expectTopLevelDecl

fn expectTopLevelDecl(p: *Parse) !?Node.Index {
    const extern_export_inline_token = p.nextToken();
    var is_extern: bool = false;
    var expect_fn: bool = false;
    var expect_var_or_fn: bool = false;
    switch (p.tokenTag(extern_export_inline_token)) {
        .keyword_extern => {
            _ = p.eatToken(.string_literal);
            is_extern = true;
            expect_var_or_fn = true;
        },
        .keyword_export => expect_var_or_fn = true,
        .keyword_inline, .keyword_noinline => expect_fn = true,
        else => p.tok_i -= 1,
    }
    const opt_fn_proto = try p.parseFnProto();
    if (opt_fn_proto) |fn_proto| {
        switch (p.tokenTag(p.tok_i)) {
            .semicolon => {
                p.tok_i += 1;
                return fn_proto;
            },
            .l_brace => {
                if (is_extern) {
                    try p.warnMsg(.{ .tag = .extern_fn_body, .token = extern_export_inline_token });
                    return null;
                }
                const fn_decl_index = try p.reserveNode(.fn_decl);
                errdefer p.unreserveNode(fn_decl_index);

                const body_block = try p.parseBlock();
                return p.setNode(fn_decl_index, .{
                    .tag = .fn_decl,
                    .main_token = p.nodeMainToken(fn_proto),
                    .data = .{ .node_and_node = .{
                        fn_proto,
                        body_block.?,
                    } },
                });
            },
            else => {
                // Since parseBlock only return error.ParseError on
                // a missing '}' we can assume this function was
                // supposed to end here.
                try p.warn(.expected_semi_or_lbrace);
                return null;
            },
        }
    }
    if (expect_fn) {
        try p.warn(.expected_fn);
        return error.ParseError;
    }

    const thread_local_token = p.eatToken(.keyword_threadlocal);
    if (try p.parseGlobalVarDecl()) |var_decl| return var_decl;
    if (thread_local_token != null) {
        return p.fail(.expected_var_decl);
    }
    if (expect_var_or_fn) {
        return p.fail(.expected_var_decl_or_fn);
    }
    return p.fail(.expected_pub_item);
}