Championed issue: dotnet/csharplang#1888

• Specification checked in to csharplang.
• attribute on declaration
• attributes on declaration are emitted
• attribute on return type
• attributes on return type are emitted
• attribute on type parameters
• attributes on type parameters are emitted
• attribute on parameters
• attributes on parameters are emitted
• Correctness of AttributeUsage
  • On declaration and return](https://github.com/dotnet/roslyn/blob/features/local-function-attributes/src/Compilers/CSharp/Test/Semantic/Semantics/LocalFunctionTests.cs#L631)
  • On parameters and type parameters
• interaction with potentially ambiguous syntax?
• speculative semantic model Speculative semantic model produces different behavior for local functions  #24135
• semantic model on attribute arguments for method/return/param/type param attributes
• expected diagnostics when binding a local function statement in speculative semantic model
  • Speculative semantic models don't give diagnostics
• Scrub references (skipped tests and such) referencing this issue
• test Obsolete attribute (and related)
  • Diagnostics about attributes occur in local function locations
• test Conditional attribute
  • test Conditional with bad target method (non-void returning)
• test various nullability attributes
• test [EnumeratorCancellation] attribute on parameter of async-iterator local function Support EnumeratorCancellationAttribute in local functions #40959
• MethodImplAttribute
• DllImportAttribute
• static extern local functions
• local functions with no body
• ExcludeFromCodeCoverageAttribute Support ExcludeFromCodeCoverageAttribute in local functions #41129
• MarshalAsAttribute Support more well-known attributes in local functions #41299
• SkipLocalsInitAttribute Support SkipLocalsInitAttribute in local functions #41183
• test restricted attributes such as IsReadOnlyAttribute, IsUnmanaged, ExtensionAttribute, etc.
• Caller...Attribute Support more well-known attributes in local functions #41299
• LanguageVersion (tests added in Parse attributes on local function declarations #38808)
  • Including in parameters
• Scoping inside full syntax (See LookupTests.cs)
  • Scoping for attribute lists on incomplete statements Fill in test gaps for local-function-attributes #41379 (comment)
• Can't use locals inside attribute creation, e.g. calling other local functions, using captured variables
  • Local constants should probably be valid
  • Attribute argument with declaration pattern
  • Attribute argument with call containing 'out var'
  • Attribute constructor with out parameter (disallowed)
    • Out to a local
    • Out var
• Referencing local function with ConditionalAttribute only in a "dead" condition #if should not produce unused warning
• ConditionalAttribute should be disallowed on non-static local functions Disallow ConditionalAttribute on non-static local functions #41333

IDE:

• Test interactive typing
• Start with an attribute
• Completion
• Keyword recommendation (extern)

Before merge:

• ConditionalAttribute disallowed on non-static local functions Disallow ConditionalAttribute on non-static local functions #41333
• IDE interactive testing
• AttributeUsage
• Prevent regressing EndToEndTests
• Draft spec checked into proposals, awaiting validation by Neal