Add documentation for clang-tidy plugin

Signed-off-by: Mike Crowe <mac@mcrowe.com>

docs/mkdocs/docs/api/macros/json_use_implicit_conversions.md

@@ -23,6 +23,8 @@ By default, implicit conversions are enabled.
 
     You can prepare existing code by already defining `JSON_USE_IMPLICIT_CONVERSIONS` to `0` and replace any implicit
     conversions with calls to [`get`](../basic_json/get.md).
+    The [clang-tidy modernize-nlohmann-json-explicit-conversions](../../integration/clang-tidy.md#modernize-nlohmann-json-explicit-conversions)
+	check can help to do this automatically.
 
 !!! hint "CMake option"
 

docs/mkdocs/docs/integration/clang-tidy.md

@@ -0,0 +1,81 @@
+# Clang-Tidy
+
+The library comes with a [clang-tidy](https://clang.llvm.org/extra/clang-tidy/) plugin.
+It is disabled by default but can be enabled by enabling the `JSON_ClangTidyPlugin` [CMake option](cmake.md#json_clangtidyplugin).
+Clang-tidy [Plugins](https://clang.llvm.org/extra/clang-tidy/ExternalClang-TidyExamples.html) are only supported by Clang 16 and later.
+
+## Building the plugin
+
+You will need to have the development files matching your version of clang-tidy installed to build the plugin.
+For example, if you are running on a Debian-derived Linux distribution:
+
+```sh
+apt install clang-tidy libclang-dev
+```
+but if this installs a version that is older than Clang 16 then you might be able to specify a newer version. For example:
+```sh
+apt install clang-tidy-19 libclang-19-dev
+```
+
+```sh
+mkdir build
+cd build
+cmake -DJSON_ClangTidyPlugin=ON ..
+cmake -build .
+```
+
+# Running the plugin
+To tell clang-tidy to use the plugin you must pass a path to it as an argument to the `-load` option.
+For example, you can run clang-tidy with only the _modernize-nlohmann-json-explicit-conversion_ check using the plugin on a single file with:
+```sh
+clang-tidy -load .../path/to/build/clang_tidy_plugin/libNlohmannJsonClangTidyPlugin.so -checks='-*,modernize-nlohmann-json-explicit-conversions` -fix source.cpp
+clang-tidy
+```
+or you can create a `.clang-tidy` file to enable the checks you require.
+
+# Checks
+
+At the moment the plugin contains only a single check.
+
+## modernize-nlohmann-json-explicit-conversions
+
+This check converts code that takes advantage of [implicit conversions](../api/macros/json_use_implicit_conversions.md) to use explicit `get()` calls using the correct templated type.
+For example, it turns:
+```c++
+void f(const nlohmann::json &j1, const nlohmann::json &j2)
+{
+  int i = j1;
+  double d = j2.at("value");
+  bool b = *j2.find("valid");
+  std::cout << i << " " << d << " " << b << "\n";
+}
+```
+into
+```c++
+void f(const nlohmann::json &j1, const nlohmann::json &j2)
+{
+  int i = j1.get<int>();
+  double d = j2.at("value").get<double>();
+  bool b = j2.find("valid")->get<bool>();
+  std::cout << i << " " << d << " " << b << "\n";
+}
+```
+by knowing what the target type is for the implicit conversion and turning
+that into an explicit call to the `get()` method with that type as the
+template parameter.
+
+Unfortunately the check does not work very well if the implicit conversion
+occurs in templated code or in a system header. For example, the following
+won't be fixed because the implicit conversion will happen inside
+`std::optional`'s constructor:
+```c++
+void f(const nlohmann::json &j)
+{
+  std::optional<int> oi;
+  const auto &it = j.find("value");
+  if (it != j.end())
+    oi = *it;
+  // ...
+}
+```
+After you have run this check you can set [JSON_USE_IMPLICIT_CONVERSIONS=0](../api/macros/json_use_implicit_conversions.md) to find any occurrences that the check have not been fixed automatically.

docs/mkdocs/docs/integration/cmake.md

@@ -131,6 +131,10 @@ Build the unit tests when [`BUILD_TESTING`](https://cmake.org/cmake/help/latest/
 
 Enable CI build targets. The exact targets are used during the several CI steps and are subject to change without notice. This option is `OFF` by default.
 
+### `JSON_ClangTidyPlugin`
+
+Enable building the [clang-tidy plugin](clang-tidy.md). This option is `OFF` by default.
+
 ### `JSON_Diagnostics`
 
 Enable [extended diagnostic messages](../home/exceptions.md#extended-diagnostic-messages) by defining macro [`JSON_DIAGNOSTICS`](../api/macros/json_diagnostics.md). This option is `OFF` by default.

docs/mkdocs/mkdocs.yml

@@ -93,6 +93,7 @@ nav:
   - Integration:
       - integration/index.md
       - integration/migration_guide.md
+      - integration/clang-tidy.md
       - integration/cmake.md
       - integration/package_managers.md
       - integration/pkg-config.md