Run WebAssembly applications using CoreCLR #134337

Goal

Run WebAssembly applications on CoreCLR in .NET 12.

Plan

R2R

Publish the WebAssembly template and other apps with full R2R by default in Release.

• Add WASM SDK support for R2R
• Split lazy native code from eager R2R code
• Fix generated P/Invoke support for customer libraries
• Validate Release publishing in macrobenchmarks
• Make R2R the default for Release publish
• Define Debug and Release native-linking modes

Performance

Reduce app size and startup time with PGO and lazy R2R loading.

• Establish macrobenchmark baselines, CPU profiles, and performance targets
• Add PGO for interpreter configurations through profile collection and WASM PGO support
• Add PGO for R2R configurations
• Reduce eager R2R code and startup download size using a Blazor startup profile
• Support and measure lazy R2R loading and reload

SDK

Ship CoreCLR WebAssembly packs and simplify the SDK for building and publishing apps.

• Build and publish CoreCLR WebAssembly packs, including correct workload runtime-pack resolution
• Simplify the WASM SDK for CoreCLR, covering browser composite mode, IL and debug-data stripping, Blazor Router lazy loading, IL release after the lazy R2R complement loads, and validation of trimming, globalization, timezone removal, and optional components
• [browser][coreclr] Strip workload content #134074
• Set CoreCLR as the default runtime for Debug, Release, and CI

Diagnostics

Support debugging interpreted code, Hot Reload, and runtime diagnostics.

• Enable end-to-end debugging and complete debugger scenario coverage, including WebCIL/DAC compatibility
• Enable Hot Reload with and without an attached debugger
• Enable R2R CPU profiling, tracing, crash dumps, and native exception symbolication
• Collect runtime and configuration telemetry using native runtime metadata, with producers and build IDs

The cDAC integration remains optional.

Testing

Test CoreCLR and full R2R across runtime, libraries, WebAssembly builds, Blazor, and developer tooling.

• Complete CoreCLR WebAssembly test infrastructure and coverage, including unblocking crossgen in the baseline SDK, reducing publishing costs, disabled browser runtime tests within the supported runtime configuration, and R2R library-test CI configuration
• Provide builds and testing instructions, and validate first-party and third-party applications against agreed scenarios and pass criteria

Consumer and tooling integration

Integrate CoreCLR WebAssembly packs into consumers and tools with passing builds and checks.

• Integrate CoreCLR WebAssembly packs into consumer repositories
• Complete Visual Studio integration
• Complete VS Code integration

Mono removal

Stop shipping Mono WebAssembly packs.

• Remove .NET 12 Mono runtime and cross-AOT packs #134397

WASI

Deliver the agreed WASI support.

• Define the WASI scope, including composite R2R publishing and runtime-test CI and CoreCLR-WASI library tests
• Complete WASI SDK and Wasmtime integration within the agreed scope

Documentation and maintenance

Document migration and known issues.

• Publish migration guidance, breaking changes, known issues, and announcements
• Define ongoing ownership and maintenance costs for the runtime, SDK, tooling, and CI