config: add replaceStdenv, cudaCapabilities, cudaForwardCompat options (#479234)

pkgs/stdenv/default.nix

@@ -43,7 +43,8 @@ in
 # Select the appropriate stages for the platform `system'.
 if crossSystem != localSystem || crossOverlays != [ ] then
   stagesCross
-else if config ? replaceStdenv then
+# The `or null` fallback is needed for contexts that don't use the module system (e.g. tarball builds).
+else if (config.replaceStdenv or null) != null then
   stagesCustom
 else if localSystem.isLinux then
   stagesLinux

pkgs/top-level/config.nix

@@ -266,6 +266,38 @@ let
       feature = "build packages with CUDA support by default";
     };
 
+    cudaCapabilities = mkOption {
+      type = types.listOf types.str;
+      default = [ ];
+      description = ''
+        A list of CUDA capabilities to build for.
+
+        Packages may use this option to control device code generation to
+        take advantage of architecture-specific functionality, speed up
+        compile times by producing less device code, or slim package closures.
+
+        For example, you can build for Ada Lovelace GPUs with
+        `cudaCapabilities = [ "8.9" ];`.
+
+        If not provided, the default value is calculated per-package set,
+        derived from a list of GPUs supported by that CUDA version.
+
+        See the [CUDA section](https://nixos.org/manual/nixpkgs/stable/#cuda) in
+        the Nixpkgs manual for more information.
+      '';
+    };
+
+    cudaForwardCompat = mkOption {
+      type = types.bool;
+      default = true;
+      description = ''
+        Whether to enable PTX support for future hardware.
+
+        When enabled, packages will include PTX code that can be JIT-compiled
+        for GPUs newer than those explicitly targeted by `cudaCapabilities`.
+      '';
+    };
+
     replaceBootstrapFiles = mkMassRebuild {
       type = types.functionTo (types.attrsOf types.package);
       default = lib.id;
@@ -294,6 +326,22 @@ let
       '';
     };
 
+    replaceStdenv = mkMassRebuild {
+      type = types.nullOr (types.functionTo types.package);
+      default = null;
+      defaultText = literalExpression "null";
+      description = ''
+        A function to replace the standard environment (stdenv).
+
+        The function receives an attribute set with `pkgs` and should return
+        a stdenv derivation.
+
+        This can be used to globally replace the stdenv with a custom one,
+        for example to use ccache or distcc.
+      '';
+      example = literalExpression "{ pkgs }: pkgs.ccacheStdenv";
+    };
+
     rocmSupport = mkMassRebuild {
       feature = "build packages with ROCm support by default";
     };