Wasmtime 47 enables WebAssembly GC and exceptions by default

The Wasmtime 47 release enables the WebAssembly GC and Exceptions proposals by default. These features allow high-level languages to use the runtime's built-in garbage collector and exception handling instead of embedding custom, heavy implementations. This change aims to make WebAssembly a more efficient target for languages with object-and-reference data models.

Technical details of Wasmtime's GC

• Uses a Cheney-style semi-space copying collector.
• The GC heap is implemented using WebAssembly linear memory, using 32-bit indices instead of native pointers for safety and compactness.
• Allocation is handled via a simple bump pointer within the active semi-space.
• The implementation avoids read or write barriers.
• Uses virtual-memory guard pages to eliminate explicit bounds checks.

Why it matters

By supporting these proposals, Wasmtime enables more high-level languages to target WebAssembly efficiently without the overhead of custom memory management or exception-handling logic.