GHC developers can see how their changes will affect a curated collection of packages that build against a very recent mainline version of GHC.
Clone horizon-core. This is a package set that comes equipped with a collection of several hundred packages built against two different flavours of GHC, with and without GHC’s profiling enabled. Simply edit the flake.nix to point to a different revision of GHC. You can then do for example:
nix build .#lens