Stable MIR's thread local not properly handled on unwind

[programmerjake]

rust/compiler/rustc_smir/src/stable_mir/mod.rs

Lines 113 to 114 in 7f74ae5

	f();
	TLV.replace(std::ptr::null_mut());

TLV is left set to a dangling pointer if f unwinds. This is unsound because a later call to with will use that dangling pointer. There is a similar issue with with and unwinding.

since you're on the Stable MIR cc list:
cc @oli-obk, @celinval

[programmerjake]

if you can add it as a dependency, using scoped-tls would probably be better than rolling your own version.

[oli-obk]

using scoped-tls

Yea, that makes sense. I think we should look into using it for the tls module, too, which is how we get a TyCtxt within the compiler itself