update infrastructure.rst/compilers with current setup

src/maintainer/infrastructure.rst

@@ -311,6 +311,77 @@ These compiler generations may have some unofficial names for our internally. We
 the existence of these names does not imply any level of support or stability for the compilers
 that form the given stack.
 
+The more recent status quo is that compilers can be upgraded in an ABI-compatible manner,
+meaning that we can just increase the version in our global pinning, and it will slowly
+roll out in a compatible manner to the ecosystem as feedstocks get rerendered.
+For such ABI-compatible upgrades, similar but looser principles apply:
+
+* The pins are similarly in the global pinning, see :ref:`globally_pinned_packages`.
+* We provide no support of any kind in terms of the long-term availability of a given compiler generation.
+* We generally provide notice in the form of an announcement when a compiler is going to be upgraded.
+* In general, our compilers on Linux and OSX are using very recent compilers, whereas
+  on windows, we generally use the last supported VS version.
+
+Despite the lack of explicit support, we try to keep the compilers in their various versions
+working also outside of conda-forge, and even provide an easy way to install them
+(through the `compilers<https://github.com/conda-forge/compilers-feedstock>`_ feedstock).
+
+In more detail, our default compiler stack is made up very differently on each platform.
+More specifically, each compiler needs an _activation_ that makes the difference
+between it being merely present in a build environment, and it being used by default.
+These will be installed when using `{{ compiler('xyz') }}` in `meta.yaml`, where `'xyz'`
+is one of `'c', 'cxx', 'fortran', 'cuda', 'rust', 'go-cgo', 'go-nocgo'`.
+
+Linux:
+* [C, C++, Fortran] Activation: https://github.com/conda-forge/ctng-compiler-activation-feedstock/
+* [C, C++, Fortran] Implementation: https://github.com/conda-forge/ctng-compilers-feedstock
+* Note that when used in conjunction with CUDA, compiler versions are restricted by the
+  maximum GCC version supported by nvcc (which is also reflected in the global pinning).
+
+OSX:
+* [C, C++] Activation: https://github.com/conda-forge/clang-compiler-activation-feedstock/
+* [C, C++] Required feedstocks:
+  `llvmdev<https://github.com/conda-forge/llvmdev-feedstock>`_,
+  `clangdev<https://github.com/conda-forge/clangdev-feedstock>`_,
+  `compiler-rt<https://github.com/conda-forge/compiler-rt-feedstock>`_,
+  `libcxx<https://github.com/conda-forge/libcxx-feedstock>`_,
+  `openmp<https://github.com/conda-forge/openmp-feedstock>`_,
+  `lld<https://github.com/conda-forge/lld-feedstock>`_,
+  `cctools<https://github.com/conda-forge/cctools-and-ld64-feedstock>`_
+* [Fortran] Activation: https://github.com/conda-forge/gfortran_osx-64-feedstock/
+* [Fortran] Implementation: https://github.com/conda-forge/gfortran_impl_osx-64-feedstock/
+
+Windows:
+* [C, C++] Activation: https://github.com/conda-forge/vc-feedstock
+* [C, C++] We cannot redistribute the actual MSVC compilers due to licensing constraints
+* [Fortran] Activation & Implementation: https://github.com/conda-forge/flang-feedstock
+
+There exists an alternative, MinGW-based, compiler stack on windows, which is available
+with a `m2w64_` prefix (e.g. `{{ compiler('m2w64_c') }}`), which is however falling out
+of use now that most projects will natively support compilation also with MSVC, in addition
+to several complications arising from mixing compiler stacks.
+
+Additionally, there is a possibility to use clang as a compiler also on Linux & Windows:
+* Activation (Linux): https://github.com/conda-forge/clang-compiler-activation-feedstock/
+* Activation (Windows): https://github.com/conda-forge/clang-win-activation-feedstock/
+
+Aside from the main C/C++ compilers, these are the feedstocks for other compilers:
+* [CUDA] https://github.com/conda-forge/nvcc-feedstock (CUDA infra currently being overhauled)
+* [Rust] `Activation<https://github.com/conda-forge/rust-activation-feedstock>`_
+  and `Implementation<https://github.com/conda-forge/rust-feedstock>`_
+* [Go] https://github.com/conda-forge/go-feedstock
+
+To upgrade the compiler version of our default compilers in the global pinning for
+Linux or OSX, ensure that all the above feedstocks have been rebuilt for the respective
+new major version, that all interrelated versions are lifted at the same time,
+and obviously that the compilers work (e.g. by testing them on some feedstocks by
+specifying the new version through the feedstock-local `conda_build_config.yaml`).
+
+For Windows, we stay on older compilers for longer, because using a newer toolchain
+would force everyone wanting to locally develop with conda-forge artefacts to use
+a toolchain that's at least as new. A more in-depth explanation can be found
+`here<https://github.com/conda-forge/conda-forge.github.io/issues/1732>`_.
+
 CentOS ``sysroot`` for ``linux-*`` Platforms
 ---------------------------------------------