Make PartialEq a const_trait

[oli-obk]

r? @fee1-dead or @compiler-errors

something generally useful but also required for #142789

[rustbot]

fee1-dead is currently at their maximum review capacity.
They may take a while to respond.

[fmease]

Dupe of #133995

[fee1-dead]

this is no longer testing the issue - can we change this to another one that still shows the errors? or just remove this test completely (or just have const_trait_impl removed in this test)

[fee1-dead]

This also means

rust/compiler/rustc_const_eval/src/check_consts/ops.rs

Lines 281 to 316 in 9972ebf

	let mut sugg = None;
	if ccx.tcx.is_lang_item(trait_id, LangItem::PartialEq) {
	match (args[0].kind(), args[1].kind()) {
	(GenericArgKind::Type(self_ty), GenericArgKind::Type(rhs_ty))
	if self_ty == rhs_ty
	&& self_ty.is_ref()
	&& self_ty.peel_refs().is_primitive() =>
	{
	let mut num_refs = 0;
	let mut tmp_ty = self_ty;
	while let rustc_middle::ty::Ref(_, inner_ty, _) = tmp_ty.kind() {
	num_refs += 1;
	tmp_ty = *inner_ty;
	}
	let deref = "*".repeat(num_refs);
	if let Ok(call_str) = ccx.tcx.sess.source_map().span_to_snippet(span) {
	if let Some(eq_idx) = call_str.find("==") {
	if let Some(rhs_idx) =
	call_str[(eq_idx + 2)..].find(|c: char| !c.is_whitespace())
	{
	let rhs_pos =
	span.lo() + BytePos::from_usize(eq_idx + 2 + rhs_idx);
	let rhs_span = span.with_lo(rhs_pos).with_hi(rhs_pos);
	sugg = Some(errors::ConsiderDereferencing {
	deref,
	span: span.shrink_to_lo(),
	rhs_span,
	});
	}
	}
	}
	}
	_ => {}
	}

should be deleted (or with a FIXME to delete once we stabilize stuff)

[fee1-dead]

@bors r+ rollup

[bors]

📌 Commit 9933442 has been approved by fee1-dead

It is now in the queue for this repository.

[RalfJung]

Why is this library trait guarded by a language feature and its tracking issue?
It doesn't even match the attribute on the impls, which is quite confusing (I think the impl attributes are completely ignored then?)