[lightbeam] panic due to unreachable! in backend.rs

[pventuzelo]

Issue description

wasmtime with lightbeam backend panic due to an unreachable!. This issue will make program calling lightbeam as 3rd party library to crash. Instead an error should be through to let the caller handle it properly.

Reproduction

wasmtime commit: 3558d41

Compilation with debug info:

$ RUSTFLAGS=-g cargo build --release --features lightbeam

Download & run testcase:
panic_internal_unreachable_code.zip

$ unzip panic_internal_unreachable_code.zip
$ ./target/release/wasmtime --lightbeam panic_internal_unreachable_code.wasm
thread 'main' panicked at 'internal error: entered unreachable code', crates/lightbeam/src/backend.rs:3559:22
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

Related piece of code

lightbeam panics because of a calling to unreachable! macro.

wasmtime/crates/lightbeam/src/backend.rs

Lines 3558 to 3560 in 3558d41

	}
	_ => unreachable!(),
	}

Fix proposal

After applying the pull request 672, wasmtime will return:

$ ./target/release/wasmtime --lightbeam panic_internal_unreachable_code.wasm
Error: failed to process main module `panic_internal_unreachable_code.wasm`

Caused by:
    0: WebAssembly failed to compile
    1: Compilation error: Failed to translate function: Microwasm error: i32_extend_s unreachable code

[pventuzelo]

@fitzgen can you add fuzz-bug label on this one? Thanks

[pventuzelo]

Merged in master.

[eira-fransham]

I'd like to reopen this issue, as it appears to be because ValueLocation::CondCode isn't covered. The correct fix is to support ValueLocation::CondCode, and furthermore to remove all _ => branches from matches in the backend.

[alexcrichton]

Lightbeam was removed in #3390 as explained in RFC 14, so I'm going to close this.