Use fetchTree / github fetcher for CI

This seems to be much more space-efficient: ~/.cache/nix/tarball-cache
is about 700MiB uncompressed, under 300MiB as tzst (using tar's
defaults, CI uses zstdmt but I assume will be in the same ballpark). The
gitv3 cache is multiple GiB.

CI will still build doom-example using fetchGit. I intend to shrink the
number of modules enabled in the example to keep gitv3 cache size under
control.

checks.nix

@@ -31,6 +31,7 @@ let
     # TODO: drop after NixOS 24.05 release.
     emacs = emacs29;
     doomLocalDir = "~/.local/share/nix-doom-unstraightened";
+    experimentalFetchTree = true;
   };
   mkDoom = args: (makeDoomPackages (common // args)).doomEmacs;
   mkDoomDir = args: writeTextDir "init.el" (toInit args);

default.nix

@@ -39,6 +39,8 @@
   profileName ? "nix",
   /* Disable profile early in startup, so "normal" cache/state dirs are used. */
   noProfileHack ? false,
+  /* Use fetchTree instead of fetchGit for package fetches. */
+  experimentalFetchTree ? false,
 
   callPackages,
   git,
@@ -233,10 +235,7 @@ let
                   else (throw "${name}: cannot derive url from recipe ${p.recipe or "<missing>"}"));
             # Use builtins.fetchGit instead of nixpkgs's fetchFromGitHub because
             # fetchGit allows fetching a specific git commit without a hash.
-            # TODO: port to fetchTree once (mostly) stable
-            # (in particular the github fetcher may be noticably more efficient)
-            src = builtins.fetchGit (
-              {
+            fetchGitArgs = {
               inherit url;
               rev = pin;
               allRefs = true;
@@ -252,8 +251,24 @@ let
               # Always specify a ref to work around https://github.com/NixOS/nix/issues/10773
               ref = p.recipe.branch or "HEAD";
             }
-              // optionalAttrs (p ? recipe.depth) { shallow = p.recipe.depth == 1; }
-            );
+            // optionalAttrs (p ? recipe.depth) { shallow = p.recipe.depth == 1; };
+            src =
+              if experimentalFetchTree
+              then builtins.fetchTree (
+                if lib.hasPrefix "https://github.com/" url
+                then let
+                  tail = lib.removePrefix "https://github.com/" url;
+                  split = lib.splitString "/" tail;
+                  owner = lib.head split;
+                  repo = lib.removeSuffix ".git" (lib.elemAt split 1);
+                in {
+                  type = "github";
+                  inherit owner repo;
+                  rev = pin;
+                } else ({
+                  type = "git";
+                } // fetchGitArgs))
+              else builtins.fetchGit fetchGitArgs;
             # Run locally to avoid a network roundtrip.
             reqfile = runCommandLocal "${name}-deps" { } ''
               ${lib.getExe emacs} -Q --batch --script \

flake.nix

@@ -101,6 +101,7 @@
             doomDir = ./doomdirs/minimal;
             doomLocalDir = "~/.local/share/nix-doom-unstraightened";
             full = true;
+            experimentalFetchTree = true;
           }).doomEmacs.emacsWithPackages.deps;
         };
       });