ci: remove cache steps in nix jobs

.github/workflows/workflow.yml

@@ -98,12 +98,6 @@ jobs:
     steps:
       - uses: actions/checkout@v6
       - uses: nixbuild/nix-quick-install-action@v34
-      - uses: nix-community/cache-nix-action@v7
-        with:
-          primary-key: |
-            nix-${{ runner.os }}-${{ github.job }}--${{ hashFiles('**/*.nix', '**/flake.lock') }}
-          restore-prefixes-first-match: nix-${{ runner.os }}-${{ github.job }}--
-          gc-max-store-size-linux: 2G
       - run: nix develop -i .#bootstrap-check_4_14 -c make release
 
   nix-build-ox:
@@ -119,12 +113,6 @@ jobs:
     steps:
       - uses: actions/checkout@v6
       - uses: nixbuild/nix-quick-install-action@v34
-      - uses: nix-community/cache-nix-action@v7
-        with:
-          primary-key: |
-            nix-${{ runner.os }}-${{ github.job }}--${{ hashFiles('**/*.nix', '**/flake.lock') }}
-          restore-prefixes-first-match: nix-${{ runner.os }}-${{ github.job }}--
-          gc-max-store-size-linux: 2G
       - run: nix develop -i .#bootstrap-ox -c make release
 
 #
@@ -308,12 +296,6 @@ jobs:
     steps:
       - uses: actions/checkout@v6
       - uses: nixbuild/nix-quick-install-action@v34
-      - uses: nix-community/cache-nix-action@v7
-        with:
-          primary-key: |
-            nix-${{ runner.os }}-${{ github.job }}--${{ hashFiles('**/*.nix', '**/flake.lock') }}
-          restore-prefixes-first-match: nix-${{ runner.os }}-${{ github.job }}--
-          gc-max-store-size-linux: 5G
       - run: nix develop .#rocq -c make test-rocq
 
   rocq-native:
@@ -323,12 +305,6 @@ jobs:
     steps:
       - uses: actions/checkout@v6
       - uses: nixbuild/nix-quick-install-action@v34
-      - uses: nix-community/cache-nix-action@v7
-        with:
-          primary-key: |
-            nix-${{ runner.os }}-${{ github.job }}--${{ hashFiles('**/*.nix', '**/flake.lock') }}
-          restore-prefixes-first-match: nix-${{ runner.os }}-${{ github.job }}--
-          gc-max-store-size-linux: 5G
       - run: nix develop .#rocq-native -c make test-rocq-native
 
   wasm:

.github/workflows/workflow.yml.in

@@ -97,12 +97,6 @@ jobs:
     steps:
       - uses: actions/checkout@v6
       - uses: nixbuild/nix-quick-install-action@v34
-      - uses: nix-community/cache-nix-action@v7
-        with:
-          primary-key: |
-            nix-${{ runner.os }}-${{ github.job }}--${{ hashFiles('**/*.nix', '**/flake.lock') }}
-          restore-prefixes-first-match: nix-${{ runner.os }}-${{ github.job }}--
-          gc-max-store-size-linux: 2G
       - run: nix develop -i .#bootstrap-check_4_14 -c make release
 
   nix-build-ox:
@@ -118,12 +112,6 @@ jobs:
     steps:
       - uses: actions/checkout@v6
       - uses: nixbuild/nix-quick-install-action@v34
-      - uses: nix-community/cache-nix-action@v7
-        with:
-          primary-key: |
-            nix-${{ runner.os }}-${{ github.job }}--${{ hashFiles('**/*.nix', '**/flake.lock') }}
-          restore-prefixes-first-match: nix-${{ runner.os }}-${{ github.job }}--
-          gc-max-store-size-linux: 2G
       - run: nix develop -i .#bootstrap-ox -c make release
 
