Championed issue: dotnet/csharplang#2691
Speclet: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-12.0/primary-constructors.md
Feature branch: https://github.com/dotnet/roslyn/tree/features/PrimaryConstructors

Compiler

• language version
• syntax changes:
  • type Name; for class, struct
  • type Name(...) ... for class, struct
  • type Name(...) : Base(...) ... for class
  • error for type Name(...) : Interface(...) ...
  • error for type Name : Base(...) ...
• public primary constructor emitted
  • (Add Primary Constructor tests for some scenarios from Test Plan #70026) explicit parameterless constructor for class C0(); or struct S0() { }
• declared constructors must call primary constructor
• error for declared constructor with matching signature
• error for primary constructor on multiple partial declarations
• parameters:
  • (Add Primary Constructor tests for some scenarios from Test Plan #70026) generic types
  • (Add Primary Constructor tests for some scenarios from Test Plan #70026) pointer types with/without unsafe
  • default parameter values
  • params modifier
  • this modifier (error)
• attributes on parameters:
  • with no target modifier; with param: target; with other target (disallowed)
  • [CallerMemberName]
  • synthesized attributes: NativeIntegerAttribute, DynamicAttribute, TupleElementNames (see ParameterCapturing_*_SynthesizedAttributes)
• parameter capturing in class, [readonly] struct, [readonly] ref struct (see ParameterCapturing_*)
• parameter referenced in:
  • instance methods and property or event accessors (capturing)
  • instance field and property initializers (not capturing)
  • static members (error)
  • const field (error)
  • other constructor (error)
  • this(x) or base(x) initializer (error)
  • : Base(x) initializer (not capturing)
  • lambdas (capturing based on containing context)
  • local function (capturing based on containing member)
  • static lambdas and local functions (error if captured)
  • partial method
• parameter referenced in:
  • is x
  • x()
  • @x (see ParameterCapturing_011_EscapedIdentifier)
  • nameof(x)
  • lock (x) (see ParameterCapturing_098_Lock)
  • using (x) (see ParameterCapturing_099_Using)
  • x switch { ... } (see ParameterCapturing_099_MultiplePathsToNode_SwitchDispatch_02)
  • deconstruction (see ParameterCapturing_103_Deconstruction)
  • &x (see *AddressOf*)
  • x.F fixed size field (see ParameterCapturing_*FixedSizeBuffer)
  • Color Color (see ParameterCapturing_*_ColorColor_*)
• error capturing ref, in, out, or ref struct parameters (see IllegalCapturingDueToRefness_*)
• name shadowing (see ParameterScope) from:
  • field, property, event, method names
  • nested type
  • local function name
  • (Add Primary Constructor tests for some scenarios from Test Plan #70026) type parameter name
  • (Add Primary Constructor tests for some scenarios from Test Plan #70026) parameter in instance member, constructor, lambda, or local function
  • (Add Primary Constructor tests for some scenarios from Test Plan #70026) local in instance member, lambda, or local function
  • local in : Base(...): error (see BaseArguments_17)
  • (Add Primary Constructor tests for some scenarios from Test Plan #70026) attribute
  • LINQ expression identifier (see ParameterCapturing_010_ShadowingInMethodBody, ParameterCapturing_016_Query)
• definite assignment (see DefiniteAssignment_*)
  • out parameters must be assigned
  • use of out parameters before assignment
  • warning for parameters that are not read
• dynamic dispatch not allowed for : Base(@dynamic) initializer (see BaseArguments_10)
• (Add Primary Constructor tests for some scenarios from Test Plan #70056) order of evaluation: field initializers, : Base()
• Semantic model:
  • primary constructor symbol available from containing type
  • GetDeclaredSymbol()
  • GetSymbolInfo()
  • GetSpeculativeSymbolInfo(), GetSpeculativeTypeInfo()
• LookupSymbols and LookupNames for parameters
  • in initializers
  • in : Base(x, y)
  • in attributes
  • in members (see ParameterScope)
• analyzer events:
  • syntax events for primary constructor parameters
  • syntax events for : Base(x, y) arguments
  • symbol events for primary constructor parameters
  • IOperation events for synthesized constructor bodies, parameters, default values
• XML doc comments:
  • param on containing type and members
  • paramref on containing type and members
  • across partial declarations
• capturing managed parameter makes struct managed (see ParameterCapturing_056_CapturingOfAManagedParameterMakesStructManaged)
• struct cycles from capture (see ParameterCapturing_*_Cycle)
• synthesized constructor not allowed on [ComImport] types
• parameters (captured or not) referenced in lambda (see *ParameterUsedInLambda_InPrimaryConstructor)
• warning for captured parameter passed to base constructor (see ParameterCapturing_*CapturedAndPassedToBase*)
• nullable analysis (see ParameterCapturing_*NullableAnalysis*)
• parameters are readonly in readonly struct (see ParameterCapturing_*_ReadonlyContext)
• parameter assignment in readonly struct or readonly method (see ParameterCapturing_*_ReadonlyContext)
• ref safety analysis of parameter references (see ParameterCapturing_145_ReturnByRef, etc.)
• (Add "Primary Constructors" to Compiler Test Plan.md #70050) add feature to Compiler Test Plan.md
• (Add Primary Constructor tests for some scenarios from Test Plan #70056) order of emitted fields with/without explicit fields
• (Add Primary Constructor tests for some scenarios from Test Plan #70056) error for primary constructor on static class
• report unused parameters
• SyntaxNormalizer (SyntaxNormalizerTests.TestSpacingOnPrimaryConstructor/TestSemicolonBody)
• (Add Primary Constructor tests for some scenarios from Test Plan #70056) structs fulfilling the unmanaged constraint requires binding all members first

