[MIR] span_mirbug! should be an error

[scottcarr]

IIUC, span_mirbug! was an warning while MIR was in development. It's time to make span_mirbug! an error. (Or if it's really supposed to be a warning, rename the macro)

https://github.com/rust-lang/rust/blob/master/src/librustc_mir/transform/type_check.rs#L29-L36

[nagisa]

Not before we’re absolutely sure there’s really no more warnings produced by this.

[eddyb]

@nagisa We can check crater after turning them into ICEs.

[durka]

@nagisa I'm not 100% sure I understand that logic -- since we're in orbit now, and MIR is really being used, it seems to me that broken MIR has to stop compilation. For example see #35883 (comment) -- crash in safe code caused by generating a binary from bad MIR (indicating a bug in typeck).

[Mark-Simulacrum]

Can someone give an update here? Should we change mirbug to ICE? It seems reasonable...

[arielb1]

This is already done.