rustdoc: correct negative-to-implicit discriminant display

[eval-exec]

This PR want to fix #145125

In:

rust/compiler/rustc_middle/src/ty/util.rs

Lines 33 to 38 in 7f7b8ef

	#[derive(Copy, Clone, Debug)]
	pub struct Discr<'tcx> {
	/// Bit representation of the discriminant (e.g., `-1i8` is `0xFF_u128`).
	pub val: u128,
	pub ty: Ty<'tcx>,
	}

the Discr's val field is u128, so we can't use discr.val as i128 to represent Discr's signed value.

We should use Discr's Display trait to display signed value.

rust/compiler/rustc_middle/src/ty/util.rs

Lines 60 to 73 in 7f7b8ef

	impl<'tcx> fmt::Display for Discr<'tcx> {
	fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
	match *self.ty.kind() {
	ty::Int(ity) => {
	let size = ty::tls::with(|tcx| Integer::from_int_ty(&tcx, ity).size());
	let x = self.val;
	// sign extend the raw representation to be an i128
	let x = size.sign_extend(x) as i128;
	write!(fmt, "{x}")
	}
	_ => write!(fmt, "{}", self.val),
	}
	}
	}

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