Mostly written by Ralf

Roadmap.md

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+All version numbers listed here are very approximate.
+
+# Targets for (unspecified, soonish):
+
+* Integration of smart_holder into refactored code as optional add-on
+    * `#include <pybind11/pybind11.h>` will only see is_smart_holder_base_tag but not any other smart_holder code.
+    * `#include <pybind11/smart_holder.h>` needed to actually bring in the smart_holder code.
+    * refactoring of smart_holder code itself, e.g. trim `smart_holder_poc.h` into a smaller `memory/smart_holder.h`, move rest to split out `tests/memory/smart_holder_poc.h`.
+
+## Targets for 2.8:
+
+* Decoupling of unit tests
+    * Currently test A cannot run until all tests A-Z are compiled.
+        * Blocking in parallel build environments
+    * Link time with optimization is currently ~63 seconds (gcc 10.2, `-Os -flto`).
+        * Also very long startup times (>15s seconds) in distributed build environments because of very large `.so` size in debug mode (~100M).
+    * Tricks needed for fast development-test iterations.
+    * Fix: minimize use of TEST_SUBMODULE, use only if the submodule feature itself is being tested.
+* Systematic refactoring
+    * **Unit tests must not be modified** (with only very rare, narrow exceptions).
+    * **No change to internals** (no exceptions).
+    * Refactoring into subdirectories.
+    * Existing header files will mostly be shims, provided for backward compatibility.
+        * While we're at it: leave nothing considered private in namespace detail.
+* Optional precompilation
+    * WRT PR #2445: jumping between 2 x the number of files is a permanent tax on development activity and an impediment to continuous refactoring.
+    * Keep code together, use `#ifndef PYBIND11_USE_PRECOMPILE_MODE`.
+    * Develop (small-ish) tool to split out `.cpp` files, integrate with CMake.
+
+
+# Targets for 3.0 / Jan 2022, whichever comes first:
+
+* Drop Python 2.7/3.5
+
+# Not on the roadmap (yet?)
+
+* Better MyPy integration
+* Better Scikit-build integration (waiting on Scikit-build developments)
+* Future of `type_caster_base`
+* Multiple-inheritance overhaul (to fix highly unobvious (to pybind11 users) UB)
+    * Porting Boost.Python inheritance code.
+    * Will probably be another round of refactoring, much smaller scale, but significant.