Post "Synchronize flake inputs"

Author: tinybeachthor

posts/flakes-sync.md

@@ -0,0 +1,82 @@
+---
+title: Synchronize flake inputs
+description: How to not end up with 20 different compiler versions
+date: Jun 22, 2022
+tags: [ nix, flakes ]
+
+---
+
+# Synchronize flake inputs
+
+When working across multiple rust crates, of course as nix flakes, 
+it is very easy to end up with multiple minor versions of the toolchain.
+Which is annoying and does not help all that much at all - 
+the strong nix reproducibility is a nuisance in this case.
+
+## Solution
+
+Using a super-flake to re-export the packages we can get the same version
+of the toolchain for multiple crate flakes.
+
+The super-flake will look something like this:
+
+```nix
+{
+  inputs = {
+    flake-utils.url = github:numtide/flake-utils/master;
+    nixpkgs.url = github:NixOS/nixpkgs/nixos-22.05;
+    rust-overlay = {
+      url = github:oxalica/rust-overlay/master;
+      inputs = {
+        nixpkgs.follows = "nixpkgs";
+        flake-utils.follows = "flake-utils";
+      };
+    };
+  };
+
+  outputs = { self, flake-utils, nixpkgs, rust-overlay }:
+    {
+      lib = flake-utils.lib;
+    }
+    //
+    flake-utils.lib.eachDefaultSystem (system: rec
+    {
+      packages = (import nixpkgs {
+        inherit system;
+        overlays = [
+          rust-overlay.overlay
+        ];
+      });
+    }); 
+}
+```
+
+We re-export the `nixpkgs` with `rust-overlay` applied, 
+and also `flake-utils.lib` for convenience.
+
+The usage is simple:
+
+```nix
+{
+  inputs = {
+    nix.url = github:${REPO_OWNER}/${REPO_NAME}/master;
+  };
+
+  outputs = { self, nix }:
+    with nix.lib;
+    eachSystem [ system.x86_64-linux ] (system: {
+      devShell = import ./shell.nix {
+        pkgs = nix.packages.${system};
+      };
+  });
+}
+```
+
+## Bonus: auto-update
+
+To auto update the input we can add the following command to the end of 
+[.envrc](https://direnv.net/):
+
+```sh
+nix flake update nix --commit-lock-file
+```