 #
@@ -307,12 +295,6 @@ jobs:
     steps:
       - uses: actions/checkout@v6
       - uses: nixbuild/nix-quick-install-action@v34
-      - uses: nix-community/cache-nix-action@v7
-        with:
-          primary-key: |
-            nix-${{ runner.os }}-${{ github.job }}--${{ hashFiles('**/*.nix', '**/flake.lock') }}
-          restore-prefixes-first-match: nix-${{ runner.os }}-${{ github.job }}--
-          gc-max-store-size-linux: 5G
       - run: nix develop .#rocq -c make test-rocq
 
   rocq-native:
@@ -322,12 +304,6 @@ jobs:
     steps:
       - uses: actions/checkout@v6
       - uses: nixbuild/nix-quick-install-action@v34
-      - uses: nix-community/cache-nix-action@v7
-        with:
-          primary-key: |
-            nix-${{ runner.os }}-${{ github.job }}--${{ hashFiles('**/*.nix', '**/flake.lock') }}
-          restore-prefixes-first-match: nix-${{ runner.os }}-${{ github.job }}--
-          gc-max-store-size-linux: 5G
       - run: nix develop .#rocq-native -c make test-rocq-native
 
   wasm:

doc/hacking.rst

@@ -256,6 +256,28 @@ We have the following shells for specific tasks:
   + ``make test-rocq``: these work well on a regular Dune opam dev switch
   + ``make test-rocq-native``: these require the Rocq native compiler to run, and thus need OCaml 4.x
 
+Warming the Cachix Cache
+========================
+
+Stable devShell closures are cached at ``https://ocaml-dune.cachix.org`` so
+contributors and CI don't rebuild them from scratch. The contents are
+declared by the ``cache-bundle`` package in ``flake.nix``.
+
+Maintainers with write access can refresh the cache after a ``flake.lock``
+change:
+
+.. code:: console
+
+   $ export CACHIX_AUTH_TOKEN=...
+   $ out=$(nix build --no-link --print-out-paths .#cache-bundle)
+   $ cachix push ocaml-dune "$out"
+
+The bundle excludes the ``ox`` and ``scope`` devShells because they override
+``dune_3`` to the repo-built binary, so their closures change per commit and
+would churn the cache without giving cross-PR hits. When adding a new
+devShell, include it in ``cache-bundle`` only if its closure is stable per
+``flake.lock``.
+
 Testing Reverse Dependencies
 ============================
 

flake.nix

@@ -40,8 +40,14 @@
   };
 
   nixConfig = {
-    extra-substituters = "https://anmonteiro.nix-cache.workers.dev";
-    extra-trusted-public-keys = "ocaml.nix-cache.com-1:/xI2h2+56rwFfKyyFVbkJSeGqSIYMC/Je+7XXqGKDIY=";
+    extra-substituters = [
+      "https://ocaml-dune.cachix.org"
+      "https://anmonteiro.nix-cache.workers.dev"
+    ];
+    extra-trusted-public-keys = [
+      "ocaml-dune.cachix.org-1:kDYe5JY2FtD9pPxkikNA9QgtPrLvl6ryizPxW+0BsqA="
+      "ocaml.nix-cache.com-1:/xI2h2+56rwFfKyyFVbkJSeGqSIYMC/Je+7XXqGKDIY="
+    ];
   };
 
   outputs =
@@ -266,6 +272,33 @@
                 touch $out/success
               '';
             };
+          cache-bundle =
+            let
+              system = pkgs.stdenv.hostPlatform.system;
+              shells = self.devShells.${system};
+              # Skip shells using repo-built dune; closure changes per commit.
+              shellNames =
+                if system == "x86_64-linux" then
+                  [
+                    "default"
+                    "fmt"
+                    "doc"
+                    "microbench"
+                    "bootstrap-check"
+                    "bootstrap-check_4_14"
+                    "bootstrap-ox"
+                    "rocq"
+                    "rocq-native"
+                  ]
+                else
+                  [ ];
+            in
+            pkgs.linkFarm "dune-cache-bundle-${system}" (
+              map (n: {
+                name = "shell-${n}";
+                path = shells.${n}.inputDerivation;
+              }) shellNames
+            );
         }
       );