672866cc

Compile an instrumented library once, not twice

Independent of #156, #157 and #158: this is `nix/mutationCheck.nix` only.

A library whose test suites a check runs is built twice from the same source with the same flags. `addManifest` builds it for its manifest, and `builtTestPkg` builds it again with `--enable-tests`, which is what produces the test executables. The second derivation does not reference the first at all, so nothing is reused and a cold check pays for the whole library twice.

The second build writes a manifest of its own, because the plugin runs over the library there exactly as it does in the first. For the same revision the two are byte-identical, which I checked by realising both outputs of one package and diffing them:

```
/nix/store/…-instrument-nix-ci-leader-data-manifest
/nix/store/…-testsuite-nix-ci-leader-data-manifest
IDENTICAL for the same revision
```

So the manifest now comes from whichever build the check needs anyway, and for a package whose suites are run here the instrument-only build is never realised. Verified on a caller that uses the `packages` argument: its mutation-report derivation went from referencing an `instrument` derivation to referencing none, while the test-suite derivation it reuses kept the same store path.

A library whose suites belong to another package (`libraries` plus `tests`, which is how this repository's own checks are written) has no `--enable-tests` build, so it keeps the instrument-only one. Those check derivations are unchanged, byte for byte:

```
development:                        /nix/store/gxya59g3ka85nnqkbd217axbv0c77fak-sydtest-mutation-example-mutation-check.drv
mutation-compile-instrumented-once: /nix/store/gxya59g3ka85nnqkbd217axbv0c77fak-sydtest-mutation-example-mutation-check.drv
```

Which is also the limitation worth stating: no check in this repository exercises the merged path, because none of them instruments a package and runs that package's own suites. The evidence for it is the manifest diff and the reference check above, both done against an out-of-tree caller.

Total

3m12s

Created 1 week ago Started 1 week ago Finished 1 week ago

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