Emit SDK-derived assembly attributes from the G# SDK

[DavidObando]

Summary

• render @(AssemblyAttribute) as file-level G# @assembly: annotations from a new WriteGsharpAssemblyInfoTask
• generate and fold $(GeneratedAssemblyInfoFile) into @(Compile) before CoreCompile, keyed off the SDK's own inputs cache for incrementality
• add unit coverage for rendering, G# escaping, positional-gap detection and write-only-when-changed

Problem

Microsoft.NET.GenerateAssemblyInfo.targets:189 gates CoreGenerateAssemblyInfo on '$(Language)'=='VB' or '$(Language)'=='C#'. build/Gsharp.NET.Sdk.props:4 sets <Language>Gsharp</Language>, so the target silently no-ops and every .gsproj shipped an assembly with no AssemblyTitle, AssemblyProduct, AssemblyCompany or AssemblyConfiguration. @(AssemblyAttribute) was already populated correctly by the standard GetAssemblyAttributes target — only the emit step was missing.

This violated ADR-0143 §D, which makes reproducing those attributes the G# SDK's job precisely because cs2gs deliberately drops the SDK-generated AssemblyInfo.cs and preserves only the project properties.

It surfaced in the migrated Oahu app, whose window title and header rendered as "Oahu.Foundation" instead of "Oahu":

App.axaml.gs:33   viewModel.Title = ApplEnv.AssemblyTitle ?? "Oahu"

ApplEnv.cs:27     AssemblyTitle =
                    GetAttribute<AssemblyTitleAttribute>()?.Title           // off EntryAssembly
                    ?? Path.GetFileNameWithoutExtension(ExecutingAssembly.Location);

With no AssemblyTitleAttribute on the entry assembly, ApplEnv fell through to GetExecutingAssembly() — which is Oahu.Foundation.dll, because ApplEnv lives there. The ?? "Oahu" guard never fired because AssemblyTitle was non-null.

Approach

WriteCodeFragment cannot be reused here: it is CodeDom-based and only emits C# and VB. Routing a generated .cs AssemblyInfo through _GsharpPartitionForeignCompile + gsgen back-translation does not work either — GeneratedDocTranslator drops compilation-unit-level [assembly: ...] lists and emits nothing at all for a type-free file (filed as #2815).

So the task renders the attributes as G# directly. It honors the same item metadata conventions WriteCodeFragment defines — positional _Parameter1…_ParameterN, any other metadata as a named argument (name: value per ADR-0080), and a <name>_IsLiteral sibling for raw code — so a project that already hand-authors <AssemblyAttribute> items for a C# build keeps working unchanged after migration.

String escaping is G#-specific rather than inherited from C#: every G# interpreted string interpolates $ident/${expr}, so a literal $ is escaped as $$ (docs/lexical.md).

$(GeneratedAssemblyInfoFile) needs no override — the SDK already defaults it to $(IntermediateOutputPath)$(MSBuildProjectName).AssemblyInfo$(DefaultLanguageSourceExtension) and Sdk.props sets that extension to .gs.

Testing

• dotnet test test/Sdk.Tests/Sdk.Tests.csproj — 57 passed
• dotnet test tools/cs2gs/Cs2Gs.Tests/Cs2Gs.Tests.csproj — 1765 passed
• all 10 e2etests/*.sh suites pass (sdk, gsgen, foreign-cs, nuget-pack, projectref, packageref, property, multitarget, multifile, debug-info, templates)

Verified end to end against the reported symptom with a G# app named Oahu referencing the real Oahu.Foundation.dll:

without fix:  ApplEnv.AssemblyTitle = Oahu.Foundation
with fix:     ApplEnv.AssemblyTitle = Oahu

Values round-trip through MetadataLoadContext with escaping intact (Probe $Company\Sub, My Probe "Title"), multi-parameter attributes (AssemblyMetadata) and InternalsVisibleTo are emitted correctly, a no-op rebuild does not rewrite the file, and changing <AssemblyTitle> regenerates it and reaches the emitted assembly.

Follow-ups

• gsgen: back-translation drops compilation-unit-level [assembly:] attributes and emits nothing for type-free files #2815 — gsgen back-translation drops [assembly:] attributes and emits nothing for type-free files.
• cs2gs: EmitFriendAssemblyAnnotations double-emits InternalsVisibleTo sourced from MSBuild items #2816 — cs2gs EmitFriendAssemblyAnnotations now double-emits InternalsVisibleTo that came from MSBuild items, since the SDK reproduces those. Benign (AllowMultiple = true) and left as-is here because MigrationPipelineTests.L2_Translate_PreservesInternalsVisibleTo pins the current behavior.

Closes #2814