Tracking Expression evaluator and debugger improvements

[vzarytovskii]

Note
This is a "meta" issue for tracking purposes, since GitHub does not support special types of issues.

This is a list of the expression evaluator and debugging experience improvements we want/plan to do.

Note, that right now, a standard expression evaluator (link above) is used, so some F#-specific features are not working.
More info about EE extensibility here

• Have a separate expression evaluator for F#, based on C# one, hidden under a preview feature flag in VS #12820
• Support F# expressions in debugger eval.
• Support F# expressions in watch window. #12821
• Support F# expressions in immediate window.
• Support for "Autos" window for F#
• Emit open namespaces, modules, and types.
• Show uncaptured locals in closures.
• Show locals when debugging.
• Capture type-level let binds
• Properly show unit arguments in locals.
• Properly show variables with special names.
• Emit compile-time values in PDB.
• Emit StateMachineMethod in PDB
• Emit DynamicLocalVariables in PDB
• Emit CompilationOptions in PDB
• Emit ExcludeFromCodeCoverage attribute for generated methids.

[dsyme]

We could add issues for debugging improvements that require adding more tables, consumed by an F# debug engine. For example

• Emit a table containing a mapping that allows a "compiler inlined" section of one method to be associated with the actual originating code which has been inlined, including showing multiple stack frames and using different locals mappings.

• Emit tables that improve the debug experience for computation expressions. I'd need to formulate exact proposals.

• Probably a bunch of other things too