Tracking Issue for WebAssembly exception handling

This is a tracking issue for wasm native exception handling, which would allow panic=unwind to function properly for code targeting wasm.

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

• Initial compiler support (#111322)
• Standard library support (#121438)
• LLVM support for newest iteration of try/catch instructions (see WebAssembly/exception-handling#280, WebAssembly/exception-handling#296)
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• -C target-feature=+exception-handling vs -C panic=unwind -- does one imply the other, or is one of them redundant?
  • panic=unwind should enable exception-handling
• Should -C llvm-args=-wasm-enable-eh be passed by default on all wasm targets, to be stripped out if the exception-handling target feature is disabled?
  • yes: afaict -wasm-enable-eh does not affect codegen in general, only isel for the wasm.throw instruction.
• Should exception-handling instructions (e.g. from prebuilt libstd) be stripped out if panic=abort? If so, by whom? wasm-ld? wasm-opt does currently implement a --strip-eh pass; maybe that's good enough if users want to target wasm MVP?
  • this problem is similar to the one that atomics (#77839) faces, of a single prebuilt libstd coming into conflict with some users needing backwards compatibility with wasm-MVP and others wanting to use more recently stabilized features

Implementation history

• #111322
• #121438

The issue names no source files or tests. Start by reading the linked WebAssembly exception-handling issues and the rustc-dev-guide stabilization and documentation guidance; completion means resolving the remaining LLVM support, documentation, stabilization, and panic=abort checklist items.