Rustlantis generates MIR with UB since typed copies retag

[saethlin]

Since rust-lang/rust#154341 typed copies now implicitly retag. Rustlantis doesn't know about this yet so some of the programs it generates execute UB.

I think the fix for this should be relatively simple? But I'm not an expert on the codebase.

[cbeuw]

I think the issue is that there isn't a way to mark a copy as no_retag when writing custom MIR, even though the syntax exists in MIR output:
https://github.com/rust-lang/rust/pull/154341/changes#diff-5c6152a48af352df422de191432412d9e915bec97a1c115db29727246591f5f0

I'll try to add this to custom MIR on rustc side, then the fix in rustlantis would be simply "if this Use candidate cannot be retagged, then emit no_retag"

[saethlin]

That would be one way to fix it, but it would be rather unfortunate to never exercise typed copies.