Some code in the TLS implementation in libstd stores `Some(val)` into an
`&mut Option<T>` (effectively) and then pulls out `&T`, but it currently
uses `.unwrap()` which can codegen into a panic even though it can never
panic. With sufficient optimizations enabled (like LTO) the compiler can
see through this but this commit helps it along in normal mode
(`--release` with Cargo by default) to avoid codegen'ing the panic path.
This ends up improving the optimized codegen on wasm by ensuring that a
call to panic pulling in more file size doesn't stick around.
This commit removes allocation of the panic message in instances like
`panic!("foo: {}", "bar")` if we don't actually end up needing the message. We
don't need it in the case of wasm32 right now, and in general it's not needed
for panic=abort instances that use the default panic hook.
For now this commit only solves the wasm use case where with LTO the allocation
is entirely removed, but the panic=abort use case can be implemented at a later
date if needed.
This commit applies a few code size optimizations for the wasm target to
the standard library, namely around panics. We notably know that in most
configurations it's impossible for us to print anything in
wasm32-unknown-unknown so we can skip larger portions of panicking that
are otherwise simply informative. This allows us to get quite a nice
size reduction.
Finally we can also tweak where the allocation happens for the
`Box<Any>` that we panic with. By only allocating once unwinding starts
we can reduce the size of a panicking wasm module from 44k to 350 bytes.
