integer overflow in CTFE implementation of rotate_left

[RalfJung]

The following code ICEs when compiled with a rustc with debug-assertions enabled:

const X: i16 = 0i16.rotate_left(124);

The problem is in the implementation of rotate_left, specifically in this line:

rust/src/librustc_mir/interpret/intrinsics.rs

Line 197 in 75f4644

let inv_shift_bits = (width_bits - raw_shift_bits) % width_bits;

The subtraction overflows here.

Cc @oli-obk

[nikic]

Fix should be:

// replace
let inv_shift_bits = (width_bits - raw_shift_bits) % width_bits;
// with
let inv_shift_bits = (width_bits - shift_bits) % width_bits;

[hgallagher1993]

Ya I'll take this on 👍

[RalfJung]

@hgallagher1993 awesome! @nikic above already pretty much gave the mentoring instructions, except for one thing: please also add a test case. Or rather, please extend the existing test case at src/test/ui/consts/const-int-rotate.rs.

Let me know if you need any more assistance.

[RalfJung]

@lzutao @hgallagher1993 looks like you are both on this now? Not sure how that happened...