lib.modules.mkRenamedOption*: warn as eagerly as possible

... i.e. without adding _any_ new strictness

This adds support for warnings in submodules.

Fixes NixOS#96006

To recap, NixOS#97023 didn't make it
because of unexpected infinite recursions. Context:
  - Attaching a warning to an expression can create an otherwise
    unnecessary data dependency, which can lead to infinite mutual
    recursion when a (typically very necessary) dependency exists in
    the other direction.
  - The `warnings` option is largely independent of the evaluation of the
    configuration, because it only reads from it. Nothing except
    `system.build.toplevel` reads from `warnings`.
    Not cyclical, everyone happy.
  - NixOS#97023 created data dependencies at submodule roots, not just `toplevel`

While something like NixOS#97023 is a very general solution that would solve
this `mkRenamedOptionModule` with ease, we don't need it for this particular
problem.
Specifically, in this case, we already have a data dependency where the new
option has a definition based on the old option. So, we can attach the
 warning to the `mkIf` condition of that definition and make it trigger when
the old option is about to be read anyway.

As a side effect of this change, these particular warnings don't appear in
the `warnings` option anymore. Maybe someone used those for testing, but
`mkRenamed`* usages aren't really worth testing, so this is a small price
to pay for support for renames in submodules.

lib/modules.nix

@@ -1341,6 +1341,21 @@ let
       toOf = attrByPath to
         (abort "Renaming error: option `${showOption to}' does not exist.");
       toType = let opt = attrByPath to {} options; in opt.type or (types.submodule {});
+      prefix = lib.take (lib.length options._module.args.loc - 2) options._module.args.loc;
+      # Memoized warning
+      # This can be triggered in two ways, and this `let` binding ensures that it
+      # only triggers once.
+      # We trigger the evaluation of this warning in two places:
+      #   - When the new option is accessed.
+      #     This ensures that a warning is shown also in submodules that don't have
+      #     a `warnings` option.
+      #   - When the `warnings` option is evaluated.
+      #     This ensures that even if the value is not used, a warning is shown.
+      definitionWarning =
+        lib.warnIf
+          (warn && fromOpt.isDefined)
+          "The option `${showOption (prefix ++ from)}' defined in ${showFiles fromOpt.files} has been renamed to `${showOption (prefix ++ to)}'."
+          null;
     in
     {
       options = setAttrByPath from (mkOption {
@@ -1352,12 +1367,14 @@ let
       });
       config = mkIf condition (mkMerge [
         (optionalAttrs (options ? warnings) {
-          warnings = optional (warn && fromOpt.isDefined)
-            "The option `${showOption from}' defined in ${showFiles fromOpt.files} has been renamed to `${showOption to}'.";
+          # NOTE: We use the warning option to trigger the warning immediately, instead of adding to the list.
+          #       Otherwise, we could show the warning twice; see `definitionWarning`.
+          warnings = mkIf (builtins.seq definitionWarning false) (lib.mkOverride 10000 []);
         })
         (if withPriority
-          then mkAliasAndWrapDefsWithPriority (setAttrByPath to) fromOpt
-          else mkAliasAndWrapDefinitions (setAttrByPath to) fromOpt)
+          then mkAliasAndWrapDefsWithPriority (d: setAttrByPath to (builtins.seq definitionWarning d)) fromOpt
+          else mkAliasAndWrapDefinitions (d: setAttrByPath to (builtins.seq definitionWarning d)) fromOpt
+        )
       ]);
     };
 

lib/tests/modules.sh

@@ -51,11 +51,13 @@ logFailure() {
     printf '\033[1;31mTEST FAILED\033[0m at %s\n' "$(loc 2)"
 }
 
+extraArgs=""
+
 evalConfig() {
     local attr=$1
     shift
     local script="import ./default.nix { modules = [ $* ];}"
-    nix-instantiate --timeout 1 -E "$script" -A "$attr" --eval-only --show-trace --read-write-mode --json
+    nix-instantiate --timeout 1 -E "$script" -A "$attr" --eval-only --show-trace --read-write-mode --json $extraArgs
 }
 
 reportFailure() {
@@ -107,6 +109,34 @@ checkConfigError() {
     fi
 }
 
+checkConfigWarning() {
+    local outputContains=$1
+    local errContains=$2
+    local err=""
+    shift
+    shift
+    if err="$((evalConfig "$@" | grep -E --silent "$outputContains") 2>&1 >/dev/null)"; then
+        if echo "$err" | grep -zP --silent "$errContains" ; then
+            ((++pass))
+        else
+            logStartFailure
+            echo "ACTUAL:"
+            reportFailure "$@"
+            echo "EXPECTED: log matching '$errContains'"
+            logFailure
+            logEndFailure
+        fi
+
+    else
+        logStartFailure
+        echo "ACTUAL:"
+        reportFailure "$@"
+        echo "EXPECTED: result matching '$outputContains'"
+        logFailure
+        logEndFailure
+    fi
+}
+
 # Shorthand meta attribute does not duplicate the config
 checkConfigOutput '^"one two"$' config.result ./shorthand-meta.nix
 
@@ -139,6 +169,12 @@ checkConfigOutput '^true$' config.assertion ./gvariant.nix
 
 checkConfigOutput '"ok"' config.result ./specialArgs-lib.nix
 
+# mkRenamedOptionModule
+checkConfigOutput '^true$' config.result ./mkRenamedOptionModule.nix
+# Check the warning. This abuses checkConfigError to check for a warning.
+extraArgs="--strict" \
+checkConfigWarning '{"a":1,"b":2}' '.*The option .basic\.old. defined in .*/modules/mkRenamedOptionModule\.nix. has been renamed to .basic\.new..*' config.basic.new ./mkRenamedOptionModule.nix
+
 # https://github.com/NixOS/nixpkgs/pull/131205
 # We currently throw this error already in `config`, but throwing in `config.wrong1` would be acceptable.
 checkConfigError 'It seems as if you.re trying to declare an option by placing it into .config. rather than .options.' config.wrong1 ./error-mkOption-in-config.nix