Add JSON parser rule for custom_unwind_resume.

[pierzchalski]

No description provided.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Aatch (or someone else) soon.

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[alexcrichton]

Is there a reason for including this beyond completeness? The field is basically "extra unstable" and I'm curious if it's actually needed anywhere else.

[pierzchalski]

cargo rustc doesn't pass extra flags transitively to dependencies, so being able to emulate -Z no-landing-pads using the platform description - even in a round-about way - comes in handy.

[alexcrichton]

@bors: r+ 054e409

[bors]

⌛ Testing commit 054e409 with merge af6e413...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt