update infrastructure.rst/compilers with current setup

Author: h-vetinari

src/maintainer/infrastructure.rst

@@ -312,6 +312,82 @@ internal use (e.g. ``comp7``). We note again that 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 (GCC):
+
+* [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 (Clang):
+
+* [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 (MSVC):
+
+* [C, C++] Activation: https://github.com/conda-forge/vc-feedstock
+  (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++/Fortran compilers, these are the feedstocks for the 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 the respective above-mentioned feedstocks have been rebuilt
+for the 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
 ---------------------------------------------