Iterator

        pub const Iterator = struct {
            tree: *const Ast,
            fn_proto: *const FnProto,
            param_i: usize,
            tok_i: TokenIndex,
            tok_flag: bool,

            pub fn next(it: *Iterator) ?Param {
                const tree = it.tree;
                while (true) {
                    var first_doc_comment: ?TokenIndex = null;
                    var comptime_noalias: ?TokenIndex = null;
                    var name_token: ?TokenIndex = null;
                    if (!it.tok_flag) {
                        if (it.param_i >= it.fn_proto.ast.params.len) {
                            return null;
                        }
                        const param_type = it.fn_proto.ast.params[it.param_i];
                        var tok_i = tree.firstToken(param_type) - 1;
                        while (true) : (tok_i -= 1) switch (tree.tokenTag(tok_i)) {
                            .colon => continue,
                            .identifier => name_token = tok_i,
                            .doc_comment => first_doc_comment = tok_i,
                            .keyword_comptime, .keyword_noalias => comptime_noalias = tok_i,
                            else => break,
                        };
                        it.param_i += 1;
                        it.tok_i = tree.lastToken(param_type) + 1;
                        // Look for anytype and ... params afterwards.
                        if (tree.tokenTag(it.tok_i) == .comma) {
                            it.tok_i += 1;
                        }
                        it.tok_flag = true;
                        return Param{
                            .first_doc_comment = first_doc_comment,
                            .comptime_noalias = comptime_noalias,
                            .name_token = name_token,
                            .anytype_ellipsis3 = null,
                            .type_expr = param_type,
                        };
                    }
                    if (tree.tokenTag(it.tok_i) == .comma) {
                        it.tok_i += 1;
                    }
                    if (tree.tokenTag(it.tok_i) == .r_paren) {
                        return null;
                    }
                    if (tree.tokenTag(it.tok_i) == .doc_comment) {
                        first_doc_comment = it.tok_i;
                        while (tree.tokenTag(it.tok_i) == .doc_comment) {
                            it.tok_i += 1;
                        }
                    }
                    switch (tree.tokenTag(it.tok_i)) {
                        .ellipsis3 => {
                            it.tok_flag = false; // Next iteration should return null.
                            return Param{
                                .first_doc_comment = first_doc_comment,
                                .comptime_noalias = null,
                                .name_token = null,
                                .anytype_ellipsis3 = it.tok_i,
                                .type_expr = null,
                            };
                        },
                        .keyword_noalias, .keyword_comptime => {
                            comptime_noalias = it.tok_i;
                            it.tok_i += 1;
                        },
                        else => {},
                    }
                    if (tree.tokenTag(it.tok_i) == .identifier and
                        tree.tokenTag(it.tok_i + 1) == .colon)
                    {
                        name_token = it.tok_i;
                        it.tok_i += 2;
                    }
                    if (tree.tokenTag(it.tok_i) == .keyword_anytype) {
                        it.tok_i += 1;
                        return Param{
                            .first_doc_comment = first_doc_comment,
                            .comptime_noalias = comptime_noalias,
                            .name_token = name_token,
                            .anytype_ellipsis3 = it.tok_i - 1,
                            .type_expr = null,
                        };
                    }
                    it.tok_flag = false;
                }
            }
        }