From cd5e16917d86334e6801ccb2387d49a5eb21d1f6 Mon Sep 17 00:00:00 2001
From: Robert Hensing <robert@roberthensing.nl>
Date: Sat, 11 Mar 2023 12:19:42 +0100
Subject: [PATCH] Add trivial source filter to avoid rebuilds

---
 CHANGELOG.md            |  1 +
 nix/haskell-project.nix | 29 +++++++++++++++++++++++------
 2 files changed, 24 insertions(+), 6 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 267b8a1..29701eb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,7 @@
   - #92: Add `devShell.mkShellArgs` to pass custom arguments to `mkShell`
   - #111: Add `devShell.extraLibraries` to add custom Haskell libraries to the devshell.
   - #100: `source-overrides` option now supports specifying Hackage versions.
+  - #114: Prevent Nix rebuilds of packages in sub-directories when parent contents change.
 - API changes
     - #37: Group `buildTools` (renamed to `tools`), `hlsCheck` and `hlintCheck` under the `devShell` submodule option; and allow disabling them all using `devShell.enable = false;` (useful if you want haskell-flake to produce just the package outputs).
     - #64: Remove hlintCheck (use [treefmt-nix](https://github.com/numtide/treefmt-nix#flake-parts) instead)
diff --git a/nix/haskell-project.nix b/nix/haskell-project.nix
index deb7981..0afc23d 100644
--- a/nix/haskell-project.nix
+++ b/nix/haskell-project.nix
@@ -58,14 +58,31 @@ in
                   hpack
                 ''
             else root;
+          setSrc = name: root: pkg: hlib.overrideSrc {
+                src =
+                  lib.cleanSourceWith {
+                    name = "source-${name}-${pkg.version}";
+                    src =
+                      # NOTE: Even though cabal2nix does run hpack automatically,
+                      # buildFromCabalSdist does not. So we must run hpack ourselves at
+                      # the original source level.
+                      realiseHpack name root;
+                  };
+              } pkg;
         in
         lib.mapAttrs
-          (name: value:
-            # NOTE: Even though cabal2nix does run hpack automatically,
-            # buildFromCabalSdist does not. So we must run hpack ourselves at
-            # the original source level.
-            let root = realiseHpack name value.root;
-            in fromSdist (self.callCabal2nix name root { })
+          (name: pkgCfg:
+            lib.pipe pkgCfg
+            [
+              (pkgCfg: self.callCabal2nix name pkgCfg.root { })
+
+              # Avoid rebuilding because of changes in parent directories
+              (setSrc name pkgCfg.root)
+
+              # Make sure all files we use are included in the sdist, as a check
+              # for release-worthiness.
+              fromSdist
+            ]
           )
           config.packages;