Bitfield macro has problems with overflow in release mode

[jounathaen]

Hi,

I cannot really say whether this is a bug, as I do not have a windows machine and only executed that very module, but I discovered that the bitfield128! macro has issues in --release mode.

To reproduce, run cargo test --release.
The error I get is

error: attempt to shift left with overflow
   --> src/xxxxxxxxx
    |
65  | /                     self.$field.High64 <<
66  | |                         (128 - $r.end) >> (64 - $r.end + $r.start)
    | |______________________________________^
...
183 | /         bitfield128!(TEST_STRUCT_U128 AsUINT128: u64[
184 | |             a set_a[0..32],
185 | |             b set_b[32..64],
186 | |             c set_c[64..96],
187 | |             d set_d[96..128],
188 | |         ]);
    | |___________- in this macro invocation
    |
    = note: `#[deny(exceeding_bitshifts)]` on by default

Might be only due to the fact that I execute only the macro and the test on my Linux machine, but I observe kind of similar behaviour in one of our Windows pipelines as well, since around Feb 22.

[petrutlucian94]

Thanks for reporting this issue. I think previous rustc versions treated this as a warning.

Such overflows would occur in case of incorrectly defined bitfields, using values outside the [0, 128] range or having r.end - r.start > 64. We should probably add an assertion checking those values and have rustc ignore this warning since it won't be able to tell that we're already avoiding overflows with that assertion.

[epakskape]

I created a new proposed PR for this that does not rely on allow(const_err): #22