 /// renderPubMember prints the given declaration using ais.
 /// it outputs anything only if the declaration is public.
 pub fn renderPubMember(gpa: Allocator, ais: *Ais, tree: Ast, decl: Ast.Node.Index, space: Space) !void {