Rollup merge of #70189 - RalfJung:is_signed, r=eddyb

Abi::is_signed: assert that we are a Scalar

A bit more sanity checking, suggested by @eddyb. This makes this method actually "safer" than `TyS::is_signed`, so I made sure Miri consistently uses the `Abi` version.

Though I am not sure if this would have caught the mistake where the layout of a zero-sized enum was asked for its sign.

r? @eddyb

src/librustc_mir/interpret/operand.rs

@@ -604,7 +604,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
  .not_undef()
  .and_then(|raw_discr| self.force_bits(raw_discr, discr_val.layout.size))
  .map_err(|_| err_ub!(InvalidDiscriminant(raw_discr.erase_tag())))?;
- let real_discr = if discr_val.layout.ty.is_signed() {
+ let real_discr = if discr_val.layout.abi.is_signed() {
  // going from layout tag type to typeck discriminant type
  // requires first sign extending with the discriminant layout
  let sexted = sign_extend(bits_discr, discr_val.layout.size) as i128;

src/librustc_target/abi/mod.rs

@@ -751,7 +751,7 @@ impl Abi {
  Primitive::Int(_, signed) => signed,
  _ => false,
  },
- _ => false,
+ _ => panic!("`is_signed` on non-scalar ABI {:?}", self),
  }
  }