Championed issue: dotnet/csharplang#2765
Speclet: https://github.com/dotnet/csharplang/blob/master/proposals/top-level-statements.md

Compiler:

• Language version errors (LanguageVersion_01, Scope_03, Scope_06)
• Top-level statements across multiple compilation units (Simple_03, Simple_05)
  • Locals and local functions defined and used in distinct compilation units (Simple_06_02, Simple_07, LocalUsedBeforeDeclaration_01, LocalUsedBeforeDeclaration_02, LocalUsedBeforeDeclaration_03, LocalUsedBeforeDeclaration_04, NullableRewrite_01, Scope_02, Scope_03, Scope_05, Scope_06, Scope_16, Scope_18, Scope_20, Scope_22, Scope_24)
  • Locals re-defined in distinct compilation units (LocalDeclarationStatement_07)
• Top-level statements before using declarations and after namespaces and type declarations
• Statements as top-level statements including:
  • ;, { }
  • labels, goto, break, continue, return
  • yield return, yield break, await
• Member declarations as top-level statements including: fields, events, methods, properties, indexers, operators, explicit implementations
• $Program.$Main has expected signature:
  • Parameter: string[] args
  • Return type: void, int, Task, or Task<int> depending on explicit return statements and use of await
  • Use-site errors for types used in $Program and $Main declarations
• Command-line arguments are available from string[] args
• Top-level statements combined with explicit declaration of valid Main entry point
  • Consider -main:<type> option
• Top-level statements when compiling a library, a net module, a .winmd
• SymbolDisplay and diagnostics containing $Program and $Main
• Variable scope:
  • Locals, local functions, and args parameter names used and redefined in top-level statements and local functions
  • Locals, local functions, and args parameter names used and redefined in using statements and type declarations, in same and distinct compilation units
• All valid and invalid modifiers for locals and local functions
• Attributes on locals and local functions
• Capturing of locals and args parameter in local functions
• Emit and execution of top-level statements and local functions
• Local function with extension method this argument
• Flow analysis of top-level statements
  • Definite assignment for single and multiple compilation units
  • Unreachable code
• Nullable analysis for single and multiple compilation units
• Unsafe code in top-level statements with/without unsafe block, with/without -unsafe command-line option
  • LDM question: should unsafe code be allowed without unsafe block?
• $Main is used for [CallerMemberName] arguments
• Explicit return statement in some branches only: if (expr) { return 1; }
• Locals named await and async
• Add "Top-level statements" to compiler test plan
• ref locals with await
• ref escaping (e.g. storing Span<int> from a stackalloc local in a field)

Public API:

• ISymbol instances for $Program and $Main
• IOperation for single and multiple compilation units
• CFG for locals and local functions in single and multiple compilation units
• Analyzer actions for single an multiple compilation units
• LookupSymbols and LookupNames:
  • For single and multiple compilation units
  • In using statements before top-level statements
  • In type declarations after top-level statements

Productivity:

• Completion on args
• QuickInfo and GoToDefinition on args
• Semantic classification of args (maybe should be colored similarly to value in property setter)

Debugging (compiler):

• $Main is correctly marked as an entry point in the PDB
• F10 on a console app project with top-level code, the debugger stops at the first statement of the top-level code (for both async and sync code)
• Expression Evaluator works in the context of top-level code (usings, args and locals are in scope when evaluating expression in context of top-level code)
• Edit and Continue - emit delta works (adding a top-level type, top-level local function, updating body of a top-level local function, modifying statements in top-level code) (EnC is currently not working in a compilation with top-level statements A change in a top-level statement or an addition of a new top-level statement is not picked up by ENC #44196)

Debugging (IDE):

• Edit and Continue works end-to-end (EnC is currently not working in a compilation with top-level statements A change in a top-level statement or an addition of a new top-level statement is not picked up by ENC #44196)

FYI @AlekseyTs