Rust 1.56 produces unstable wasm for Safari

[futursolo]

The same binary for syntax highlight worker produced under 1.55 and 1.56.

When 1.56 binary is used under Safari, it will generate the following error:

panicked at 'regex string should be pre-tested: InnerError(Syntax(
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
regex parse error:
    \b(?:(pub)\s+)?\b(mod)\s+((?:[\p{L}][_\p{L}\p{N}]*|_[_\p{L}\p{N}]+)\b)\b
                                  ^^^^^
error: Unicode not allowed here
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
))', /home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/syntect-4.6.0/src/parsing/regex.rs:72:57

This apparently only affects Safari.

I have chosen to downgrade to 1.55 for now as I don't know how to reproduce this error.

[futursolo]

Some more information:

• Only 1.56+ is affected
• Only happens if wasm-opt is used with -O (any optimisation level)
• Only happens on Safari