Packages built with `haskellPackages.callHackage` won't be rebuilt when
updating `all-cabal-hashes`.
The removed comment was keeping a reference to the `cabal2nix` call,
which itself depends on `all-cabal-hashes`, in order to keep this file
during a garbage collection.
The tradeoff is between:
- The current behavior: a mass rebuild, any change of `all-cabal-hashes`
triggers a rebuild of all the packages built with `callHackage` and
packages which depend on them. This can take hours, and may happen
after a "small" unrelated change (i.e. an user is bumping
`all-cabal-hashes` in order to use a new package from hackage). It
also have global impacts in a project (long rebuild in CI, new entries
in cache, developers need to fetch the new entries, ...). In this
context, `cabal2nix` entries are not garbage collected.
- The new behavior: No mass rebuild, but `cabal2nix` derivations need to
be recomputed after a garbage collection. This is usually fast (a few
seconds by call), linear with the number of calls and should not
happen a lot (i.e. users are not garbage collecting everyday).
See https://github.com/NixOS/nixpkgs/issues/194751 for details.
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