Use dynamic dispatch in `mozjs::panic::wrap_panic`

SimonSapin commented 4 years ago

Servo has generated code that calls it with thousands of different closures. With monomorphization, this function alone contributed significantly to code size (millions of lines of unoptimized LLVM IR) and compilation time: https://github.com/servo/servo/issues/26585

By switching to dynamic dispatch, compilation time for Servo’s script crate (which is pathologically long) is reduced by about a quarter on my machine.

Other changes are:

Removing return value handling, which required a generic type parameter. Instead, the provided callback needs to assign to a captured variable. (See test for an example.)
The callback needs to implement FnMut rather than only FnOnce. We could instead take Box<dyn FnOnce() + '_> (with allocation overhead), but none of the callers is affected.
The UnwindSafe constraint is removed, because multi-trait objects do not support up-casting yet. It wasn’t much help anyway since the code generator always used AssertUnwindSafe regardless of what each closure does.

Dynamic dispatch has non-zero runtime cost. Perhaps here it is small enough, or compensated by better instruction cache utilization thanks to reduced code size?