cmake, ci: updates for recent nixpkgs #238

ryanofsky commented at 7:12 PM on January 9, 2026: collaborator

This PR fixes two problems that happen with recent nixpkgs. The first commit works around compilation errors that happen when IWYU is enabled with (-DMP_ENABLE_IWYU=ON) that were caused by https://github.com/NixOS/nixpkgs/pull/445095. The second commit fixes problems building old versions of Cap'n Proto with CMake 4.0 in shell.nix.

Neither problem has showed up so far running in github actions, only running locally with unstable nixpkgs. More details about the problems and fixes are in the commit messages.

cmake: Fix IWYU in nix by adding CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES

Without this change, building with -DMP_ENABLE_IWYU=ON and newer versions of
nixpkgs (after 2025-11-12) fails with mysterious errors that look like:

make[2]: *** [CMakeFiles/mputil.dir/build.make:79: CMakeFiles/mputil.dir/src/mp/util.cpp.o] Error 1

with no other error messages. The are no messages because cmake -E
__run_co_compile hides the output from IWYU and does not display anything itself.

The actual problem is missing include directories on the compiler command line,
and the fix here is to extend a workaround previously added for clang-tidy to
be used for IWYU as well.

Breakage seems to have been caused by
https://github.com/NixOS/nixpkgs/pull/445095/changes/76a8ffa6d8d2f93b81d4e9be327a01cab1c893c0
from https://github.com/NixOS/nixpkgs/pull/445095, which I found by bisecting
with --first-parent to a change between the following merge commit:

https://github.com/NixOS/nixpkgs/commits/de21549a840be528ef4763dd7dca2006caf9c11f (bad)
https://github.com/NixOS/nixpkgs/commits/c4d1151093af5d483d1900d9214a65e68bc21560 (good)

If you check out the bad commit above (or any later commit) and revert
https://github.com/NixOS/nixpkgs/commit/76a8ffa6d8d2f93b81d4e9be327a01cab1c893c0
the problem disappears.

I suspect the problem happens because cmake is not handling -cxx-isystem
properly, but did not debug further. The later change
https://github.com/NixOS/nixpkgs/pull/462747 which followed up on
https://github.com/NixOS/nixpkgs/pull/445095 did not seem to have any effect on
this issue in my testing.

91a7759a9a

DrahtBot commented at 7:12 PM on January 9, 2026: none

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Conflicts

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#276 (build: prepare for subtree split by Sjors)

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ci: Fix shell.nix compatibility with CMake 4.0

CMake 4.0 dropped support for older policies, so versions of Cap'n Proto <=
v1.1.0 (before https://github.com/capnproto/capnproto/pull/2272) require an
additional option to build with CMake 4.0. Without this change, there are
errors like:

> CMake Error at CMakeLists.txt:1 (cmake_minimum_required):
>   Compatibility with CMake < 3.5 has been removed from CMake.
>
>   Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
>   to tell CMake that the project requires at least <min> but has been updated
>   to work with policies introduced by <max> or earlier.
>
>   Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.

This change fixes the problem by specifying -DCMAKE_POLICY_VERSION_MINIMUM=3.5
when needed.

db7acb3ce2

ryanofsky renamed this:
~~cmake: Fix IWYU in nix by adding CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES~~
cmake, ci: updates for recent nixpkgs
on Jan 9, 2026

hebasto approved

hebasto commented at 7:24 PM on January 12, 2026: member

ACK b66a6d35526f69a9f3cdb39e2cd85480732555ee, I have reviewed the code and it looks OK.

ryanofsky force-pushed on Jan 13, 2026

ryanofsky commented at 12:07 PM on January 13, 2026: collaborator

Updated b66a6d35526f69a9f3cdb39e2cd85480732555ee -> db7acb3ce279196607fd628126c58588ba582483 (pr/iwyufu.2 -> pr/iwyufu.3, compare) to remove an incorrect comment in the b66a6d35526f69a9f3cdb39e2cd85480732555ee commit message about the cmake 4.0 problem not affecting the olddeps CI job. The problem does affect the olddeps job when I run it locally. It doesn't affect github actions CI just because it is currently using CMake 3.31.6.

sedited approved

sedited commented at 10:18 AM on February 2, 2026: collaborator

ACK db7acb3ce279196607fd628126c58588ba582483

DrahtBot requested review from hebasto on Feb 2, 2026

hebasto approved

hebasto commented at 12:22 PM on February 2, 2026: member

re-ACK db7acb3ce279196607fd628126c58588ba582483.

in CMakeLists.txt:113 in 91a7759a9a

 108 | +  # Workaround for nix from https://gitlab.kitware.com/cmake/cmake/-/issues/20912#note_793338
 109 | +  # Nix injects header paths via $NIX_CFLAGS_COMPILE; CMake tags these as
 110 | +  # CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES and omits them from the compile
 111 | +  # database, so clang-tidy, which ignores $NIX_CFLAGS_COMPILE, can't find capnp
 112 | +  # headers. Setting them as standard passes them to clang-tidy.
 113 | +  set(CMAKE_CXX_STANDARD_INCLUDE_DIRECTORIES ${CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES})

ryanofsky commented at 2:43 PM on June 2, 2026:

In commit "cmake: Fix IWYU in nix by adding CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES" (91a7759a9abe4700d466adb94024dce81fa230ab)

Note: I might merge this PR anyway since it has two acks, but this comment isn't exactly right since newer versions of clang-tidy in nixpkgs fix the issue and make this workaround unnecessary. The fix was https://github.com/NixOS/nixpkgs/commit/d46fa2640e3d3f05b4da137be9b4f016cf043c37 (https://github.com/NixOS/nixpkgs/pull/462747).

The problem with IWYU still exists but can be fixed with a similar change, which should probably be a PR:

--- a/pkgs/development/tools/analysis/include-what-you-use/wrapper
+++ b/pkgs/development/tools/analysis/include-what-you-use/wrapper
@@ -4,7 +4,7 @@ buildcpath() {
   local path after
   while (( $# )); do
     case $1 in
-        -isystem)
+        -isystem|-cxx-isystem)
             shift
             path=$path${path:+':'}$1
             ;;

ryanofsky merged this on Jun 2, 2026

ryanofsky closed this on Jun 2, 2026