Determine if the tag is a ref or a rev

Test the `tag` attribute in the `cabal.project` file. If it looks like a
git hash, then assign it to `rev`. Otherwise, assign it to `ref`.

Author: parsonsmatt

lib/cabal-project-parser.nix

@@ -74,9 +74,15 @@ let
   #   --shar256: 003lm3pm0000hbfmii7xcdd9v20000flxf7gdl2pyxia7p014i8z
   # will be trated like a field and returned here
   # (used in call-cabal-project-to-nix.nix to create a fixed-output derivation)
-  extractSourceRepoPackageData = cabalProjectFileName: sha256map: repo: {
+  extractSourceRepoPackageData = cabalProjectFileName: sha256map: repo:
+    let
+      refOrRev =
+        if builtins.match "[0-9a-f](40)" repo.tag
+          then "rev"
+          else "ref";
+    in {
     url = repo.location;
-    ref = repo.tag;
+    "${refOrRev}" = repo.tag;
     sha256 = repo."--sha256" or (
       if sha256map != null
         then sha256map."${repo.location}"."${repo.tag}"

lib/call-cabal-project-to-nix.nix

@@ -228,7 +228,7 @@ let
                   then throw "${inputMap.${repoData.url}.rev} may not match ${repoData.ref} for ${repoData.url} use \"${repoData.url}/${repoData.ref}\" as the inputMap key if ${repoData.ref} is a branch or tag that points to ${inputMap.${repoData.url}.rev}."
                   else inputMap.${repoData.url})
             else if repoData.sha256 != null
-              then fetchgit { inherit (repoData) url sha256; rev = repoData.ref; }
+              then fetchgit repoData
             else
               let drv = builtins.fetchGit { inherit (repoData) url ref; };
               in __trace "WARNING: No sha256 found for source-repository-package ${repoData.url} ${repoData.ref} download may fail in restricted mode (hydra)"