Remove `Forward constructor

[Leonidas-from-XIV]

I was trying to understand what `Forward does. To do so figure out where a value of `Forward is constructed (from scratch, not as part of a map operation) and see what context this is useful. But it seems that such a value is never constructed. To test my hypothesis I removed the `Forward constructor from the codebase and it seems to compile fine and the tests also pass.

Thus my investigation seems to point to the fact that `Forward is unnecessary and can be removed. I'm opening a draft PR and invite maintainers to take a look - maybe I am missing some context in which `Forward is used, but if anything then at least it points at the fact that this functionality is not covered by any tests.

[art-w]

I agree it looks like pure dead code!

Probably dead code too (?):

odoc/src/xref2/tools.ml

Lines 956 to 958 in 1c3102c

	| `Forward f ->
	resolve_module env (`Root (ModuleName.make_std f))
	|> map_error (fun e -> `Parent (`Parent_module e))

odoc/src/xref2/tools.mli

Lines 33 to 37 in 1c3102c

	If the path is a 'Forward' path, that is, a path to a module that has not
	yet been compiled, then it may not be possible to resolve the path if this
	is being called during the 'compile' phase, in which case the function will
	return an unresolved path with no component. Resolution should be attempted
	again during the link phase.

(env.ml also have some Forward which might be removable, and the UnresolvedForwardPath error seems guarded by is_module_forward = const false)

[Leonidas-from-XIV]

Good points, I removed some more unreachable code.

I hoped I could remove the UnresolvedForwardPath error, but it is emitted by

odoc/src/xref2/tools.ml

Lines 953 to 954 in 1c3102c

	| Some Env.Forward ->
	Error (`Parent (`Parent_sig `UnresolvedForwardPath))

. I looked up Forward and through some layers it seems this can only happen if

odoc/src/odoc/resolver.ml

Line 328 in 1c3102c

if important_digests then Ok Odoc_xref2.Env.Forward_reference

evaluates to a a Forward_reference which is dependent on the function called with important_digests = true. This seems to be set when the odoc binary is launched with the -r/--resolve-fwd-refs option. Unfortunately this path seems to be untested, so I am a bit cautious removing things that might have a user-visible effect.