IIRC LLVM's IR verification is not enabled in release build. In other words, if you're using rustc with debug version of LLVM the error message should pop up.

EDIT: "...LLVM's IR verification is not enabled in release build..." this is wrong, LLVM doesn't turn off verification based on its build mode. It is up to the user of LLVM, namely rustc in this case, to enable verification. For instance, you can verify IR after each optimization Pass (which is pretty expensive) by configuring `llvm::StandardInstrumentation` properly. Or you can verify the IR before codegen by switching up one of `llvm::TargetMachine`'s options. Clang always enables the latter (verification) by default but disables the former regardless of the optimization level or its build mode.

I don't know how these checks compare to what clang is doing.

If I understand correctly, it's undefined behaviour to do this:

    int *ptr = (int*)42;

It's possible things are different when the type is void*, I'm not certain.

Is it documented undefined behavior to feed bad IR into LLVM?

For example, LLVM has a pointer type and a void type, but you may not make a pointer-to-void type, see https://llvm.org/docs/LangRef.html#pointer-type . If you do call `Type::getVoidTy(C)->getPointerTo()` then LLVM will hit an assertion only in a build with assertions enabled. Without assertions LLVM may silently execute UB.

Probably not. As the author describes, LLVM has to tool to check for invalid IR, which they used to investigate the issue and generate an explanatory error message.