Compile modules that are needed by template haskell, even with -fno-code.

This patch relates to Trac #8025

The goal here is to enable typechecking of packages that contain some
template haskell. Prior to this patch, compilation of a package with
-fno-code would fail if any functions in the package were called from
within a splice.

downsweep is changed to do an additional pass over the modules,
targetting any ModSummaries transitively depended on by a module that
has LangExt.TemplateHaskell enabled. Those targeted modules have
hscTarget changed from HscNothing to the default target of the platform.

There is a small change to the prevailing_target logic to enable this.

A simple test is added.

I have benchmarked with and without a patched haddock
(available:https://github.com/duog/haddock/tree/wip-no-explicit-th-compi
lation).  Running cabal haddock on the wreq package results in a 25%
speedup on my machine:

time output from patched cabal haddock:

real    0m5.780s
user    0m5.304s
sys     0m0.496s
time output from unpatched cabal haddock:

real    0m7.712s
user    0m6.888s
sys     0m0.736s

Reviewers: austin, bgamari, ezyang

Reviewed By: bgamari

Subscribers: bgamari, DanielG, rwbarton, thomie

GHC Trac Issues: #8025

Differential Revision: https://phabricator.haskell.org/D3441

1 files changed, 1 insertions, 0 deletions

diff --git a/compiler/backpack/DriverBkp.hs b/compiler/backpack/DriverBkp.hs
index 86e84d13f9..a82e66b7b0 100644
--- a/compiler/backpack/DriverBkp.hs
+++ b/compiler/backpack/DriverBkp.hs
@@ -155,6 +155,7 @@ withBkpSession cid insts deps session_type do_this = do
       -- turn on interface writing.  However, if the user also
       -- explicitly passed in `-fno-code`, we DON'T want to write
       -- interfaces unless the user also asked for `-fwrite-interface`.
+      -- See Note [-fno-code mode]
       (case session_type of
         -- Make sure to write interfaces when we are type-checking
         -- indefinite packages.