add announcement for upcoming stdlib migration

news/2024-03-20-stdlib-migration.md

@@ -0,0 +1,47 @@
+# Upcoming migration for stdlib("c")
+
+Almost since the inception of conda-forge, the baseline version of our standard
+library ("stdlib") for C has not changed. This library comes with extra
+complications because it is an essential part of the operating system, and one
+of the few things that conda/mamba/etc. cannot ship safely.
+
+As the ecosystem has moved on and many packages are starting to require newer
+baseline versions, we need to follow suit at some point. However, to avoid
+breaking users on older systems, we need to have infrastructure in place that
+allows our packages to have sufficiently accurate metadata, such that conda can
+avoid the installation of a package requiring a newer stdlib on an old system.
+
+After many discussions across conda-forge stakeholders, the solution we arrived
+at is the introduction of a new Jinja2-function `{{ stdlib("c") }}`, which
+reflects that a given recipe requires a C stdlib. Making this relationship
+explicit will make it easy to correctly reflect the requirement for newer
+stdlib versions per feedstock, as well as in our global pinning.
+
+Up until now, the stdlib was handled implicitly as part of the compiler stack.
+In order to allow this transition to happen, we need to introduce this function
+to essentially all compiled (i.e. not pure-Python) packages. This will be done
+in stages, first for a single migration, and then attached to _all_ ongoing
+migrations in conda-forge.
+
+The logic of the piggyback migrator tries to correctly handle most scenarios,
+but it is impossible to cover all corner cases. As for some general rules that
+all feedstock maintainers are free to apply independently:
+
+- if a feedstock uses a `- {{ compiler(...) }}` jinja in the build section,
+  add a line with `- {{ stdlib("c") }}` to the build environment.
+- if a feedstock uses `- sysroot_linux-64 2.17  # [linux64]` (or a variation),
+  remove this line and add the following to your `conda_build_config.yaml`:
+  ```
+  c_stdlib_version:              # [linux]
+    - 2.17                       # [linux]
+  ```
+- if a feedstock sets `MACOSX_DEPLOYMENT_TARGET` in `conda_build_config.yaml`,
+  for example to 10.13 for `x86_64`, replace that section with the following
+  (note, this does _not_ apply to `MACOSX_SDK_VERSION`!):
+  ```
+  c_stdlib_version:              # [osx and x86_64]
+    - 10.13                      # [osx and x86_64]
+  ```
+  You should then also remove any line involving `__osx` from `meta.yaml`.
+
+For more details, see this [issue](https://github.com/conda-forge/conda-forge.github.io/issues/2102).