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<@U054MQUPL>: only to be used as the key in a `Get`

`Get(OutputType, InputType, thing)` requires that `InputType` and `thing` are hashable

it does not require that `OutputType` is hashable

we reeeally need shorthand for "select the target that owns this file"

... intern project? <@U054MQUPL>: i think it would be a `TargetRootsCalculator` edit

will spend some time sketching that out today

the hardest bit there I think would be how to specify the files vs targets; `./pants filemap` uses passthru args like `./pants filemap -- x/y/z.py` to differentiate. in the case of `TargetRoots` form, we would use.. options?

or a net new notation? e.g. `./pants x f:x/y/z.py`

but yeah, fundamentally very similar to changed mechanics

essentially its just explicit “changed files” specification vs letting git do that

then you’d get things like transitive flags etc for free

<@U054MQUPL>: i think that it would just be a target spec... i don't think they're ambiguous with files

could see ambiguity if we ever wanted (or someone expected) that to be able to be a dir

but we could explicitly disallow that I guess

it's a parsing ambiguity, yea... but i think it's straightforward to resolve

in favor of e.g. shell-side globbing (`x/y/*`)

<@U6ZRNH0TC>: i should have noticed it earlier, but watch out for stuff like: ```@rule(SetupPyInvocationEnvironment, [Select(NativeToolchain)])
def get_setup_py_env(native_toolchain):
  joined_path = get_joined_path(
    native_toolchain.path_entries(), os.environ.copy())
  return SetupPyInvocationEnvironment(joined_path)```

it is effectively impossible to do that correctly today, because of memoization... `os.environ` is mutable

so that's going to be gone anyway -- i do recognize what you're saying though

in fact it should be gone so i missed something during the rebase already