new $nimbleDir path substitution yields unexpected search paths

[disruptek]

Given the following nim.cfg in an otherwise empty directory:

--clearNimblePath
--path="$nimbledir/foo"

nim dump now yields

#...
/home/adavidoff/.nimble/pkgs/foo
/opt/nimble/pkgs/foo
#...

Because I don't actually have any nimblePaths (due to --clearNimblePath), I expected no substitution, ie:

#...
$nimbledir/foo
#...

I tried adding --noNimblePath:

--noNimblePath
--clearNimblePath
--path="$nimbledir/foo"

and the output did not change. Then I tried adding a --nimblePath:

--clearNimblePath
--nimblePath="$config/goats"
--path="$nimbledir/foo"

and now my search paths (not lazy paths) include the following:

/home/adavidoff/git/t/goats/foo
/home/adavidoff/.nimble/pkgs/foo
/opt/nimble/pkgs/foo

and lazy paths includes /home/adavidoff/git/t/goats.

So it seems that there are two breaking changes introduced in #12750:

• we cannot remove these global paths at all
• paths (below the target) that do not exist are added via --nimblePath

nimph will just add --excludePath to the nim.cfg to exclude missing paths, but I feel a better solution would be to fix the compiler.

[genotrance]

we cannot remove these global paths at all

--clearNimblePath and --noNimblePath, only clear lazyPaths today. They do not affect what is being stored into conf.nimblePaths.

Here is what I propose: both --clearNimblePath and --noNimblePath will set conf.nimblePaths = @[]. If you then add a --path:"$nimbleDir/xyz-1.0.0" without adding a --nimblePath, it will fail since there's no $nimbleDir available to substitute with.

There isn't any error though - the path is simply not added at all and the module import fails. We can raise an error if preferred.

--nimblePath after a --noNimblePath won't get added at all so $nimbleDir will always fail. --clearNimblePath is more forgiving though and provides the behavior expected.