MIR contains StorageDead for locals that have never ben live

[RalfJung]

If I change this function to trigger an error when using StorageDead on a local that is already dead, that shows that we do generate MIR with redundant StorageDead. I am not sure if we should consider this a bug, but given that we consider redundant StorageLive a bug (due to some rather vague language on the LLVM side, that seems safer), I wonder if we should do the same for StorageDead?

Here's an execution trace (from CTFE) of such a piece of MIR:

├─┐rustc_const_eval::interpret::eval_context::frame core::num::<impl i8>::checked_div
│ ├─0ms  INFO rustc_const_eval::interpret::step // executing bb0
│ ├─0ms  INFO rustc_const_eval::interpret::step StorageLive(_3)
│ ├─0ms  INFO rustc_const_eval::interpret::step StorageLive(_4)
│ ├─0ms  INFO rustc_const_eval::interpret::step StorageLive(_5)
│ ├─0ms  INFO rustc_const_eval::interpret::step StorageLive(_6)
│ ├─0ms  INFO rustc_const_eval::interpret::step _6 = _2
│ ├─0ms  INFO rustc_const_eval::interpret::step _5 = Eq(move _6, const 0_i8)
│ ├─0ms  INFO rustc_const_eval::interpret::step StorageDead(_6)
│ ├─0ms  INFO rustc_const_eval::interpret::step switchInt(move _5) -> [false: bb2, otherwise: bb1]
│ ├─0ms  INFO rustc_const_eval::interpret::step // executing bb2
│ ├─0ms  INFO rustc_const_eval::interpret::step StorageLive(_7)
│ ├─0ms  INFO rustc_const_eval::interpret::step StorageLive(_8)
│ ├─0ms  INFO rustc_const_eval::interpret::step StorageLive(_9)
│ ├─0ms  INFO rustc_const_eval::interpret::step _9 = _1
│ ├─0ms  INFO rustc_const_eval::interpret::step _8 = Eq(move _9, const core::num::<impl i8>::MIN)
│ ├─0ms  INFO rustc_const_eval::interpret::step StorageDead(_9)
│ ├─0ms  INFO rustc_const_eval::interpret::step switchInt(move _8) -> [false: bb4, otherwise: bb5]
│ ├─0ms  INFO rustc_const_eval::interpret::step // executing bb4
│ ├─0ms  INFO rustc_const_eval::interpret::step _7 = const false
│ ├─0ms  INFO rustc_const_eval::interpret::step goto -> bb6
│ ├─0ms  INFO rustc_const_eval::interpret::step // executing bb6
│ ├─0ms  INFO rustc_const_eval::interpret::step StorageDead(_10)

As you can see, _10 was never marked as live, and yet it is marked as dead here.

Unfortunately I do not know a good way to get the CTFE MIR of a standard library function. I tried copying its code to generate the MIR separately, but that looks pretty different from the above trace.

Cc @tmandry @JakobDegen

[RalfJung]

#98896 contains an example of such MIR.

[tmandry]

@RalfJung did you mean to close this?

[RalfJung]

Yeah, it's a duplicate of #98896.