stdenv: Introduce hasCC attribute

Before, we'd always use `cc = null`, and check for that. The problem is
this breaks for cross compilation to platforms that don't support a C
compiler.

It's a very subtle issue. One might think there is no problem because we
have `stdenvNoCC`, and presumably one would only build derivations that
use that. The problem is that one still wants to use tools at build-time
that are themselves built with a C compiler, and those are gotten via
"splicing". The runtime version of those deps will explode, but the
build time / `buildPackages` versions of those deps will be fine, and
splicing attempts to work this by using `builtins.tryEval` to filter out
any broken "higher priority" packages (runtime is the default and
highest priority) so that both `foo` and `foo.nativeDrv` works.

However, `tryEval` only catches certain evaluation failures (e.g.
exceptions), and not arbitrary failures (such as `cc.attr` when `cc` is
null). This means `tryEval` fails to let us use our build time deps, and
everything comes apart.

The right solution is, as usually, to get rid of splicing. Or, baring
that, to make it so `foo` never works and one has to explicitly do
`foo.*`. But that is a much larger change, and certaily one unsuitable
to be backported to stable.

Given that, we instead make an exception-throwing `cc` attribute, and
create a `hasCC` attribute for those derivations which wish to
condtionally use a C compiler: instead of doing `stdenv.cc or null ==
null` or something similar, one does `stdenv.hasCC`. This allows quering
without "tripping" the exception, while also allowing `tryEval` to work.

No platform without a C compiler is yet wired up by default. That will
be done in a following commit.

1 files changed, 13 insertions, 3 deletions

diff --git a/pkgs/stdenv/generic/default.nix b/pkgs/stdenv/generic/default.nix
index 2f43db9cfc2..befeb450997 100644
--- a/pkgs/stdenv/generic/default.nix
+++ b/pkgs/stdenv/generic/default.nix
@@ -1,6 +1,15 @@
 let lib = import ../../../lib; in lib.makeOverridable (
 
-{ name ? "stdenv", preHook ? "", initialPath, cc, shell
+{ name ? "stdenv", preHook ? "", initialPath
+
+, # If we don't have a C compiler, we might either have `cc = null` or `cc =
+  # throw ...`, but if we do have a C compiler we should definiely have `cc !=
+  # null`.
+  #
+  # TODO(@Ericson2314): Add assert without creating infinite recursion
+  hasCC ? cc != null, cc
+
+, shell
 , allowedRequisites ? null, extraAttrs ? {}, overrides ? (self: super: {}), config
 
 , # The `fetchurl' to use for downloading curl and its dependencies
@@ -57,7 +66,8 @@ let
       ../../build-support/setup-hooks/move-sbin.sh
       ../../build-support/setup-hooks/move-lib64.sh
       ../../build-support/setup-hooks/set-source-date-epoch-to-latest.sh
-      cc
+      # TODO use lib.optional instead
+      (if hasCC then cc else null)
     ];
 
   defaultBuildInputs = extraBuildInputs;
@@ -145,7 +155,7 @@ let
 
       inherit overrides;
 
-      inherit cc;
+      inherit cc hasCC;
     }
 
     # Propagate any extra attributes.  For instance, we use this to