libexpr: warn when encountering IFD with warn-import-from-derivation

This is intended to be a softer version of
`allow-import-from-derivation`, allowing for users to only flag when IFD
is encountered rather than error out

Based-on: https://github.com/NixOS/nix/pull/13279
Co-authored-by: gustavderdrache <alex.ford@determinate.systems>
Co-authored-by: Eelco Dolstra <edolstra@gmail.com>
Change-Id: I879a444e3a24b4e82a8cea3a5a94834c78c3924a

doc/manual/change-authors.yml

@@ -99,6 +99,9 @@ goldstein:
   forgejo: goldstein
   github: GoldsteinE
 
+gustavderdrache:
+  github: gustavderdrache
+
 horrors:
   display_name: eldritch horrors
   forgejo: pennae

doc/manual/rl-next/warn-import-from-derivation.md

@@ -0,0 +1,12 @@
+---
+synopsis: "Emit warnings when encountering IFD with `warn-import-from-derivation`"
+prs: [nix#13279]
+cls: [3879]
+category: Features
+credits: [getchoo, gustavderdrache, edolstra]
+---
+
+Instead of only being able to toggle the use of [Import from
+Derivation](https://nix.dev/manual/nix/stable/language/import-from-derivation) with
+`allow-import-from-derivation`, Lix is now able to warn users whenever IFD is encountered with
+`warn-import-from-derivation`.

lix/libexpr/meson.build

@@ -31,6 +31,7 @@ libexpr_setting_definitions = files(
   'settings/restrict-eval.md',
   'settings/trace-function-calls.md',
   'settings/trace-verbose.md',
+  'settings/warn-import-from-derivation.md',
   # keep-sorted end
 )
 liblix_generated_headers += custom_target(

lix/libexpr/primops.cc

@@ -82,6 +82,13 @@ StringMap EvalState::realiseContext(const NixStringContext & context)
             drvs.begin()->to_string(*ctx.store)
         ).debugThrow();
 
+    if (evalSettings.warnImportFromDerivation) {
+        printTaggedWarning(
+            "building '%1%' during evaluation due to the use of import from derivation",
+            drvs.begin()->to_string(*ctx.store)
+        );
+    }
+
     /* Build/substitute the context. */
     std::vector<DerivedPath> buildReqs;
     for (auto & d : drvs) buildReqs.emplace_back(DerivedPath { d });

lix/libexpr/settings/warn-import-from-derivation.md

@@ -0,0 +1,14 @@
+---
+name: warn-import-from-derivation
+internalName: warnImportFromDerivation
+type: bool
+default: false
+---
+
+By default, Lix allows you to `import` from a derivation, allowing
+building at evaluation time. With this option set to true, Lix will
+throw a warning when evaluating an expression that uses this feature,
+allowing users to know when their evaluation will requires any builds
+to take place.
+
+This option has no effect when `allow-import-from-derivation` is `false`.

tests/functional/meson.build

@@ -105,6 +105,7 @@ functional_tests_scripts = [
   'extra-sandbox-profile.sh',
   'regression-484.sh',
   'redirected-filter-source.sh',
+  'warn-import-from-derivation.sh',
 ]
 
 # Plugin tests require shared libraries support.

tests/functional/warn-import-from-derivation.sh

@@ -0,0 +1,8 @@
+source common.sh
+
+clearStore
+
+nix-build ./import-derivation.nix \
+  --no-out-link \
+  --option warn-import-from-derivation true \
+  2>&1 | grepQuiet "warning: building '.*' during evaluation due to the use of import from derivation"