;re-enable doctest suite, with a workaround for cabal 3 (#1139)

cabal.project

@@ -6,3 +6,14 @@ packages: hledger-lib
 allow-newer:
   brick:base
   config-ini:containers
+
+-- generate a ghc environment file, needed by doctest 
+-- (eg: ./.ghc.environment.x86_64-darwin-8.8.3). 
+-- cabal 3 doesn't write these by default.
+-- https://github.com/simonmichael/hledger/issues/1139
+-- https://github.com/sol/doctest/issues/245#issuecomment-583171683
+-- cabal 2 doesn't understand this flag, but it's just a warning; should be ok.
+-- cabal 3 still fails with doctest the first time; you have to run it
+-- a second time. Hopefully in practice it won't fail, because of a
+-- preceding cabal build.
+write-ghc-environment-files: always

hledger-lib/hledger-lib.cabal

@@ -4,7 +4,7 @@ cabal-version: 1.12
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: 4f9a789b279703a75cecaf4e5cca835f20a5ddb9cfeb9e98e62db665a573d016
+-- hash: 3d3dab519880fdc9325ef53cd2811154ab38f1e15f6e59e3d0b2b1c935d40337
 
 name:           hledger-lib
 version:        1.17.0.1
@@ -199,7 +199,6 @@ test-suite doctest
     , transformers >=0.2
     , uglymemo
     , utf8-string >=0.3.5
-  buildable: False
   if (impl(ghc < 8.2))
     buildable: False
   default-language: Haskell2010

hledger-lib/package.yaml

@@ -164,10 +164,10 @@ tests:
     dependencies:
     - hledger-lib
 
-  # Running this test suite with cabal requires cabal exec:
-  # cabal exec -- cabal test hledger-lib:test:doctest
+  # Note when run by cabal, doctest requires a ghc environment file,
+  # so we ensure this is generated, see cabal.project.
+  # https://github.com/simonmichael/hledger/issues/1139
   doctest:
-    buildable: false   # doesn't run with cabal test, https://github.com/simonmichael/hledger/issues/1139
     source-dirs: test
     main: doctests.hs  # can't call it doctest.hs ("File name does not match module name")
     other-modules: []  # prevent double compilation, https://github.com/sol/hpack/issues/188