Compiler architecture

G# has a shared front end and one execution backend: the emit pipeline used by every command-line driver, including the interactive REPL.

End-to-end pipeline

.gs source → Lexer → Parser → Syntax tree → Binder → BoundProgram → Lowerer → Backend

The front end lives under src/Core/CodeAnalysis. The lexer and parser produce syntax trees, the binder resolves names and types into symbols and bound nodes, and lowering rewrites higher-level constructs into forms that the backends can execute or emit.

Emit backend

The production compiler path writes managed PE metadata directly with ReflectionMetadataEmitter and System.Reflection.Metadata.Ecma335.

BoundProgram (lowered) → ReflectionMetadataEmitter → PE assembly
                                      │
                                      └→ PortablePdbEmitter → sidecar or embedded PDB

gsc owns command-line parsing, response-file expansion, output stream creation, optional reference-assembly output, Portable PDB stream selection, and .runtimeconfig.json generation for executable outputs. Gsharp.NET.Sdk wraps the same driver from MSBuild by building a response file and invoking dotnet gsc.dll during CoreCompile.

No Roslyn dependency in the emit path

G# stays on the bespoke emitter and does not use Roslyn to write assemblies at runtime. The motivation is practical: NuGet-distributable assemblies and cross-language debugging require standard ECMA-335 metadata and Portable PDBs, and those are available directly through System.Reflection.Metadata.

Roslyn is used only by sibling tooling. cs2gs uses Roslyn to read C# projects for migration, and gsgen hosts Roslyn source generators out of process, projects G# declarations to C# stubs, translates generator output back to G#, and then hands generated .g.gs files to gsc. The compiler process itself remains Roslyn-free.

Unified emitted execution

Every command-line driver uses the emitter, including the interactive gsi REPL. /out:<path> controls whether gsc saves the assembly or runs the emitted program immediately. The legacy tree-walking evaluator, its Compilation.Evaluate API, and the --engine evaluator / GSI_ENGINE=evaluator escape hatches have been removed.

Lowering and generated code

Lowering prepares language constructs for execution and emit. Examples include top-level entry-point synthesis, defer and cleanup lowering, for in patterns, async state machines, and sync/async iterator state machines. Async and iterator lowering synthesize state-machine types and hidden scaffolding while preserving source sequence points for debugging.

Top-level statements are lowered to a synthesized entry point unless an explicit func Main() takes precedence. Mixing explicit and synthesized entry points is diagnosed.

Packages, namespaces, and assemblies

G# package declarations map to CLR namespaces. G# uses a .NET-style multi-package-per-project model: one .gsproj produces one assembly, and that assembly may contain multiple packages/namespaces. The assembly name comes from project metadata such as AssemblyName, RootNamespace, or the project file name, not from whichever source file the compiler sees first.

The emitter produces separate namespace/type metadata for each package. The package containing top-level statements owns the synthesized entry point for executable assemblies.

References and target frameworks

gsc can run on .NET 10 while emitting for other target frameworks. The compiler routes user and SDK references through ReferenceResolver and an isolated metadata load context. User-supplied target-framework reference assemblies shadow the compiler host assemblies, so emitted type references carry the target framework's identity.

This is why SDK builds pass the complete reference assembly closure through /r:<path> arguments and why direct gsc usage should do the same for non-trivial projects.

Portable PDBs

When debug information is enabled, ReflectionMetadataEmitter collaborates with PortablePdbEmitter. The PDB records document rows, SHA-256 hashes of raw source bytes, sequence points, local scopes, local variables, embedded source, Source Link, compilation options, and PE debug-directory data.

Reified generics model

G# generics are emitted as CLR reified generics, end-to-end. A user-declared generic type (data struct Box[T], class Pair[A, B], etc.) becomes a TypeDef with GenericParam rows; backtick-arity is part of the name (Box`1). Generic methods carry their own MVar slots; field, parameter, return, and local signatures over an in-scope T encode Var(idx) / MVar(idx). Closed CLR generics that mention an in-scope type parameter (List[T], Dictionary[string, T]) emit as honest GenericInstantiation blobs. Open-bearing delegate shapes (func(T) U) emit as a constructed Func<!T, !U> (or Action<…>) and dispatch through normal callvirt FuncN::InvokeMemberRefs parented at the constructedTypeSpec`.

Constraints (any, comparable, sealed-interface bounds) round-trip as GenericParamConstraint rows. Variance markers (in/out) on interface type parameters are emitted as the matching GenericParam variance flags. C# / F# consumers see the same shape they would see from a C#-defined equivalent: GetGenericArguments() returns the type parameters, GetField("Value").FieldType returns the parameter type, and there is no box/unbox.any boundary at member access.

Contributor file map

Area	Representative paths
CLI driver	src/Compiler/Program.cs
Lexer/parser/syntax	src/Core/CodeAnalysis/Syntax/
Binding and symbols	src/Core/CodeAnalysis/Binding/, src/Core/CodeAnalysis/Symbols/
Lowering	src/Core/CodeAnalysis/Lowering/
Interactive emitted sessions	src/Repl/Engine/EmittedSessionEngine.cs, src/Repl/ (gsi REPL host)
PE emit	src/Core/CodeAnalysis/Emit/ReflectionMetadataEmitter.cs
Portable PDB emit	src/Core/CodeAnalysis/Emit/PortablePdbEmitter.cs
SDK integration	src/Sdk/Gsharp.NET.Sdk/
Source-generator host	src/GeneratorHost/, tools/gsgen/
C# migration tooling	tools/cs2gs/
Language server	src/LanguageServer/