`.cabal` files: Make a trailing colon for stanzas a parse failure

[9999years]

#10525 made this mistake an error in cabal.project files:

source-repository-package:
    type: git
    location: https://github.com/parsonsmatt/foundation
    tag: 688c32ccd9a951bc96dd09423a6e6684f091d510
    subdir: basement
    subdir: foundation

Now, it prints this error:

Error: [Cabal-7090]
Error parsing project file cabal.project:52:
'source-repository-package' is a stanza, not a field. Remove the trailing ':' to parse a stanza.

Unfortunately, .cabal files use a ~completely different system for parsing, so the fix in #10525 doesn't apply to those!

Implementation strategy

For parsing cabal.project, the parseFieldsAndSections function (which #10525 modifies to error when stanzas are used as fields) is used with a list of top-level fields and a list of top-level stanzas (which themselves contain a grammar) to parse the file:

cabal/cabal-install/src/Distribution/Client/ProjectConfig/Legacy.hs

Lines 1250 to 1256 in 5ec4dd1

	parseLegacyProjectConfigFields :: ProjectConfigPath -> [ParseUtils.Field] -> ParseResult LegacyProjectConfig
	parseLegacyProjectConfigFields (ConstraintSourceProjectConfig -> constraintSrc) =
	parseFieldsAndSections
	(legacyProjectConfigFieldDescrs constraintSrc)
	legacyPackageConfigSectionDescrs
	legacyPackageConfigFGSectionDescrs
	mempty

E.g. Here's the section description for source-repository-package:

cabal/cabal-install/src/Distribution/Client/ProjectConfig/Legacy.hs

Lines 1805 to 1826 in 5ec4dd1

	packageRepoSectionDescr
	:: ( FieldGrammar c g
	, Applicative (g SourceRepoList)
	, c (Identity RepoType)
	, c (List NoCommaFSep FilePathNT String)
	, c (NonEmpty' NoCommaFSep Token String)
	)
	=> FGSectionDescr g LegacyProjectConfig
	packageRepoSectionDescr =
	FGSectionDescr
	{ fgSectionName = "source-repository-package"
	, fgSectionGrammar = sourceRepositoryPackageGrammar
	, fgSectionGet = map (\x -> ("", x)) . legacyPackagesRepo
	, fgSectionSet =
	\lineno unused pkgrepo projconf -> do
	unless (null unused) $
	syntaxError lineno "the section 'source-repository-package' takes no arguments"
	return
	projconf
	{ legacyPackagesRepo = legacyPackagesRepo projconf ++ [pkgrepo]
	}
	}

And here's the grammar for the fields contained in a source-repository-package:

cabal/cabal-install/src/Distribution/Client/Types/SourceRepo.hs

Lines 101 to 120 in 5ec4dd1

	sourceRepositoryPackageGrammar
	:: ( FieldGrammar c g
	, Applicative (g SourceRepoList)
	, c (Identity RepoType)
	, c (List NoCommaFSep FilePathNT String)
	, c (NonEmpty' NoCommaFSep Token String)
	)
	=> g SourceRepoList SourceRepoList
	sourceRepositoryPackageGrammar =
	SourceRepositoryPackage
	<$> uniqueField "type" srpTypeLens
	<*> uniqueFieldAla "location" Token srpLocationLens
	<*> optionalFieldAla "tag" Token srpTagLens
	<*> optionalFieldAla "branch" Token srpBranchLens
	<*> monoidalFieldAla "subdir" (alaList' NoCommaFSep FilePathNT) srpSubdirLens -- note: NoCommaFSep is somewhat important for roundtrip, as "." is there...
	<*> fmap (maybe [] toList) pcc
	where
	pcc = optionalFieldAla "post-checkout-command" (alaNonEmpty' NoCommaFSep Token) srpCommandLensNE
	{-# SPECIALIZE sourceRepositoryPackageGrammar :: ParsecFieldGrammar' SourceRepoList #-}
	{-# SPECIALIZE sourceRepositoryPackageGrammar :: PrettyFieldGrammar' SourceRepoList #-}

