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The underlying implementation of `LLVMCreateConstantRangeAttribute` assumes that each of `LowerWords` and `UpperWords` points to enough u64 values to define an integer of the specified bit-length, and will encounter UB if that is not the case. Our safe wrapper function always passes pointers to `[u64; 2]` arrays, regardless of the bit-length specified. That's fine in practice, because scalar primitives never exceed 128 bits, but it is technically a soundness hole in a safe function. We can close the soundness hole by explicitly asserting `size_bits <= 128`. This is effectively just a stricter version of the existing check that the value must be small enough to fit in `c_uint`.
The codegen crate contains the code to convert from MIR into LLVM IR,
and then from LLVM IR into machine code. In general it contains code
that runs towards the end of the compilation process.
For more information about how codegen works, see the rustc dev guide.