DataflowConstProp trips LLVM assertion "Invalid constexpr bitcast!"

[saethlin]

Reduced from core:

pub struct State {
    inner: bool
}

pub fn make() -> State {
    State {
        inner: true,
    }
}

rustc +master-stage1 -Zmir-opt-level=0 -Zmir-enable-passes=+DataflowConstProp --crate-type=lib

Compilation should crash with this error:

rustc: /checkout/src/llvm-project/llvm/lib/IR/Constants.cpp:2216: static Constant *llvm::ConstantExpr::getBitCast(Constant *, Type *, bool): Assertion `CastInst::castIsValid(Instruction::BitCast, C, DstTy) && "Invalid constantexpr bitcast!"' failed.
error: could not compile `core` (lib)

Because we are trying to generate an LLVM bitcast from i1 to i8.

The MIR dump looks like this:

fn make() -> State {
    let mut _0: State;

    bb0: {
        _0 = const State {{ inner: true }};
        return;
    }
}

alloc3 (size: 1, align: 1) {
    01                                              │ .
}

I haven't been able to figure out how to produce this MIR without DataflowConstProp.

[saethlin]

Thes standard library also works as a reproducer. Simply building a hello world with this will assert:

RUSTFLAGS="-Zmir-opt-level=4 -Cdebuginfo=2 -Zcross-crate-inline-threshold=yes \
-Zinline-mir-hint-threshold=285 cargo +master-stage1build --release \
-Zbuild-std --target=x86_64-unknown-linux-gnu

[saethlin]

@tmiasko You asked about this crash before when I had a messy reproducer. Pinging you now because it's a lot smaller. Still not elegant though. it is now