Championed issue: dotnet/csharplang#2608
Specification: https://github.com/dotnet/csharplang/blob/master/proposals/module-initializers.md

• LangVer
• missing module init type: [ModuleInit(typeof(Missing))]
• module init type has a use-site error [ModuleInit(typeof(A))] where A can't be fully loaded (A defined in a library, but it's base type is referenced in another library which is not referenced in current/client library)
• test some netmodule scenarios (adding multiple modules within an assembly)
• attribute on local functions
• attribute on static and non-static methods
• attribute on methods with parameters
• attribute on non-void methods
• attribute on generic methods
• attribute on methods directly or indirectly in a generic type, method is generic and not generic
• attribute on methods inaccessible on the module level
• attribute on method inside an added .NET module compiled from IL. Expect the method to be not executed.
• attribute on multiple methods in source. Expect methods are executed in lexical order. The spec should clarify the order.
• different methods decorated with different attributes (extern aliases). Should make no difference, all methods should be executed.
• methods decorated with the attribute using constructor other than default are ignored.
• multiple attributes on the same method (change attribute definition to allow multiple applications)
• attributes on legacy partial methods: on definition, on implementation, on both (change attribute definition to allow multiple applications)
• attributes on new-style partial methods: on definition, on implementation, on both (change attribute definition to allow multiple applications)
• attributes on a static constructor
• attributes on methods in an interface
• attributes on non-method declarations (change attribute definition to allow), should be ignored.
• attribute on conditional method
• attribute is conditional
• attribute on an obsolete method
• Error when attribute is used in VB
• No warning when module initializer method initializes a static field with a non-nullable reference type seems to be a difficult analysis to reason about correctly. Test plan for "module initializers" feature #40500 (comment)
• Expected semantics for static int field = default; when module initializer also writes to field
  • the optimization should not be a problem because the static constructor will run before the module initializer can write anything.

BCL

• Add type to BCL
  • API request New API: ModuleInitializerAttribute runtime#35749
  • API review New API: ModuleInitializerAttribute runtime#35749 (comment)
  • API implementation New API: ModuleInitializerAttribute runtime#35749

LDM

• write and review speclet
• decide what to do for VB
  • verdict: no support and error on usage of well-known attribute Codegen for module initializers feature #43281 (comment)

FYI @jnm2