Allow custom printing in nix repl

[infinisil]

Is your feature request related to a problem? Please describe.

Sometimes there are attribute sets with a public interface but an internal structure, which when passed to nix repl currently print their internal structure by default. A good example is the lib.fileset library:

nix-repl> lib.fileset.unions [ ./. ]
{ _internalBase = /home/tweagysil/src/nixpkgs/filesets; _internalBaseComponents = [ ... ]; _internalBaseRoot = /; _internalIsEmptyWithoutBase = false; _internalTree = "directory"; _internalVersion = 3; _type = "fileset"; }

Related to that: Nixpkgs derivations are currently always printed as their .drv:

nix-repl> hello
«derivation /nix/store/g2mxdrkwr1hck4y5479dww7m56d1x81v-hello-2.12.1.drv»

When really, there's a lot more to the interface of a package than just its .drv, things like passthru, meta, .override attributes, etc. (notably not all attributes).

Describe the solution you'd like

A special attribute like __nixReplShowString, which defines a __toString-like function. When an attribute set with such an attribute gets passed to the REPL, instead of printing it recursively, it should just evaluate that attribute and print the resulting string:

nix-repl> { name = "Paul"; __nixReplShowString = self: "Hello, ${self.name}!"; }
Hello, Paul!

This should take precedence over the existing derivation printing, such that Nixpkgs can customise it more.

Describe alternatives you've considered

Additional context

This issue is sponsored by Antithesis ✨

Priorities

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[roberth]

Custom functions are more fragile when it comes to possibly cyclic structures. Those are detected by the current printing routine. To recover that functionality, the special show function could return values in a tree form with value attributes for automatic deduplication.
Other reasons why returning structured data is nice, is that Nix could implement pretty printing, layouts, more semantically useful messages, etc.

• See also Structured markup for throw and trace messages that make Nix more robust #7795

[infinisil]

I think it's fine to keep it simple in this case: __nixReplShowString must return a string, which is directly printed. I don't think we have a need for anything more powerful for now. And if we do in the future we could add another special more general special attribute for it.

[SuperSandro2000]

We should definitely add a cli flag to turn this off in case of any issues for better debugging.