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Make command infra less stateful and more regular
Already, we had classes like `BuiltPathsCommand` and `StorePathsCommand` which provided alternative `run` virtual functions providing the implementation with more arguments. This was a very nice and easy way to make writing command; just fill in the virtual functions and it is fairly clear what to do. However, exception to this pattern were `Installable{,s}Command`. These two classes instead just had a field where the installables would be stored, and various side-effecting `prepare` and `load` machinery too fill them in. Command would wish out those fields. This isn't so clear to use. What this commit does is make those command classes like the others, with richer run functions. Not only does this restore the pattern making commands easier to write, it has a number of other benefits: - `prepare` and `load` are gone entirely! One command just hands just hands off to the next. - We can use `ref` instead of `std::shared_ptr`. The former must be initialized (so it is like Rust's `Box` rather than `Option<Box>`, This expresses the invariant that the installable are in fact initialized much better. This is possible because since we just have local variables not fields, we can stop worrying about the not-yet-initialized case. - Fewer lines of code! (Finally I have a large refactor that makes the number go down not up...) - `nix repl` is now implemented in a clearer way. The last item deserves further mention. `nix repl` is not like the other installable commands because instead working from once-loaded installables, it needs to be able to load them again and again. To properly support this, we make a new superclass `RawInstallablesCommand`. This class has the argument parsing and completion logic, but does *not* hand off parsed installables but instead just the raw string arguments. This is exactly what `nix repl` needs, and allows us to instead of having the logic awkwardly split between `prepare`, `useDefaultInstallables,` and `load`, have everything right next to each other. I think this will enable future simplifications of that argument defaulting logic, but I am saving those for a future PR --- best to keep code motion and more complicated boolean expression rewriting separate steps. Finally do note that I stopped overloading the `run` functions. The reason was we are liable to get `-Woverloaded-virtual` warnings from this. Yes, there is a workaround in putting `using BaseClass::virtual_method;` next to overrides, but since I made the `run`-delegation chain cheaper that would have required *far* more `using` statements. I figured it was less obnoxious to just slightly vary the names than force all the commands to do that. Helps with NixOS/rfcs#134
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