[Thorium]

This is split from #4774 and is dependent on #4780 (so that should be merged first).

Copilot generated description:
This pull request significantly improves Fable's support for F# quotations, especially for statically typed targets like Rust. The changes ensure that quoted expressions preserve .NET member metadata, generate portable representations for values like null and unit, and improve compatibility and fidelity for pattern matching, options, lists, and type information. Several helpers and code paths are refactored to produce correct and target-agnostic quotation trees.

Quotations emission and representation improvements:

• Quoted expressions now preserve .NET member calls and metadata by introducing a CapturingQuotation flag in the Context, ensuring that member calls are not replaced or inlined when capturing quotations. This preserves necessary metadata for downstream consumers like the QuotationEmitter. [1] [2] [3] [4]
• The quotation emitter now emits runtime-constructed null nodes (via mkNullExpr) for instance-less calls, unit, null, option None, and empty lists, improving compatibility with statically typed targets such as Rust and Dart. [1] [2] [3]
• Pattern matching and decision trees in quotations are now faithfully inlined and transformed into trees of IfThenElse and Let expressions, matching F#'s quotation semantics.
• Quotation emission for pattern tests (union case, option, list, type tests) now generates explicit method/property calls, using the correct type names and method names, so that the structure is preserved and recognized in all targets. [1] [2]

Target-specific and cross-target compatibility:

• Rust-specific handling for quotations: union cases, records, and options are now emitted with additional type and case information, and empty quotation-expr arrays are routed through a runtime helper to ensure correct typing. [1] [2]
• All targets now receive a four-argument form for mkCall in quotations, ensuring compatibility with Rust's stricter function arity and maintaining back-compatibility with other runtimes.

General improvements and refactoring:

• Type and number kind stringification for quotations is made more robust and comprehensive, covering all relevant F# and .NET types.
• The Rust backend now supports casting from bool to integer types, matching Rust's native casting semantics.

These changes together make Fable's quotation support more robust, portable, and faithful to F# semantics across all supported targets.

References:
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

[Thorium]

Let me know if you need any help rebasing these. I just don't want to do it at the same time as you, because of the "force".

[MangelMaxime]

Thank you for proposing.

For this PR, it was easy enough as just dropping the commit. I am now working on reviewing the changes, adding tests if needed etc.

Hopefully, the other issues will be as simple to rebase 🤞