cabal check flag names

[phadej]

Related to #4686

---

Please include the following checklist in your PR:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.

Please also shortly describe how you tested your change. Bonus points for added tests!

[phadej]

Do we have cabal check tests in cabal test-suite? @ezyang ?

[hvr]

why not warn about any non-ascii flags?

invalidFlagName cs = any (not . isAscii) cs

[phadej]

Currently

[FL973] ~/mess/lead-dash % cat lead-dash.cabal 
name:                lead-dash
version:             0.1.0.0
synopsis:            Lorem
description:         Lorem Ipsum
license:             BSD3
license-file:        LICENSE
author:              Oleg Grenrus
maintainer:          oleg.grenrus@iki.fi
category:            Testing
build-type:          Simple
extra-source-files:  ChangeLog.md
cabal-version:       >=1.10

flag -foobar
  manual: True
  default: False

library
  exposed-modules:     LeadDash
  build-depends:       base >=4.9 && <4.10
  hs-source-dirs:      src
  default-language:    Haskell2010
[FL973] ~/mess/lead-dash % cabal check                                                                                                                          
No errors or warnings could be found in the package.
[FL973] ~/mess/lead-dash % /home/ogre/Documents/other-haskell/cabal/dist-newstyle/build/x86_64-linux/ghc-8.0.2/cabal-install-2.1.0.0/build/cabal/cabal check     
The following errors will cause portability problems on other environments:
* Suspicious flag names: -foobar. To avoid ambiguity in command line
interfaces, flag shouldn't start with a dash. Also for better compatibility,
flag names shouldn't contain non-ascii characters.

Hackage would reject this package.
[FL973] ~/mess/lead-dash % cabal configure                                                                                                                      
Resolving dependencies...
Configuring lead-dash-0.1.0.0...
[FL973] ~/mess/lead-dash % /home/ogre/Documents/other-haskell/cabal/dist-newstyle/build/x86_64-linux/ghc-8.0.2/cabal-install-2.1.0.0/build/cabal/cabal configure
Resolving dependencies...
Invalid flag assignment: --foobar
CallStack (from HasCallStack):
  error, called at ./Distribution/ReadE.hs:46:24 in Cabal-2.1.0.0-inplace:Distribution.ReadE

I.e. -foobar is still allowed in GenericPackageDescription parser, but you cannot really use it.

ATM it's simpler to disallow leading dash, as you cannot use it with current --constraint syntax, alternatively one could make --constraint +-flag work. I'm not sure it's worth the trouble.

Let's see what Travis says.

[23Skidoo]

ATM it's simpler to disallow leading dash

+1 on that.

[hvr]

That limitation only applies to Hackage as a point of distribution, but outside of Hackage developers are free to name their packages, module names and flags and descriptions in a more i18n way.

Here's a perfectly valid .cabal file including unicode package names and flag names:

name:                無
version:             0
synopsis:            The canonical non-package
build-type:          Simple
cabal-version:       >=1.10

source-repository head
  Type:     git
  Location: https://github.com/hvr/-.git

flag 無
  description: 無

library
  default-language:    Haskell2010

  exposed-modules: Ω

  if !flag(無)
    buildable:False

[phadej]

I'll change it to [[:alnum:]_][[:alnum:]_-]* and add a note about Hackage format.