Integer overflow in rotate_left

[RalfJung]

Running the following program in Miri when rustc was compiled with debug assertions leads to an integer overflow:

fn main() {
    assert_eq!(0i16.rotate_left(124), 0);
}

Relevant part of the backtrace:

  12: core::panicking::panic
             at src/libcore/panicking.rs:49
  13: rustc_mir::interpret::intrinsics::<impl rustc_mir::interpret::eval_context::InterpretCx<M>>::emulate_intrinsic
             at /home/r/src/rust/rustc/src/librustc_mir/interpret/intrinsics.rs:197
  14: miri::intrinsic::EvalContextExt::call_intrinsic
             at src/intrinsic.rs:20
  15: <miri::Evaluator as rustc_mir::interpret::machine::Machine>::call_intrinsic
             at src/lib.rs:421
  16: rustc_mir::interpret::terminator::<impl rustc_mir::interpret::eval_context::InterpretCx<M>>::eval_fn_call
             at /home/r/src/rust/rustc/src/librustc_mir/interpret/terminator.rs:250
  17: rustc_mir::interpret::terminator::<impl rustc_mir::interpret::eval_context::InterpretCx<M>>::eval_terminator
             at /home/r/src/rust/rustc/src/librustc_mir/interpret/terminator.rs:95
  18: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpretCx<M>>::terminator
             at /home/r/src/rust/rustc/src/librustc_mir/interpret/step.rs:287
  19: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpretCx<M>>::step
             at /home/r/src/rust/rustc/src/librustc_mir/interpret/step.rs:69
  20: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpretCx<M>>::run
             at /home/r/src/rust/rustc/src/librustc_mir/interpret/step.rs:40
  21: miri::eval_main::{{closure}}
             at src/lib.rs:229
  22: miri::eval_main
             at src/lib.rs:228

[RalfJung]

The bug is actually on the rustc side, in the implementation of rotate_left.

                let layout = self.layout_of(substs.type_at(0))?;
                let val_bits = self.read_scalar(args[0])?.to_bits(layout.size)?;
                let raw_shift_bits = self.read_scalar(args[1])?.to_bits(layout.size)?;
                let width_bits = layout.size.bits() as u128;
                let shift_bits = raw_shift_bits % width_bits;
                let inv_shift_bits = (width_bits - raw_shift_bits) % width_bits;
                let result_bits = if intrinsic_name == "rotate_left" {
                    (val_bits << shift_bits) | (val_bits >> inv_shift_bits)
                } else {
                    (val_bits >> shift_bits) | (val_bits << inv_shift_bits)
                };
                let truncated_bits = self.truncate(result_bits, layout);
                let result = Scalar::from_uint(truncated_bits, layout.size);

The computation of "width_bits - raw_shift_bits" can overflow.

Looks like our CTFE intrinsic test suite is insufficient. ;)

[RalfJung]

Closing in favor of rust-lang/rust#61406.