Public API

• API review (New APIs for "Primary Constructors" #66914)

IDE test pass

• Language version upgrade
• Type and paste new constructs
• Completion
  • Type completion for parameters
  • Parameter name completion
    • In base initializer
    • In field initializer
    • In member body
  • Dotting off of parameter name
    • In base initializer
    • In field initializer
    • In member body
  • Identifier highlighting
    • Prefix matching
    • Full identifier matching
• Formatting
  • Format command
• Automatic Brace Completion
  • Parameter list
  • Argument list
• Indentation (typing Enter in an unfinished statement indents the next line)
  • Parameter list
  • Argument list
• Go To definition
  • For parameter
  • For constructor
• Find All References
  • For parameters
  • For constructor
• Colorization
  • Types in parameter list
  • Parameter names
• Error Squiggles
• Rename of parameter
• Change signature (for constructor)
• Extract method/local function for code referencing parameter

• Debugging
  • Stepping
  • Non-captured parameters
    • Show up in Locals window
    • Can be modified
  • Captured parameters
    • In initializers
      • Capture fields are hidden
      • Show up in Locals window, the values reflect the actual captured state. Modifications go to the the capture field. (Implement support for captured Primary Constructor parameters in EE when portable PDB is used. #67266)
      • Same in Watch window. (Implement support for captured Primary Constructor parameters in EE when portable PDB is used. #67266)
    • Inside instance methods
      • Capture fields are hidden
      • Show up in Locals window, the values reflect the actual captured state. Modifications go to the the capture field. (Implement support for captured Primary Constructor parameters in EE when portable PDB is used. #67266)
      • Same in Watch window. (Implement support for captured Primary Constructor parameters in EE when portable PDB is used. #67266)

• Edit and Continue (EnC)
  • Implement EnC support for Primary Constructors #67108, EnC: Implement support for editing types with primary constructors #68477.

Filed issues:

• Formatting is not triggered after ending class declaration with a semicolon #67011
• Unexpected "IDE0060 Remove unused parameter" suggestion for a Primary Constructor parameter #67013
• Feature "Diagnostic analyzer runner" is currently unavailable due to an internal error (due to an assert) #67015
• Change signature refactoring doesn't update argument list in class_base #67016
• Extract method refactoring is not offered in the base class argument #67017
• No completion for primary constructor parameters in Watch window in initializers #67021