util: Add ConstevalFormatString

The type is used to wrap a format string once it has been compile-time
checked to contain the right number of format specifiers.

src/test/util_string_tests.cpp

@@ -0,0 +1,86 @@
+// Copyright (c) 2024-present The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <util/string.h>
+
+#include <boost/test/unit_test.hpp>
+
+using namespace util;
+
+BOOST_AUTO_TEST_SUITE(util_string_tests)
+
+// Helper to allow compile-time sanity checks while providing the number of
+// args directly. Normally PassFmt<sizeof...(Args)> would be used.
+template <unsigned NumArgs>
+inline void PassFmt(util::ConstevalFormatString<NumArgs> fmt)
+{
+    // This was already executed at compile-time, but is executed again at run-time to avoid -Wunused.
+    decltype(fmt)::Detail_CheckNumFormatSpecifiers(fmt.fmt);
+}
+template <unsigned WrongNumArgs>
+inline void FailFmtWithError(std::string_view wrong_fmt, std::string_view error)
+{
+    using ErrType = const char*;
+    auto check_throw{[error](const ErrType& str) { return str == error; }};
+    BOOST_CHECK_EXCEPTION(util::ConstevalFormatString<WrongNumArgs>::Detail_CheckNumFormatSpecifiers(wrong_fmt), ErrType, check_throw);
+}
+
+BOOST_AUTO_TEST_CASE(ConstevalFormatString_NumSpec)
+{
+    PassFmt<0>("");
+    PassFmt<0>("%%");
+    PassFmt<1>("%s");
+    PassFmt<0>("%%s");
+    PassFmt<0>("s%%");
+    PassFmt<1>("%%%s");
+    PassFmt<1>("%s%%");
+    PassFmt<0>(" 1$s");
+    PassFmt<1>("%1$s");
+    PassFmt<1>("%1$s%1$s");
+    PassFmt<2>("%2$s");
+    PassFmt<2>("%2$s 4$s %2$s");
+    PassFmt<129>("%129$s 999$s %2$s");
+    PassFmt<1>("%02d");
+    PassFmt<1>("%+2s");
+    PassFmt<1>("%.6i");
+    PassFmt<1>("%5.2f");
+    PassFmt<1>("%#x");
+    PassFmt<1>("%1$5i");
+    PassFmt<1>("%1$-5i");
+    PassFmt<1>("%1$.5i");
+    // tinyformat accepts almost any "type" spec, even '%', or '_', or '\n'.
+    PassFmt<1>("%123%");
+    PassFmt<1>("%123%s");
+    PassFmt<1>("%_");
+    PassFmt<1>("%\n");
+
+    // The `*` specifier behavior is unsupported and can lead to runtime
+    // errors when used in a ConstevalFormatString. Please refer to the
+    // note in the ConstevalFormatString docs.
+    PassFmt<1>("%*c");
+    PassFmt<2>("%2$*3$d");
+    PassFmt<1>("%.*f");
+
+    auto err_mix{"Format specifiers must be all positional or all non-positional!"};
+    FailFmtWithError<1>("%s%1$s", err_mix);
+
+    auto err_num{"Format specifier count must match the argument count!"};
+    FailFmtWithError<1>("", err_num);
+    FailFmtWithError<0>("%s", err_num);
+    FailFmtWithError<2>("%s", err_num);
+    FailFmtWithError<0>("%1$s", err_num);
+    FailFmtWithError<2>("%1$s", err_num);
+
+    auto err_0_pos{"Positional format specifier must have position of at least 1"};
+    FailFmtWithError<1>("%$s", err_0_pos);
+    FailFmtWithError<1>("%$", err_0_pos);
+    FailFmtWithError<0>("%0$", err_0_pos);
+    FailFmtWithError<0>("%0$s", err_0_pos);
+
+    auto err_term{"Format specifier incorrectly terminated by end of string"};
+    FailFmtWithError<1>("%", err_term);
+    FailFmtWithError<1>("%1$", err_term);
+}
+
+BOOST_AUTO_TEST_SUITE_END()