However, for .cabal files, the top-level grammar is defined manually:

cabal/Cabal-syntax/src/Distribution/PackageDescription/FieldGrammar.hs

Lines 97 to 161 in 5ec4dd1

	packageDescriptionFieldGrammar
	:: ( FieldGrammar c g
	, Applicative (g PackageDescription)
	, Applicative (g PackageIdentifier)
	, c (Identity BuildType)
	, c (Identity PackageName)
	, c (Identity Version)
	, forall from to. c (List FSep (RelativePathNT from to) (RelativePath from to))
	, forall from to. c (List VCat (RelativePathNT from to) (RelativePath from to))
	, c (List FSep TestedWith (CompilerFlavor, VersionRange))
	, c CompatLicenseFile
	, c CompatDataDir
	)
	=> g PackageDescription PackageDescription
	packageDescriptionFieldGrammar =
	PackageDescription
	<$> optionalFieldDefAla "cabal-version" SpecVersion L.specVersion CabalSpecV1_0
	<*> blurFieldGrammar L.package packageIdentifierGrammar
	<*> optionalFieldDefAla "license" SpecLicense L.licenseRaw (Left SPDX.NONE)
	<*> licenseFilesGrammar
	<*> freeTextFieldDefST "copyright" L.copyright
	<*> freeTextFieldDefST "maintainer" L.maintainer
	<*> freeTextFieldDefST "author" L.author
	<*> freeTextFieldDefST "stability" L.stability
	<*> monoidalFieldAla "tested-with" (alaList' FSep TestedWith) L.testedWith
	<*> freeTextFieldDefST "homepage" L.homepage
	<*> freeTextFieldDefST "package-url" L.pkgUrl
	<*> freeTextFieldDefST "bug-reports" L.bugReports
	<*> pure [] -- source-repos are stanza
	<*> freeTextFieldDefST "synopsis" L.synopsis
	<*> freeTextFieldDefST "description" L.description
	<*> freeTextFieldDefST "category" L.category
	<*> prefixedFields "x-" L.customFieldsPD
	<*> optionalField "build-type" L.buildTypeRaw
	<*> pure Nothing -- custom-setup
	-- components
	<*> pure Nothing -- lib
	<*> pure [] -- sub libs
	<*> pure [] -- executables
	<*> pure [] -- foreign libs
	<*> pure [] -- test suites
	<*> pure [] -- benchmarks
	-- * Files
	<*> monoidalFieldAla "data-files" (alaList' VCat RelativePathNT) L.dataFiles
	<*> optionalFieldDefAla "data-dir" CompatDataDir L.dataDir sameDirectory
	^^^ fmap (\x -> if null (getSymbolicPath x) then sameDirectory else x) -- map empty directories to "."
	<*> monoidalFieldAla "extra-source-files" formatExtraSourceFiles L.extraSrcFiles
	<*> monoidalFieldAla "extra-tmp-files" (alaList' VCat RelativePathNT) L.extraTmpFiles
	<*> monoidalFieldAla "extra-doc-files" formatExtraSourceFiles L.extraDocFiles
	<*> monoidalFieldAla "extra-files" formatExtraSourceFiles L.extraFiles
	^^^ availableSince CabalSpecV3_14 []
	where
	packageIdentifierGrammar =
	PackageIdentifier
	<$> uniqueField "name" L.pkgName
	<*> uniqueField "version" L.pkgVersion
	licenseFilesGrammar =
	(++)
	-- TODO: neither field is deprecated
	-- should we pretty print license-file if there's single license file
	-- and license-files when more
	<$> monoidalFieldAla "license-file" CompatLicenseFile L.licenseFiles
	<*> monoidalFieldAla "license-files" (alaList' FSep RelativePathNT) L.licenseFiles
	^^^ hiddenField

And the goSections parsing function just handles each field name manually:

cabal/Cabal-syntax/src/Distribution/PackageDescription/Parsec.hs

Line 286 in 5ec4dd1

| name == "library" && null args = do

So we should adapt those into the system used by cabal.project.