Proposal: ​
dotnet/csharplang#4487

Spec:
https://github.com/dotnet/csharplang/blob/main/proposals/csharp-10.0/improved-interpolated-strings.md

Feature Branch:
https://github.com/dotnet/roslyn/tree/features/interpolated-string

Compiler:

• LangVersion
• DefaultInterpolatedStringHandler
  • Not present
  • Present
  • Malformed
  • In an async method with no await holes
  • In an async method with await holes
• Binary operator combination of interpolated strings
  • + operator
  • + operator with non interp string
• Custom InterpolatedStringHandler
  • Incorrect ctor
    • implicit conversions to int?
• Missing AppendLiteral
• Incorect AppendLiteral
  • Not return bool or void
• Missing AppendFormatted
• Incorrect AppendFormatted
  • Not return bool or void
• Mismatched AppendFormatted and AppendLiteral return types
• Handler overloads
  • Pick handler over regular string for non-const
  • Regular string over handler for const
  • Handler over conversion to string
  • Handler overloads with arguments:
    • val over in
    • val over ref
• Handler args as:
  • val
  • ref
  • in
  • implicit ref (allow mismatch)
  • out
  • normal / expanded form
  • other params after handler param
  • default values
  • reordered at call site
• InterpolatedStringHandlerArgumentAttribute
  • Missing
  • Malformed
• Parameter names
  • Single parameter name
  • Multiple parameters
  • ""
    • Passes this
    • On a static method
    • On a ctor
  • Incorrect parameter name
  • Mix of correct and incorrect param names
  • Referring to self
• Nullable analysis for format holes and handler constructor (issue Proper nullable support for interpolated string handler constructors and conversions #54583)
• Await in interpolation hole
  • Ref parameter
• Dynamic handler
• IOperation
• Control Flow Graph
• Semantic model
  • GetConstantValue on additive expressions

Productivity:

• Intellisense for methods that have builder arguments
• Verify experience on some methods/overloads that will get builders (string.Create, Debug.Assert, Debug.Write{Line}If, StringBuilder.Append{Line}, MemoryExtensions.TryWrite)
• Extract interpolated string to a local (risks changing meaning/perf)
• Find all refs on custom handler type and constructor, and append methods
• go to definition

LDM:

• conversion operator to string? (details) (answer: no)
• parentheses and additive expressions (details) (answer: allow parens and suppressions)