stdenv: perform checks only when evaluating .drv and .out

This pushes check-meta evaluation to derivation evaluation step, leaving all other
attributes accessible.

Before this commit:

> $ HOME=/homeless-shelter NIX_PATH=nixpkgs=$(pwd) nix-instantiate --eval --strict ./default.nix -A xen --argstr system aarch64-linux
> Package ‘xen-4.5.5’ in pkgs/applications/virtualization/xen/generic.nix:226 is not supported on ‘aarch64-linux’, refusing to evaluate.

as expected

> $ HOME=/homeless-shelter NIX_PATH=nixpkgs=$(pwd) nix-instantiate --eval --strict ./default.nix -A xen.name --argstr system aarch64-linux
> Package ‘xen-4.5.5’ in pkgs/applications/virtualization/xen/generic.nix:226 is not supported on ‘aarch64-linux’, refusing to evaluate.

> $ HOME=/homeless-shelter NIX_PATH=nixpkgs=$(pwd) nix-instantiate --eval --strict ./default.nix -A xen.meta.description --argstr system aarch64-linux
> Package ‘xen-4.5.5’ in pkgs/applications/virtualization/xen/generic.nix:226 is not supported on ‘aarch64-linux’, refusing to evaluate.

which is unfortunate since its impossible to use packages in autogenerated
documentation on all platforms.

After this commit:

> $ HOME=/homeless-shelter NIX_PATH=nixpkgs=$(pwd) nix-instantiate --eval --strict ./default.nix -A xen --argstr system aarch64-linux

still fails

> $ HOME=/homeless-shelter NIX_PATH=nixpkgs=$(pwd) nix-instantiate --eval --strict ./default.nix -A xen.name --argstr system aarch64-linux
> "xen-4.5.5"

> $ HOME=/homeless-shelter NIX_PATH=nixpkgs=$(pwd) nix-instantiate --eval --strict ./default.nix -A xen.meta.description --argstr system aarch64-linux
> "Xen hypervisor and related components (vanilla)"

2 files changed, 29 insertions, 29 deletions

diff --git a/pkgs/stdenv/generic/check-meta.nix b/pkgs/stdenv/generic/check-meta.nix
index cd20c4a3cef..0a8dc006dc5 100644
--- a/pkgs/stdenv/generic/check-meta.nix
+++ b/pkgs/stdenv/generic/check-meta.nix
@@ -1,6 +1,5 @@
-# Extend a derivation with checks for brokenness, license, etc.  Throw a
-# descriptive error when the check fails; return `derivationArg` otherwise.
-# Note: no dependencies are checked in this step.
+# Checks derivation meta and attrs for problems (like brokenness,
+# licenses, etc).
 
 { lib, config, system, meta, derivationArg, mkDerivationArg }:
 
@@ -196,13 +195,11 @@ let
       { valid = false; reason = "unknown-meta"; errormsg = "has an invalid meta attrset:${lib.concatMapStrings (x: "\n\t - " + x) res}"; }
     else { valid = true; };
 
+   validity = checkValidity attrs;
+
+in validity // {
   # Throw an error if trying to evaluate an non-valid derivation
-  validityCondition =
-         let v = checkValidity attrs;
-         in if !v.valid
-           then handleEvalIssue (removeAttrs v ["valid"])
-           else true;
-
-in
-  assert validityCondition;
-  derivationArg
+  handled = if !validity.valid
+    then handleEvalIssue (removeAttrs validity ["valid"])
+    else true;
+}
diff --git a/pkgs/stdenv/generic/make-derivation.nix b/pkgs/stdenv/generic/make-derivation.nix
index 2d702ab389e..6038e2c339b 100644
--- a/pkgs/stdenv/generic/make-derivation.nix
+++ b/pkgs/stdenv/generic/make-derivation.nix
@@ -182,6 +182,14 @@ rec {
           outputs = outputs';
         } else { }));
 
+      validity = import ./check-meta.nix {
+        inherit lib config meta derivationArg;
+        mkDerivationArg = attrs;
+        # Nix itself uses the `system` field of a derivation to decide where
+        # to build it. This is a bit confusing for cross compilation.
+        inherit (stdenv) system;
+      };
+
       # The meta attribute is passed in the resulting attribute set,
       # but it's not part of the actual derivation, i.e., it's not
       # passed to the builder and is not a dependency.  But since we
@@ -207,21 +215,16 @@ rec {
 
     in
 
-      lib.addPassthru
-        (derivation (import ./check-meta.nix
-          {
-            inherit lib config meta derivationArg;
-            mkDerivationArg = attrs;
-            # Nix itself uses the `system` field of a derivation to decide where
-            # to build it. This is a bit confusing for cross compilation.
-            inherit (stdenv) system;
-          }))
-        ( {
-            overrideAttrs = f: mkDerivation (attrs // (f attrs));
-            inherit meta passthru;
-          } //
-          # Pass through extra attributes that are not inputs, but
-          # should be made available to Nix expressions using the
-          # derivation (e.g., in assertions).
-          passthru);
+      lib.extendDerivation
+        validity.handled
+        ({
+           overrideAttrs = f: mkDerivation (attrs // (f attrs));
+           inherit meta passthru;
+         } //
+         # Pass through extra attributes that are not inputs, but
+         # should be made available to Nix expressions using the
+         # derivation (e.g., in assertions).
+         passthru)
+        (derivation derivationArg);
+
 }