[Core] Add fmtlib dependency

Closes #1070. fmtlib allows for easy, safe string substitution in C++,
adding advanced formatting functionality (e.g. rounding), and reducing
bloat.

So add as a new private build-only header-only dependency.

Using header-only increases compile times, but subjectively it's not
noticeable, and has the benefit of avoiding a possible public library
dependency (if OpenAssetIO is built as a static library).

Tweak a single existing, tested, log string to make use of libfmt, to
ensure basic usage is validated.

Signed-off-by: David Feltell <david.feltell@foundry.com>

RELEASE_NOTES.md

@@ -1,6 +1,14 @@
 Release Notes
 =============
 
+v1.0.0-alpha.x
+---------------
+
+### Breaking changes
+
+- Added `fmtlib` as a new header-only private dependency.
+  [#1070](https://github.com/OpenAssetIO/OpenAssetIO/issues/1070)
+
 v1.0.0-alpha.14
 ---------------
 

cmake/ThirdParty.cmake

@@ -3,6 +3,13 @@
 
 find_package(tomlplusplus REQUIRED)
 
+
+#-----------------------------------------------------------------------
+# String formatting
+
+find_package(fmt REQUIRED)
+
+
 #-----------------------------------------------------------------------
 # Python
 

resources/build/conanfile.py

@@ -40,3 +40,7 @@ def requirements(self):
         self.requires("catch2/2.13.8")
         # Mocking library
         self.requires("trompeloeil/42")
+        self.requires("fmt/10.1.1")
+
+    def configure(self):
+        self.options["fmt"].header_only = True

src/openassetio-core/CMakeLists.txt

@@ -90,9 +90,13 @@ target_include_directories(openassetio-core
     # Use includes from install tree for installed lib.
     "$<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>")
 
-target_link_libraries(openassetio-core
+target_link_libraries(
+    openassetio-core
     PRIVATE
-    $<BUILD_INTERFACE:tomlplusplus::tomlplusplus>)
+    # Header-only private dependencies:
+    $<BUILD_INTERFACE:tomlplusplus::tomlplusplus>
+    $<BUILD_INTERFACE:fmt::fmt-header-only>
+)
 
 #-----------------------------------------------------------------------
 # API export header

src/openassetio-core/src/hostApi/Manager.cpp

@@ -3,6 +3,8 @@
 #include <stdexcept>
 #include <utility>
 
+#include <fmt/format.h>
+
 #include <openassetio/Context.hpp>
 #include <openassetio/TraitsData.hpp>
 #include <openassetio/constants.hpp>
@@ -53,12 +55,11 @@ std::optional<Str> entityReferencePrefixFromInfo(const log::LoggerInterfacePtr &
   if (auto iter = info.find(Str{constants::kInfoKey_EntityReferencesMatchPrefix});
       iter != info.end()) {
     if (const auto *prefixPtr = std::get_if<openassetio::Str>(&iter->second)) {
-      std::string msg = "Entity reference prefix '";
-      msg += *prefixPtr;
-      msg +=
-          "' provided by manager's info() dict. Subsequent calls to isEntityReferenceString will"
-          " use this prefix rather than call the manager's implementation.";
-      logger->debugApi(msg);
+      logger->debugApi(
+          fmt::format("Entity reference prefix '{}' provided by manager's info() dict. Subsequent"
+                      " calls to isEntityReferenceString will use this prefix rather than call the"
+                      " manager's implementation.",
+                      *prefixPtr));
 
       return *prefixPtr;
     }