Always pass ghc-options (#8717)

* Always pass `ghc-options`

The [documentation for `ghc-shared-options`][1] states that they are
combined with `ghc-options`:

> Additional options for GHC when the package is built as shared
> library. The options specified via this field are combined with the
> ones specified via `ghc-options`, and are passed to GHC during both
> the compile and link phases.

However, _only_ `ghc-shared-options` and not `ghc-options` are passed in
many cases.

This is an issue because it requires setting `ghc-shared-options` even
if the shared (dynamic) parts of the build don't actually need different
options; this has the unpleasant side-effect of causing modules to be
compiled twice, effectively doubling compile time! See here, where any
non-empty `ghc-shared-options` causes Cabal to not set `-dynamic-too`:

https://github.com/haskell/cabal/blob/acbc0f3a5cc9faf0913ff3e270196693816cec41/Cabal/src/Distribution/Simple/GHC.hs#L1466-L1469

This issue was discovered while integrating the `mold` linker with a
Haskell project.

[1]: https://cabal.readthedocs.io/en/latest/cabal-package.html#pkg-field-ghc-shared-options

* Add documentation

* Also enhance profArgs and profDynArgs

---------

Co-authored-by: Hécate Moonlight <hecate+github@glitchbra.in>
Co-authored-by: Hécate <Kleidukos@users.noreply.github.com>

Cabal/src/Distribution/Simple/GHC.hs

@@ -734,7 +734,7 @@ libAbiHash verbosity _pkg_descr lbi lib clbi = do
           , ghcOptFPic = toFlag True
           , ghcOptHiSuffix = toFlag "dyn_hi"
           , ghcOptObjSuffix = toFlag "dyn_o"
-          , ghcOptExtra = hcSharedOptions GHC libBi
+          , ghcOptExtra = hcOptions GHC libBi ++ hcSharedOptions GHC libBi
           }
     profArgs =
       vanillaArgs
@@ -746,7 +746,7 @@ libAbiHash verbosity _pkg_descr lbi lib clbi = do
                 (withProfLibDetail lbi)
           , ghcOptHiSuffix = toFlag "p_hi"
           , ghcOptObjSuffix = toFlag "p_o"
-          , ghcOptExtra = hcProfOptions GHC libBi
+          , ghcOptExtra = hcOptions GHC libBi ++ hcProfOptions GHC libBi
           }
     profDynArgs =
       vanillaArgs
@@ -760,7 +760,7 @@ libAbiHash verbosity _pkg_descr lbi lib clbi = do
           , ghcOptFPic = toFlag True
           , ghcOptHiSuffix = toFlag "p_dyn_hi"
           , ghcOptObjSuffix = toFlag "p_dyn_o"
-          , ghcOptExtra = hcProfSharedOptions GHC libBi
+          , ghcOptExtra = hcOptions GHC libBi ++ hcProfSharedOptions GHC libBi
           }
     ghcArgs =
       let (libWays, _, _) = buildWays lbi

Cabal/src/Distribution/Simple/GHCJS.hs

@@ -583,7 +583,7 @@ buildOrReplLib mReplFlags verbosity numJobs _pkg_descr lbi lib clbi = do
             , ghcOptFPic = toFlag True
             , --  ghcOptHiSuffix    = toFlag "dyn_hi",
               --  ghcOptObjSuffix   = toFlag "dyn_o",
-              ghcOptExtra = hcSharedOptions GHC libBi
+              ghcOptExtra = hcOptions GHC libBi ++ hcSharedOptions GHC libBi
             , ghcOptHPCDir = hpcdir Hpc.Dyn
             }
 
@@ -772,7 +772,7 @@ buildOrReplLib mReplFlags verbosity numJobs _pkg_descr lbi lib clbi = do
               , ghcOptDynLinkMode = toFlag GhcDynamicOnly
               , ghcOptInputFiles = toNubListR dynamicObjectFiles
               , ghcOptOutputFile = toFlag sharedLibFilePath
-              , ghcOptExtra = hcSharedOptions GHC libBi
+              , ghcOptExtra = hcOptions GHC libBi ++ hcSharedOptions GHC libBi
               , -- For dynamic libs, Mac OS/X needs to know the install location
                 -- at build time. This only applies to GHC < 7.8 - see the
                 -- discussion in #1660.
@@ -1330,7 +1330,7 @@ gbuild verbosity numJobs pkg_descr lbi bm clbi = do
               ghcOptFPic = toFlag True
             , ghcOptHiSuffix = toFlag "dyn_hi"
             , ghcOptObjSuffix = toFlag "dyn_o"
-            , ghcOptExtra = hcSharedOptions GHC bnfo
+            , ghcOptExtra = hcOptions GHC bnfo ++ hcSharedOptions GHC bnfo
             , ghcOptHPCDir = hpcdir Hpc.Dyn
             }
       dynTooOpts =
@@ -1781,7 +1781,7 @@ libAbiHash verbosity _pkg_descr lbi lib clbi = do
           , ghcOptFPic = toFlag True
           , ghcOptHiSuffix = toFlag "js_dyn_hi"
           , ghcOptObjSuffix = toFlag "js_dyn_o"
-          , ghcOptExtra = hcSharedOptions GHC libBi
+          , ghcOptExtra = hcOptions GHC libBi ++ hcSharedOptions GHC libBi
           }
     profArgs =
       vanillaArgs

changelog.d/pr-8717

@@ -0,0 +1,25 @@
+synopsis: Always pass `ghc-options` to GHC
+packages: Cabal
+prs: #8717
+issues:
+
+description: {
+
+Previously, options set in the package field `ghc-options` would not be passed
+to GHC during the link phase for shared objects (where multiple `.o` or
+`.dyn_o` files are merged into a single object file). This made it impossible
+to use `ghc-options` to use a different linker by setting (for example)
+`ghc-options: -optl-fuse-ld=mold -optlm-fuse-ld=mold`; the options would be
+dropped in the link phase, falling back to the default linker.
+
+It was possible to work around this by duplicating the `ghc-options` to
+`ghc-shared-options`, which _are_ passed in the shared link phase, but that had
+the (undocumented and unfortunate) side-effect of disabling the GHC
+`-dynamic-too` flag, effectively doubling compilation times when
+`ghc-shared-options` are set.
+
+Now, `ghc-options` are combined with `ghc-shared-options` (to accurately
+reflect the documentation on this feature) and the fact that
+`ghc-shared-options` disables `-dynamic-too` is documented.
+
+}

doc/cabal-package-description-file.rst

@@ -1780,6 +1780,11 @@ system-dependent values for these fields.
     ones specified via :pkg-field:`ghc-options`, and are passed to GHC during
     both the compile and link phases.
 
+    Note that if any :pkg-field:`ghc-shared-options` are set, the
+    ``-dynamic-too` option will never be passed to GHC, leading to all modules
+    being compiled twice (once to generate the ``.o`` files and another to
+    generate the ``.dyn_o`` files).
+
 .. pkg-field:: ghcjs-options: token list
 
    Like :pkg-field:`ghc-options` but applies to GHCJS