Linq expansion planning/next steps

[dubiousconst282]

• Infrastructure:

  • Introduce a new IR::Lambda intrinsic to represent lambda factories in a single instruction, to make it easier to identify queries (this brings too many complications and it's wasteful; detecting queries isn't really difficult and it's easier to just rely on lambda devirtualization instead)
  • General refactor/rewrite
  • Allow single-def vars used inside regions to be SSA renamed (ILImporter.AnalyseVars())

• Support for sources other than arrays:

  • Fallback to IEnumerator
  • Specialized source for Array, List<>, string
  • Specialized source for Enumerable.Range()
  • Specialize/devirtualize and inline boxed enumerators (ImmutableArray, ArraySegment)

• Operators/stages:

  • Concretization:
    • ToArray
    • ToList
    • ToHashSet
    • ToDictionary
  • Aggregation:
    • Aggregate
      • Support for "unseeded" aggregates (throws if source is empty, somewhat tricky to implement)
    • Count (predicated only)
    • #23
    • Min, Max (vectorized in .NET 7)
  • Search:
    • Any, All
    • First[OrDefault]
    • Last[OrDefault] (will probably need some trickery to implement efficiently for random-access sources)
    • Single[OrDefault]
    • ElementAt[OrDefault] (not profitable)
  • Stages:
    • Select
    • Where
    • Cast, OfType
    • Skip, Take
    • SelectMany
    • Buffering:
      • Reverse (idea: could have an option to disable buffering for fast reverse)
      • Order* (this will be tricky since there's no other stable sort function in the BCL)

• Other:

  • Support for loop consumed queries (port for the front-to-back rewrite)
  • #24
    • Otherwise we may leak memory for e.g. File.ReadLines()
  • Offer options to preserve behavior (alt: opt-in/out attributes)
    • List enumerator version/concurrent mod checking
    • Keep null arg checks (really?)

• Known bugs:

  • Overlapping queries (.ToArray().ToArray())