Add JSON_DELETE_DEPRECATED_FUNCTIONS to delete the deprecated functions (#5755)

* Add JSON_DELETE_DEPRECATED_FUNCTIONS to delete the deprecated functions

Defining JSON_DELETE_DEPRECATED_FUNCTIONS to 1 (or the CMake option
JSON_DeleteDeprecatedFunctions) declares every deprecated function as
deleted instead of deprecated, so that code that is not ready for 4.0.0
no longer compiles. A deleted function still takes part in overload
resolution, so from_*(ptr, len) cannot silently bind len to the strict
parameter of from_*(InputType&&, bool); the roadmap now plans to keep
these overloads deleted in 4.0.0 instead of removing them.

The legacy discarded-value comparison is left to its own macro.

Also update the 4.0 roadmap: add JSON_DISABLE_TUPLE_REFERENCE_CONVERSION
and JSON_DELETE_DEPRECATED_FUNCTIONS to the macro table, add the
from_bjdata/from_bon8 (ptr, len) overloads to the deprecated functions,
document the macro in the migration guide, and fix the docs style check
findings (example titles, missing docset entry for JSON_STRICT_BINARY_UTF8).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Declare each deprecated function once and guard only its body

Instead of repeating every deprecated declaration in an
#if JSON_DELETE_DEPRECATED_FUNCTIONS branch, keep one declaration
(with its deprecation attribute) and switch only between "= delete;"
and the function body. Suggested by @gregmarr in the review.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-bon8.cpp

@@ -8,6 +8,14 @@
 
 #include "doctest_compatibility.h"
 
+// capture whether JSON_DELETE_DEPRECATED_FUNCTIONS was enabled on the command
+// line *before* including json.hpp, since the library #undefs it once the header
+// has been fully processed (see include/nlohmann/detail/macro_unscope.hpp); the
+// tests of deprecated functions are skipped if these functions are deleted
+#if defined(JSON_DELETE_DEPRECATED_FUNCTIONS) && (JSON_DELETE_DEPRECATED_FUNCTIONS == 1)
+    #define JSON_TEST_DEPRECATED_FUNCTIONS_DELETED
+#endif
+
 #include <nlohmann/json.hpp>
 using nlohmann::json;
 #ifdef JSON_TEST_NO_GLOBAL_UDLS
@@ -1028,6 +1036,7 @@ TEST_CASE("issue #5648 - from_bon8(ptr, len) must read len bytes, not treat ptr
     }
     REQUIRE(contains_nul);
 
+#ifndef JSON_TEST_DEPRECATED_FUNCTIONS_DELETED
     // before the fix, from_bon8 had no (ptr, len) overload, so this call
     // bound to from_bon8(InputType&&, bool strict) instead: ptr was read as
     // a NUL-terminated C string (stopping at the embedded 0x00 byte), and
@@ -1040,6 +1049,7 @@ TEST_CASE("issue #5648 - from_bon8(ptr, len) must read len bytes, not treat ptr
 
     // len must not collapse into the strict flag either
     CHECK(json::from_bon8(packed.data(), packed.size(), false) == j);
+#endif
 }
 
 #ifdef JSON_HAS_CPP_17