remapPathPrefixHook: disable automatic path remapping on Darwin builders (#950)

Author: ipetkov

CHANGELOG.md

@@ -9,6 +9,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 ### Changed
 * **Breaking**: dropped compatibility for Nix versions below 2.31.2
 * **Breaking**: dropped compatibility for nixpkgs-25.05
+* `remapPathPrefixHook` is now do nothing when building on Darwin since the
+  current implementation results in validating build caches due to Nix behavior
+  differences between Linux and Darwin.
 
 ### Fixed
 * `mkCargoDerivation` will now set `noCompressDebugSectionsSet` to disable

docs/API.md

@@ -1940,6 +1940,8 @@ a temporary location defined by `$postBuildInstallFromCargoBuildLogOut`
 
 ### `craneLib.remapSourcePathPrefixHook`
 
+> Note: this hook is not supported when building on Darwin and will do nothing
+
 Defines `configureRustcRemapPathPrefix()` which can be used to set up a source
 map using the [`--remap-path-prefix`] option. The output of the derivation gains
 a dependency on the source code stored within the Nix store, but in return

lib/setupHooks/remapPathPrefixHook.nix

@@ -1,9 +1,27 @@
 {
+  lib,
   makeSetupHook,
+  stdenv,
 }:
 makeSetupHook {
   name = "remapPathPrefixHook";
   substitutions = {
     storeDir = builtins.storeDir;
+
+    # Unfortunately, we cannot support automatic path remapping on Darwin. The remap paths option
+    # requires absolute paths (since rustc will basically do a blind substitution). On Linux builds,
+    # each derivation is chrooted to `/build/{name-of-src}` which ends up being the same string for
+    # both the real and the deps-only derivations. On Darwin, however, this ends up being
+    # `/nix/var/nix/builds/{name-of-derivation}` which *is* different between the main and deps-only
+    # derivations. Since this value ends up in CARGO_BUILD_RUSTFLAGS, it effectively will lead to
+    # cache invalidation when the real derivation runs if the values differ.
+    #
+    # Moreover, we can't "just" replace the bytes ourselves since there's no guarantee that the
+    # original build path is the same length as the un-neutered store path. Thus we'll noop this for
+    # now on Darwin builders and leave it up to the caller to handle path remap if needed...
+    isDarwin = lib.optionalString stdenv.buildPlatform.isDarwin /* bash */ ''
+      echo 'automatic path remapping not supported on Darwin'
+      return 0
+    '';
   };
 } ./remapPathPrefixHook.sh

lib/setupHooks/remapPathPrefixHook.sh

@@ -1,4 +1,6 @@
 configureRustcRemapPathPrefix() {
+  @isDarwin@
+
   local remapTo="${1:-${src:?not defined}}"
   local remapFrom="${2:-$(pwd)}"
   local doNeuter="${3:-neuter}"