Turn on markdown for druntime doc build

[schveiguy]

I want to use the auto tester to see what happens when we turn on markdown. Don't merge, I may have to fix things first.

[dlang-bot]

Thanks for your pull request, @schveiguy!

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + druntime#3016"

[schveiguy]

OK I think as long as the docs come out OK, this is mergeable.

[schveiguy]

Hm... the ddox changed, how is that generated? I don't see anything in the makefile. The change is slightly worse than the original, but the original sucked too.

[schveiguy]

Even with the ddox change, I think this is good to merge.

[PetarKirov]

Hm... the ddox changed, how is that generated? I don't see anything in the makefile.

https://github.com/dlang/dlang.org/tree/36c76465537501a0b34f32792f5a76f1f9b3888d/dpl-docs

https://github.com/dlang/dlang.org/blob/36c76465537501a0b34f32792f5a76f1f9b3888d/posix.mak#L706

[schveiguy]

@PetarKirov thanks!

Hm.. not sure I can do anything. It looks like json output doesn't spit out markdown results.

[PetarKirov]

@schveiguy Actually the -preview=markdown flag shouldn't affect Ddox at all, since Ddox has its own parsing based on the json files output from dmd and and libdparse. Additionally it has its own macro engine for Ddoc: https://github.com/rejectedsoftware/ddox/blob/master/source/ddox/ddoc.d

[PetarKirov]

LGTM modulo one question: