Remove build-depends from pkgDescrFieldDescrs

[ezyang]

Remove the build-depends field from the top-level parser of Cabal files.

Why do I think this is OK? Two reasons:

1. We never consult buildDepends until after Distribution/PackageDescription/Configuration.hs runs, in which case the contents of the field get overwritten with the true dependency picks. We do consult the field for some package syntax checks but as I argue in Cabal.Distribution.PackageDescription.Check should be factored per component #2065 this the wrong place to try to do the check.
2. The build-depends field already gets lifted up by special case code in sectionizeFields to propagate it to the stanzas, which is the true codepath how we account for these build-depends.

Duncan would feel better if we ran this modification on Hackage to make sure it's good. But based on code reading and light testing, I think it's right.

Signed-off-by: Edward Z. Yang [email protected]

[23Skidoo]

/cc @dcoutts

[23Skidoo]

It looks like that there are around 935 (mostly old) packages on Hackage that use this form.

So instead of just removing this field I think we should deprecate it. Top-level build-depends should trigger a warning if Cabal-version is less than 1.21 and an error otherwise.

[ezyang]

@23Skidoo Nah, that's misunderstanding what the patch does. My point is that the top-level parser for build-depends is actually DEAD code: the 935 old packages which use this form, this case is handled with sectionizeFields. So, the expected behavior if you tried to compile one of these with my patched Cabal is that they would work, and indeed, this is the case:

[ezyang@hs01 TrieMap-0.0.1.2]$ ghc --make Setup.lhs -package-db=../.cabal-sandbox/x86_64-linux-
ghc-7.8.3-packages.conf.d/                                                                     
[1 of 1] Compiling Main             ( Setup.lhs, Setup.o )
Linking Setup ...
[ezyang@hs01 TrieMap-0.0.1.2]$ ./Setup configure
Configuring TrieMap-0.0.1.2...
Setup: At least the following dependencies are missing:
containers ==0.2.0.1

[23Skidoo]

Thanks for the explanation. Merging.