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nix flake check: Skip substitutable derivations

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Since `nix flake check` doesn't produce a `result` symlink, it doesn't
actually need to build/substitute derivations that are already known
to have succeeded, i.e. that are substitutable.

This can speed up CI jobs in cases where the derivations have already
been built by other jobs. For instance, a command like

  nix flake check github:NixOS/hydra/aa62c7f7db31753f0cde690f8654dd1907fc0ce2

should no longer build anything because the outputs are already in
cache.nixos.org.

Based-on: https://github.com/DeterminateSystems/nix-src/pull/134
Based-on: https://gerrit.lix.systems/c/lix/+/3841
Co-authored-by: Eelco Dolstra <edolstra@gmail.com>
This commit is contained in:
Seth Flynn 2025-08-05 13:56:09 -04:00 • committed by Jörg Thalheim
parent 1907a3300f
commit ecdda5798c
3 changed files with 55 additions and 2 deletions
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9
doc/manual/rl-next/faster-nix-flake-check.md Normal file
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---
synopsis: "`nix flake check` now skips derivations that can be substituted"
prs: [13574]
---
Previously, `nix flake check` would evaluate and build/substitute all
derivations. Now, it will skip downloading derivations that can be substituted.
This can drastically decrease the time invocations take in environments where
checks may already be cached (like in CI).