Championed issue: dotnet/csharplang#1881
Proposal: pattern-match-span-of-char-on-string.md
Branch: features/patterns-span-char

Compiler

• -langver
• test pattern matching in:
  • is
  • switch expression
  • switch statement
• handle missing well-known type and members: System.MemoryExtensions
• with Span<char> or ReadOnlySpan<char> value:
  • default,
  • new(null)
  • new(new char[0])
  • new("")
  • new("...")
• with case:
  • null
  • default
  • (string)null
  • default(string)
  • ""
  • shorter than span, longer than span
  • different case
  • constant interpolated string
• and, or, not with constant string patterns, property patterns, list patterns
• exhaustiveness checks:
  • for constant string patterns only
  • for constant string patterns and other patterns
• obj is ReadOnlySpan<char> and "abc", for object obj and ValueType obj
• disallow in expression tree
• [Obsolete] static class MemoryExtensions { ... }
• generate string hash function and use in switch rather than separate comparisons when more than 6 cases (see <PrivateImplementationDetails>.ComputeReadOnlySpanHash() etc.)
  • hash function with (string)null case
• SemanticModel
  • GetTypeInfo() on constant string case
  • GetDeclaredSymbol(): chars is "abc" and var x checking the type of x
• IOperation
• Control Flow Graph
• Add to docs/contributing/Compiler Test Plan.md

IDE

• Convert if to switch

Open questions

• Is case (string)null allowed? Is it equivalent to case ""?
• How to handle generic cases in future? static bool F<T>(T t) where T : ref struct => t is "str";
• LDM: How much should we care about interaction with list-patterns? readOnlySPanOfChar switch { [ 'a', .. ] => 0, "abc" => 1 /*unreachable*/ }