build: UNITS_USE_LIBFMT support improved

docs/usage.rst

@@ -151,16 +151,6 @@ Additionally, enables project documentation generation when the project is being
 CMake Options
 ^^^^^^^^^^^^^
 
-UNITS_DOWNCAST_MODE
-+++++++++++++++++++
-
-**Values**: ``OFF``/``ON``/``AUTO``
-
-**Defaulted to**: ``ON``
-
-Equivalent to `downcast_mode`_.
-
-
 UNITS_AS_SYSTEM_HEADERS
 +++++++++++++++++++++++
 
@@ -171,6 +161,26 @@ UNITS_AS_SYSTEM_HEADERS
 Exports library as system headers.
 
 
+UNITS_BUILD_DOCS
+++++++++++++++++
+
+**Values**: ``ON``/``OFF``
+
+**Defaulted to**: ``ON``
+
+Enables project documentation generation.
+
+
+UNITS_DOWNCAST_MODE
++++++++++++++++++++
+
+**Values**: ``OFF``/``ON``/``AUTO``
+
+**Defaulted to**: ``ON``
+
+Equivalent to `downcast_mode`_.
+
+
 UNITS_IWYU
 ++++++++++
 
@@ -182,14 +192,14 @@ Enables include-what-you-use when compiling with a clang compiler.
 Additionally turns on `UNITS_AS_SYSTEM_HEADERS`_.
 
 
-UNITS_BUILD_DOCS
+UNITS_USE_LIBFMT
 ++++++++++++++++
 
 **Values**: ``ON``/``OFF``
 
 **Defaulted to**: ``ON``
 
-Enables project documentation generation.
+Enables usage of `{fmt} <https://github.com/fmtlib/fmt>`_ library instead of the C++20 Standard Library feature.
 
 
 Installation and Reuse

src/core-fmt/CMakeLists.txt

@@ -22,14 +22,13 @@
 
 cmake_minimum_required(VERSION 3.15)
 
-option(UNITS_USE_LIBFMT "Enables usage of libfmt instead of the one from 'std'" OFF)
+option(UNITS_USE_LIBFMT "Enables usage of libfmt instead of the one from 'std'" ON)
 message(STATUS "UNITS_USE_LIBFMT: ${UNITS_USE_LIBFMT}")
 
-# find dependencies
-find_package(fmt CONFIG REQUIRED)
-
 add_units_module(core-fmt mp-units::core)
+target_compile_definitions(mp-units-core-fmt INTERFACE UNITS_USE_LIBFMT=$<BOOL:${UNITS_USE_LIBFMT}>)
+
 if(UNITS_USE_LIBFMT)
+  find_package(fmt CONFIG REQUIRED)
   target_link_libraries(mp-units-core-fmt INTERFACE fmt::fmt)
-  target_compile_definitions(mp-units-core-fmt INTERFACE UNITS_USE_LIBFMT)
 endif()

src/core-fmt/include/units/bits/fmt_hacks.h

@@ -29,6 +29,10 @@
 
 #include <units/bits/external/hacks.h>
 
+#ifndef UNITS_USE_LIBFMT
+#define UNITS_USE_LIBFMT 1
+#endif
+
 #if UNITS_USE_LIBFMT
 
 UNITS_DIAGNOSTIC_PUSH