Support using Slice to enable slicing syntax expr1[expr2..expr3]

[brianrourkeboll]

Description

• Add support for using a Slice method to enable slicing syntax expr1[expr2..expr3].
  • Language suggestion: [Slice] Allow slice syntax to use instance Slice method instead of requiring GetSlice method fsharp/fslang-suggestions#1317
  • RFC: TODO
  • RFC discussion: TODO

Checklist

• Test cases added.
• Release notes entry updated.

Notes

• It was rather difficult to follow the existing flow of logic in the indexing/slicing typechecking, and it became even harder as I added the new logic, so I refactored it. I hope it's at least a little bit clearer: the main control flow logic is now centralized in the final pattern match.

  fsharp/src/Compiler/Checking/CheckExpressions.fs

  Lines 6971 to 7004 in b4d3224

  	match (decodedIndexArgs, setInfo), exprTy with
  	// Look for FSharp.Core array and string indexing/slicing helpers.
  	| (_, Array) & (ArrayIndexerOrSlicer (path, meth, args), _)
  	| (_, String) & (StringIndexerOrSlicer (path, meth, args), _) -> tcArrayOrStringIndexing (path, meth, args)
  	// Look for an indexer property, or else assume `Item`.
  	| (Indexing, Getting), Indexable indexer
  	| (Indexing, Getting), (Array | Nominal) & PossiblyIndexable indexer ->
  	propagateThenTcDelayed tpenv expr exprTy (mkDelayedIndexedGet indexer decodedIndexArgs @ delayed)
  	// Look for `GetSlice`.
  	| (Slicing, Getting), Nominal & GetSliceable slicer ->
  	propagateThenTcDelayed tpenv expr exprTy (mkDelayedGetSlice slicer decodedIndexArgs @ delayed)
  	// In the absence of `GetSlice`, look for `Slice`.
  	| ((Slicing, Getting), Nominal) & Sliceable (tpenv, expr, exprTy) ->
  	propagateThenTcDelayed tpenv expr exprTy delayed
  	// In the immediate absence of either, assume `GetSlice`.
  	| (Slicing, Getting), PossiblyGetSliceable slicer ->
  	propagateThenTcDelayed tpenv expr exprTy (mkDelayedGetSlice slicer decodedIndexArgs @ delayed)
  	// Look for an indexer property, or else assume `Item`.
  	| (Indexing, Setting (setArg, mOfLeftOfSet)), Indexable indexer
  	| (Indexing, Setting (setArg, mOfLeftOfSet)), (Array | Nominal) & PossiblyIndexable indexer ->
  	propagateThenTcDelayed tpenv expr exprTy (mkDelayedIndexedSet indexer decodedIndexArgs setArg mOfLeftOfSet @ delayed)
  	// Assume `SetSlice`.
  	| (Slicing, Setting (setArg, mOfLeftOfSet)), (Array | Nominal) ->
  	propagateThenTcDelayed tpenv expr exprTy (mkDelayedSetSlice decodedIndexArgs setArg mOfLeftOfSet @ delayed)
  	| _ ->
  	// deprecated constrained lookup
  	error(Error(FSComp.SR.tcObjectOfIndeterminateTypeUsedRequireTypeConstraint(), mWholeExpr))

[github-actions]

❗ Release notes required

---

✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/8.0.400.md

[T-Gro]

Hi @brianrourkeboll , I noticed this PR which was put on hold during the release freeze prior to .NET 9 ;; that is not a concern anymore.

Is it ready for review, WDYT ?

[brianrourkeboll]

@T-Gro no, this is not ready. I'll need to get back to working on the RFC and either flesh out an updated design for from-end slicing — and thus also how we should deal with System.Index and System.Range — or else see if I can somehow ignore those considerations for a pared-down version of this feature while keeping our options open for those later.

[T-Gro]

@T-Gro no, this is not ready. I'll need to get back to working on the RFC and either flesh out an updated design for from-end slicing — and thus also how we should deal with System.Index and System.Range — or else see if I can somehow ignore those considerations for a pared-down version of this feature while keeping our options open for those later.

I think a pared-down version would work here.

System.Index and System.Range can be enabled independently, assuming there are enough use cases worth supporting.