Layer variables missing from Field Calculator algorithm on QGIS 3.16.1

[spatialthoughts]

The expression engine is not able to access @layer and @layer_name variables from Field Calculator. They work fine elsewhere, i.e. via field calculator, or select by expression, but not from the Processing Toolbox. This works fine on 3.10 so it's a regression.

QGIS and OS versions

QGIS version	3.16.0-Hannover	QGIS code revision	4af1cbf
Compiled against Qt	5.14.2	Running against Qt	5.14.2
Compiled against GDAL/OGR	3.1.2	Running against GDAL/OGR	3.1.2
Compiled against GEOS	3.8.1-CAPI-1.13.3	Running against GEOS	3.8.1-CAPI-1.13.3
Compiled against SQLite	3.31.1	Running against SQLite	3.31.1
PostgreSQL Client Version	12.3	SpatiaLite Version	4.3.0a
QWT Version	6.1.4	QScintilla2 Version	2.11.4
Compiled against PROJ	6.3.2	Running against PROJ	Rel. 6.3.2, May 1st, 2020
OS Version	macOS 10.15
Active python plugins	ORStools; slyr_community; QuickOSM; ee_plugin; profiletool; DataPlotly; quick_map_services; QNEAT3; pathfinder; qdraw; minimal; GeoCoding; ViewshedAnalysis; pluginbuilder3; show_time; latlontools; Qgis2threejs; qgsAzimuth; plugin_reloader; Mergin; processing; db_manager

[PeterPetrik]

Ok, looks like the variable is expanded OK when processing algorithm is run, but it is not shown in the list of available expressions and not in the auto-complete list

[troopa81]

@PeterPetrik I didn't notice you assigned yourself the issue and I was dealing with this one too. I think I found what the problem is, can I proceed with a PR or do you want to fix it yourself?

[PeterPetrik]

Ok, feel free to assign to yourself and proceed. For future it is safer if you assign ticked to yourself when starting with it :)

I think moving the processing alg to C++ (https://github.com/qgis/QGIS/blame/master/src/analysis/processing/qgsalgorithmfieldcalculator.cpp) , @suricactus forgot to add project/layer to expression context

[suricactus]

Oops, true that. Thanks for taking care of that.

[troopa81]

For future it is safer if you assign ticked to yourself when starting with it :)

Sure, sorry about that!