tests: Introduce a way to disable options when running in tests (#1095)

In our basic template we used to provide a check based on
`mkTestDerivationFromNvim`. The issue with this check (that is handled
correctly internally) is that some plugins _can't_ be used in the test
environment, for example image.nvim like in #1085.

This commit introduces a new function to generate such checks,
`mkTestDerivationFromNixvimModule`, that wraps a nixvim configuration
instead of a built nvim instance.

Then a configuration can rely on the newly added
`helpers.enableExceptInTests` attribute to disable parts of the
configuration depending if it is evaluated in tests or in a real final
configuration.

Resolves #1085

tests/default.nix

@@ -1,11 +1,12 @@
 {
   makeNixvim,
+  makeNixvimWithModule,
   lib,
   helpers,
   pkgs,
 }: let
   fetchTests = import ./fetch-tests.nix;
-  test-derivation = import ./test-derivation.nix {inherit pkgs makeNixvim;};
+  test-derivation = import ./test-derivation.nix {inherit pkgs makeNixvim makeNixvimWithModule;};
   inherit (test-derivation) mkTestDerivation;
 
   # List of files containing configurations

tests/enable-except-in-tests.nix

@@ -0,0 +1,41 @@
+{
+  pkgs,
+  mkTestDerivationFromNixvimModule,
+  makeNixvimWithModule,
+}: let
+  module = {helpers, ...}: {
+    plugins.image.enable = helpers.enableExceptInTests;
+  };
+
+  inTest = mkTestDerivationFromNixvimModule {
+    name = "enable-except-in-tests-test";
+    inherit pkgs module;
+  };
+
+  notInTest = let
+    nvim = makeNixvimWithModule {
+      inherit pkgs module;
+    };
+  in
+    pkgs.runCommand "enable-except-in-tests-not-in-test" {
+      printConfig = "${nvim}/bin/nixvim-print-init";
+    } ''
+      if ! "$printConfig" | grep 'require("image").setup'; then
+        echo "image.nvim is not present in the configuration"
+        echo -e "configuration:\n$($printConfig)"
+        exit 1
+      fi
+
+      touch $out
+    '';
+in
+  pkgs.linkFarm "enable-except-in-tests" [
+    {
+      name = "in-test";
+      path = inTest;
+    }
+    {
+      name = "not-in-test";
+      path = notInTest;
+    }
+  ]

tests/test-derivation.nix

@@ -1,6 +1,7 @@
 {
   pkgs,
   makeNixvim,
+  makeNixvimWithModule,
 }: let
   # Create a nix derivation from a nixvim executable.
   # The build phase simply consists in running the provided nvim binary.
@@ -40,6 +41,21 @@
       '';
     };
 
+  mkTestDerivationFromNixvimModule = {
+    name ? "nixvim-check",
+    pkgs,
+    module,
+    extraSpecialArgs ? {},
+  }: let
+    nvim = makeNixvimWithModule {
+      inherit pkgs module extraSpecialArgs;
+      _nixvimTests = true;
+    };
+  in
+    mkTestDerivationFromNvim {
+      inherit name nvim;
+    };
+
   # Create a nix derivation from a nixvim configuration.
   # The build phase simply consists in running neovim with the given configuration.
   mkTestDerivation = name: config: let
@@ -56,5 +72,5 @@
       inherit (testAttributes) dontRun;
     };
 in {
-  inherit mkTestDerivation mkTestDerivationFromNvim;
+  inherit mkTestDerivation mkTestDerivationFromNvim mkTestDerivationFromNixvimModule;
 }