Re-enable -rtsopts for stack + document RTS opts via exec

[mgsloan]

I am forking this off from #2022 because that thread got rather involved. I think the summary is:

1. stack exec -- exeName +RTS ... works fine to pass RTS options because -- causes further RTS arguments to not be processed.

2. stack exec exeName -- +RTS .. doesn't work because the -- is also visible to the exeName process.

One option for resolving this would be to deprecate the stack exec exeName -- ARGS form, perhaps even the stack exec exeName form. How can we deprecate it though? That would require adding stuff to stdout or stderr.

Either way, we should re-enable -rtsopts for stack, may as well have them if they are being parsed out of the commandline. Either way, we should thoroughly document using RTS options with exec.

[AndreasPK]

This is not correct. Under windows 10 with Stack 1.3.2 and GHC 8.0.2 I get

> stack exec -- echo test +RTS -s1 -RTS -s2
test -s2

> stack exec echo -- test +RTS -s1 -RTS -s2
test -s2

> stack exec -- echo -- test +RTS -s1 -RTS -s2
-- test -s2

In ALL cases stack consumes the RTS options. Only --RTS would cause further rts arguments to be not processed according to #2022 (comment)

Passing --RTS works but is not an options since we would have to pass that to stack BEFORE the runtime can parse it:

> stack exec -- echo --RTS test +RTS -s1 -? -RTS -s2
test +RTS -s1 -? -RTS -s2

I wrote up code that adds an rts-options flag to explicitly pass RTS options along and will submit that as pull request when it's fleshed out. The result will be:

> stack exec --rts-options="-s" -- echo test
test +RTS -s -RTS

Edit: Under linux stack exec -- executable +RTS -s works it seems to be Windows specific for that to fail. Might be worth creating an Issue with GHC for this.

[rwbarton]

The original issue description is correct, except to the extent that there's a bug on Windows only (which @AndreasPK identified and reported) that causes RTS options (or more properly, options that look like RTS options) that come after a -- not to be included in the program's argument list as returned by getArgs. The bug is https://ghc.haskell.org/trac/ghc/ticket/13287.

[AndreasPK]

Can this be closed via (#3012)?

[mgsloan]

@AndreasPK Yep, thanks for noticing that!