flakes: Don't warn about meta and ellipsis except for root flake

... and fix an incorrect use of `grep -v`

doc/manual/src/release-notes/rl-next.md

@@ -34,7 +34,7 @@
 
 - The flake output function now accepts a parameter `meta`, which gives access to `sourceInfo` and such parameters in a robust manner, even when `self` can not be evaluated. This helps frameworks report error messages with locations, as trying to access the location previously resulted in an unhelpful infinite recursion when the error condition caused `self` to fail.
 
-  If an `outputs` function has a binding for all attributes (using `@`), you will be asked to add an ellipsis to avoid later confusion as to why `meta` is missing - which is necessary because of a limitation of Nix function semantics. This warning may occur in a dependency.
+  If an `outputs` function has a binding for all attributes (using `@`), you will be asked to add an ellipsis to avoid later confusion as to why `meta` is missing - which is necessary because of a limitation of Nix function semantics.
 
 - Flake follow paths at depths greater than 2 are now handled correctly, preventing "follows a non-existent input" errors.
 

src/libexpr/flake/call-flake.nix

@@ -84,7 +84,7 @@ let
               canRemove = isClosed && (builtins.functionBindsAllAttrs flake.outputs == false);
 
               removals = if acceptsMeta then [] else [ "meta" ];
-              checked = if !acceptsMeta && !canRemove then warning else x: x;
+              checked = if isRoot && !acceptsMeta && !canRemove then warning else x: x;
               warning = warn
                 "in flake ${toString outPath}: The flake's ${emphasize "outputs"} function does not accept the ${emphasize "meta"} argument.\nThis will become an error.\nPlease add ellipsis (${emphasize "..."}) to the function header for it to be compatible with both dated and upcoming versions of Flakes. Example use of ellipsis: ${emphasize "outputs = { self, ... }: "}.";
             in

tests/flakes/check.sh

@@ -2,7 +2,8 @@ source common.sh
 
 flakeDir=$TEST_ROOT/flake3
 depDir=$TEST_ROOT/flakedep
-mkdir -p $flakeDir $depDir
+depDirB=$TEST_ROOT/flakedep2
+mkdir -p $flakeDir $depDir $depDirB
 
 cat > $depDir/flake.nix <<EOF
 {
@@ -135,20 +136,39 @@ EOF
 
 expectStderr 1 nix flake check $flakeDir | grep -F "flake input name 'meta' is reserved"
 
-
-cat > $flakeDir/flake.nix <<EOF
+echo cat \> $depDirB/flake.nix
+cat > $depDirB/flake.nix <<EOF
 {
   inputs = {
   };
   outputs = args@{ self }:
-    # args won't have meta, as required by function semantics, but it is rather unfortunate, so Nix should warn about it.
+    # args won't have meta, as required by function semantics, but it is rather unfortunate,
+    # so Nix should warn about it, when this is the root flake.
     assert !args?meta; # implied by function semantics
+    {
+      packages.$system = { };
+    };
+}
+EOF
+
+nix flake check $depDirB 2>&1 | grep -F "Please add ellipsis"
+
+cat $depDirB/flake.nix
+
+# However it should not warn when the flake is used as a dependency, because in that case the user may not own the flake and can't change it. If they do own it, they only need to know about it when working on the flake itself.
+cat > $flakeDir/flake.nix <<EOF
+{
+  inputs = {
+    dep.url = "$depDirB";
+  };
+  outputs = { self, dep }:
+    builtins.seq dep.packages
     {
     };
 }
 EOF
 
-nix flake check $flakeDir 2>&1 | grep -F "Please add ellipsis"
+nix flake check $flakeDir 2>&1 | grepQuietInverse "Please add ellipsis"
 
 
 cat > $flakeDir/flake.nix <<EOF
@@ -163,4 +183,4 @@ cat > $flakeDir/flake.nix <<EOF
 }
 EOF
 
-nix flake check $flakeDir 2>&1 | grep -v "Please add ellipsis"
+nix flake check $flakeDir 2>&1 | grepQuietInverse "Please add ellipsis"