Teach miri about unsized locals

[oli-obk]

These are just unsized in the typesystem. They are actually just fat pointers

https://play.rust-lang.org/?gist=43d2f8256e22ed27e79b5e58082b3357&version=nightly&mode=debug&edition=2015 is a small example that should work

[RalfJung]

Basically I think all we have to do is remove a bunch of asserts, and teach copy_op to with with the dynamic, not static, size. Shouldn't be hard.

... famous last words...

[RalfJung]

Isn't this a duplicate of #438?

[RalfJung]

Likely also relevant (at least for tests): rust-lang/rust#54183

[RalfJung]

For the record, here's the main implementation PR for the LLVM backend: rust-lang/rust#51131

[RalfJung]

@oli-obk

These are just unsized in the typesystem. They are actually just fat pointers

Looking at the MIR, I cannot see this at all. What makes you say that?

My current impression is that the best way to support this is to actually have proper unsized locals, with the fat-ptr-meta stored in the Place that is stored in miri's locals table.

[RalfJung]

Has been fixed by rust-lang/rust#59780.