Is this basically how people are recommended to run cabal-docspec locally as well?

Pretty much, since it's not on Hackage (cf #52 (comment)).

Do you know how cabal-docspec is able to find these libraries? Does it call Cabal under the hood to install the necessary libraries listed here?

Also, does cabal-docspec work when building with stack?

Do you know how cabal-docspec is able to find these libraries? Does it call Cabal under the hood to install the necessary libraries listed here?

The package must exist in the plan.json. It is explained here

Also, does cabal-docspec work when building with stack?

cabal-docspec is looking for a cabal-install plan.json, I don't think stack provides such a thing?

Not being able to run doctests with stack or Nix seems pretty unfortunate. I'm not sure if I want to give that up.

I'll have to take a good look at the documentation you've linked to and think about how I feel about this.

@cdepillabout On my personal CI I use cabal-docspec the same way I use hlint, and I don't expect the results to diverge if Nix or stack is used. If you want I can create a third set of steps that is independent of cabal / stack.

Regarding Nix it's of course a bit more tricky since you need to patch the ELF.

@tchoutri Ah, sorry, maybe I don't quite understand because I haven't yet read the cabal-docspec manual.

I was assuming that since cabal-docspec reads the plan.json, and stack doesn't output a plan.json, that people using stack for development can't easily run cabal-docspec. (Same thing with Nix, or, for example, building password in Nixpkgs.)

I don't want to give up easy support for running doctests in stack and Nix. Although, like you say on the issue, if cabal-doctest is no longer being maintained, then maybe I have no choice here...

@cdepillabout Okay I was wrong, you indeed need to build with cabal-install (which is the thing that provides plan.json, not necessarily the Cabal library).

I don't want to give up easy support for running doctests in stack and Nix. Although, like you say on the issue, if cabal-doctest is no longer being maintained, then maybe I have no choice here...

Unfortunately as you say, this easy support might disappear one day. :/
The best path forward from here is to team up with the Cabal team so that the library provides enough features that stack and cabal-install can implement lightweight runners on top of it and reach feature parity. I don't know much about how cabal2nix would do it though.