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I've suspected that this is the case for some time but I've confirmed it this morning. I ran synchronize and the highest revision was 94004 "Changes %s to %ls in wprintf in launcher.c for C99 compatibility." As expected MSVC rebuilt the launcher. Later I reran synchronize and the highest revision was 94009 "Update bundled pip and setuptools to 6.0.6 and 11.0.". The output from the 64 bit Release build concluded "31 succeeded, 0 failed, 5 up-to-date, 1 skipped". There also seems to be a toggle operating between the Release and Debug builds such that only one rebuilds at any one time.

This is because the hg id result has changed and we embed that into python35.dll. You'll see the same thing after an edit too (when the revision has + added).

To clarify a bit, there's very little re-compiling, but everything that references the pythoncore project (every extension) is re-linked. There's no way around that short of not including the hg revision (which I won't accept :), but the re-linking only takes a fraction of a second per project.

Then we're not talking about the same thing. Maybe my setup is wrong, but a load of files were recompiled (from memory I think from sections 3 and 4 of the Release build) so it took minutes rather than fractions of a second.

Hmmm, what are sections 3 and 4? Are you building from the VS GUI or
Command Prompt?
From Command Prompt in a clean checkout, running PCbuild\build.bat -d -e
(debug build) should take several minutes. The same again should be quick,
and then just "PCbuild\build.bat" (release build) should take a while, and
the same again should be quick. If you then do another debug build (add
"-d" back), it will take a bit of time due to rebuilding (or at least
re-"installing") Tcl/Tk. Otherwise, things should be pretty quick.
I haven't tested any of that this morning, though; how far off is that from
your experience, Mark?

I build from the GUI. I've just tried the Release build, it very quickly rebuilt the first four items and said the rest were up to date. I switched to Debug and got the output in the attached file. This is what I meant earlier by the effect toggling between the two builds.
I tried what you suggested from the command line. I'll attach the end of the output from the final run of the debug build in a moment as I can't see how to put it in in one pass here.

On testing, you are correct, Mark. Sorry for the premature close.
How does this patch look to you, Steve?

Ah, I forgot to put Configuration in there. That patch is fine.
If it's only the OpenSSL projects doing this, that shouldn't cause 31 projects to rebuild (4 at most I'd have though), but it probably will cause more rebuilds than necessary.

Our original explanation accounts for the 31 projects "rebuilding".
I'll get the patch committed later tonight (hopefully).

New changeset d53506fe31e1 by Zachary Ware in branch 'default':
Closes #23154: Fix unnecessary recompilation of OpenSSL on Windows
https://hg.python.org/cpython/rev/d53506fe31e1 

I've noticed a similar problem this morning with 5 modules rebuilt under Debug but 29 under Release. I believe the change that triggered me spotting it is this fb8a093db8b1 Issue #23095. Do we need a new issue for this?

That change modified pythoncore, so I'd expect to see most projects rebuild. Especially under Release, where we don't have some of the build optimizations enabled (because they hurt runtime performance).