Generate expression trees in compiled models (query filters, value converters)

[roji]

The compiled model currently has the limitation that expression tree lambdas cannot be rendered (e.g. query filters). The ongoing work on precompiled queries includes a visitor that can generate a faithful C# code representation from a LINQ expression tree, so we could possibly use that to remove the restriction.

From a conversation with @ajcvickers

[roji]

Note that this isn't necessarily about replacing the entire current code generation mechanism with a Roslyn-based one - we can use the LINQ->C# visitor only for lambda fragments and keep the rest as it is.

Replacing the entire mechanism might be worth considering:

• Various formatting, whitespace and the rest is taken care of automatically, so the process would be considerably simplified
• User formatting and settings (e.g. .editorconfig) are automatically taken into account
• It would probably make supporting VB really easy, since Roslyn's SyntaxGenerator allows for generating documents in a language-agnostic way.
  • On the flip side, it would make it impossible to support F#, since it isn't supported by Roslyn.

[ajcvickers]

Note from triage: Query filter case is most interesting here. Make sure that captured context access is preserved.

[roji]

@AndriySvyryd we've already done work on this right? Is it useful to keep this, maybe possibly to add a task list for things not yet done?

[AndriySvyryd]

I think #24897 captures what's left.