From 51b92ee2efb8f8767d4feba7fa1f31c688fb7f1a Mon Sep 17 00:00:00 2001
From: Yvan Sraka <yvan@sraka.xyz>
Date: Tue, 29 Aug 2023 09:46:17 +0200
Subject: [PATCH] Add deprecation warnings to Nix integration

https://cabal.readthedocs.io/en/latest/nix-integration.html
---
 cabal-install/src/Distribution/Client/Nix.hs             | 5 ++++-
 cabal-install/src/Distribution/Client/Setup.hs           | 6 +++---
 .../PackageTests/Freeze/enable-benchmarks.out            | 1 +
 cabal-testsuite/PackageTests/Freeze/enable-tests.out     | 1 +
 doc/nix-integration.rst                                  | 9 +++++++++
 5 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/cabal-install/src/Distribution/Client/Nix.hs b/cabal-install/src/Distribution/Client/Nix.hs
index 34a12f9157b..a2c75e897e1 100644
--- a/cabal-install/src/Distribution/Client/Nix.hs
+++ b/cabal-install/src/Distribution/Client/Nix.hs
@@ -47,7 +47,7 @@ import Distribution.Simple.Program
   , simpleProgram
   )
 import Distribution.Simple.Setup (fromFlagOrDefault)
-import Distribution.Simple.Utils (debug, existsAndIsMoreRecentThan)
+import Distribution.Simple.Utils (debug, existsAndIsMoreRecentThan, warn)
 
 import Distribution.Client.Config (SavedConfig (..))
 import Distribution.Client.GlobalFlags (GlobalFlags (..))
@@ -141,6 +141,9 @@ nixShell verb dist globalFlags config go = do
   if alreadyInShell
     then go
     else do
+      -- Nix integration never worked with cabal-install v2 commands ...
+      warn verb "Nix integration has been deprecated and will be removed in a future release. You can learn more about it here: https://cabal.readthedocs.io/en/latest/nix-integration.html"
+
       findNixExpr globalFlags config >>= \case
         Nothing -> go
         Just shellNix -> do
diff --git a/cabal-install/src/Distribution/Client/Setup.hs b/cabal-install/src/Distribution/Client/Setup.hs
index 6a96fb54640..44224d9559b 100644
--- a/cabal-install/src/Distribution/Client/Setup.hs
+++ b/cabal-install/src/Distribution/Client/Setup.hs
@@ -502,17 +502,17 @@ globalCommand commands =
               )
               ""
               ["nix"] -- Must be empty because we need to return PP.empty from viewAsFieldDescr
-              "Nix integration: run commands through nix-shell if a 'shell.nix' file exists (default is False)"
+              "[DEPRECATED] Nix integration: run commands through nix-shell if a 'shell.nix' file exists (default is False)"
           , noArg
               (Flag True)
               []
               ["enable-nix"]
-              "Enable Nix integration: run commands through nix-shell if a 'shell.nix' file exists"
+              "[DEPRECATED] Enable Nix integration: run commands through nix-shell if a 'shell.nix' file exists"
           , noArg
               (Flag False)
               []
               ["disable-nix"]
-              "Disable Nix integration"
+              "[DEPRECATED] Disable Nix integration"
           ]
       , option
           []
diff --git a/cabal-testsuite/PackageTests/Freeze/enable-benchmarks.out b/cabal-testsuite/PackageTests/Freeze/enable-benchmarks.out
index b888d4fd6f1..b076aa8497e 100644
--- a/cabal-testsuite/PackageTests/Freeze/enable-benchmarks.out
+++ b/cabal-testsuite/PackageTests/Freeze/enable-benchmarks.out
@@ -1,4 +1,5 @@
 # cabal v2-update
 Downloading the latest package list from test-local-repo
 # cabal v1-freeze
+Warning: Nix integration has been deprecated and will be removed in a future release. You can learn more about it here: https://cabal.readthedocs.io/en/latest/nix-integration.html
 Resolving dependencies...
diff --git a/cabal-testsuite/PackageTests/Freeze/enable-tests.out b/cabal-testsuite/PackageTests/Freeze/enable-tests.out
index b888d4fd6f1..b076aa8497e 100644
--- a/cabal-testsuite/PackageTests/Freeze/enable-tests.out
+++ b/cabal-testsuite/PackageTests/Freeze/enable-tests.out
@@ -1,4 +1,5 @@
 # cabal v2-update
 Downloading the latest package list from test-local-repo
 # cabal v1-freeze
+Warning: Nix integration has been deprecated and will be removed in a future release. You can learn more about it here: https://cabal.readthedocs.io/en/latest/nix-integration.html
 Resolving dependencies...
diff --git a/doc/nix-integration.rst b/doc/nix-integration.rst
index 6edf595b6f0..5d4fa695cd4 100644
--- a/doc/nix-integration.rst
+++ b/doc/nix-integration.rst
@@ -1,6 +1,15 @@
 Nix Integration
 ===============
 
+.. warning::
+
+    Nix integration has been deprecated and will be removed in a future release.
+
+    The original mechanism can still be easily replicated with the following commands:
+
+    - for a ``shell.nix``: ``nix-shell --run "cabal ..."``
+    - for a ``flake.nix``: ``nix develop -c cabal ...``
+
 .. note::
 
     This functionality doesn't work with nix-style builds.