Spec: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-11.0/raw-string-literal.md
Championed issue: dotnet/csharplang#4304

Compiler

• Address PROTOTYPE comments
• test semantic model APIs (GetTypeInfo in literal and expressions in holes)
• Review any public API changes
• nullability
• consider improving diagnostic for bad indent
• Expression trees (see AsStringInExpressionTrees_01)
• constant interpolations (see TestInConstantFieldInitializer...).
• Update test plan
• verify order of evaluation of interpolations, definite assignment
• LangVer
• balanced and unbalanced open and close braces
• test parsing with @$$ and $$@ (errors)
• test interpolated single-line raw string with newlines in interpolation hole/expression
• nested interpolated strings of different kinds (interpolated, interpolated verbatim, interpolated raw)
• formats
• SyntaxNormalizer (see TestSpacingOnRawInterpolatedString)
• Conversion of interpolations to IFormattable and FormattableString result in calling FormattableStringFactory.Create, otherwise natural type string results in string.Format
• Newline in format specifier (see TrailingSpaceInFormatSpecifier02)
• interpolation handlers

Productivity

• Review IDE code that references SyntaxKind.InterpolatedStringStartToken (there's a few locations)
• Typing/completion
• Colorization
• QuickInfo
  • test multi-line in immediate window and QuickWatch/watch
  • QuickInfo should display constant value
• Formatting/indentation
• below minbar below:
• Display indentation line
• Convert to raw string literal (PR Add refactoring to convert normal (or verbatim) strings to raw strings. #59180)
• Add a " on both ends
• Add a $
• Interaction with regexes /*regex*/ """[0-9a-z]*"""; (should colorization correctly)
• Interaction with datetime completion DateTime.Now.ToString(... typing here ...)

IDE follow-up work: #58356
For exploratory testing, see IDE test plan: https://github.com/dotnet/roslyn/blob/main/docs/wiki/Manual-Testing.md