Document custom generators in Conan 2.0 (#3213)

* add docs

* Update reference/extensions/custom_generators.rst

reference/extensions.rst

@@ -5,8 +5,13 @@ Extensions
 
 Conan can be extended in a few ways, with custom user code:
 
-- ``python_requires`` allow to put common recipe code in a recipe package that can be reused
-  by other recipes by declaring a ``python_requires = "mypythoncode/version"``
+- ``python_requires`` allow to put common recipe code in a recipe package that can be
+  reused by other recipes by declaring a ``python_requires = "mypythoncode/version"``
+- You can create your own custom Conan commands to solve self-needs thanks to Python and
+  Conan public API powers altogether.
+- It's also possible to make your own custom Conan generators in case you are using build
+  systems that are not supported by the built-in Conan tools. Those can be used from
+  ``python_requires`` or installed globally.
 - ``hooks`` are "pre" and "post" recipe methods (like ``pre_build()`` and ``post_build()``) 
   extensions that can be used to complement recipes
   with orthogonal functionality, like quality checks, binary analyzing, logging, etc.
@@ -37,6 +42,7 @@ Contents:
 
    extensions/python_requires
    extensions/custom_commands
+   extensions/custom_generators
    extensions/python_api
    extensions/deployers
    extensions/hooks

reference/extensions/custom_generators.rst

@@ -0,0 +1,90 @@
+.. _reference_commands_custom_generators:
+
+Custom Conan generators
+=======================
+
+In the case that you need to use a build system or tool that is not supported by Conan
+off-the-shelf, you could create your own custom integrations using a custom generator.
+This can be done in two different ways.
+
+Custom generators as python_requires
+------------------------------------
+
+One way of having your own custom generators in Conan is by using them as
+:ref:`python_requires<reference_extensions_python_requires>`. You could declare a
+*MyGenerator* class with all the logic to generate some files inside the *mygenerator/1.0*
+`python_requires` package:
+
+.. code-block:: python
+
+    from conan import ConanFile
+    from conan.tools.files import save
+
+
+    class MyGenerator:
+        def __init__(self, conanfile):
+            self._conanfile = conanfile
+
+        def generate(self):
+            deps_info = ""
+            for dep, _ in self._conanfile.dependencies.items():
+                deps_info = f"{dep.ref.name}, {dep.ref.version}"
+            save(self._conanfile, "deps.txt", deps_info)
+
+
+    class PyReq(ConanFile):
+        name = "mygenerator"
+        version = "1.0"
+        package_type = "python-require"
+
+
+And then use it in the generate method of your own packages like this:
+
+.. code-block:: python
+
+    from conan import ConanFile
+
+
+    class MyPkg(ConanFile):
+        name = "pkg"
+        version = "1.0"
+
+        python_requires = "mygenerator/1.0"
+        requires = "zlib/1.2.11"
+
+        def generate(self):
+            mygenerator = self.python_requires["mygenerator"].module.MyGenerator
+            mygenerator.generate(self)
+
+This has the advantage that you can version your own custom generators as packages and
+also that you can share those generators as Conan packages.
+
+Using global custom generators
+------------------------------
+
+You can also use your custom generators globally if you store them in the
+``[CONAN_HOME]/extensions/generators`` folder. You can place them directly in that folder
+or install with the ``conan config install`` command.
+
+.. code-block:: python
+    :caption: [CONAN_HOME]/extensions/generators/mygen.py
+    
+    from conan.tools.files import save
+
+
+    class MyGenerator:
+        def __init__(self, conanfile):
+            self._conanfile = conanfile
+
+        def generate(self):
+            deps_info = ""
+            for dep, _ in self._conanfile.dependencies.items():
+                deps_info = f"{dep.ref.name}, {dep.ref.version}"
+            save(self._conanfile, "deps.txt", deps_info)
+
+Then you can use them by name in the recipes or in the command line using the *-g*
+argument:
+
+.. code-block:: bash
+
+    conan install --requires=zlib/1.2.13 -g MyGenerator