compiler-docs: Missing trait impls for type

[tesuji]

This is a regression from stable to beta, although this issue will be P-low.
The problem is #73771 ignored documenting unstable traits for non-local types,
#76571 fixes some regressions caused by it but the regression
in nightly-rustc / compiler docs still opens.

For example: Compare https://doc.rust-lang.org/nightly/nightly-rustc/rustc_expand/base/struct.ExtCtxt.html#blanket-implementations and https://doc.rust-lang.org/1.46.0/nightly-rustc/rustc_expand/base/struct.ExtCtxt.html#blanket-implementations .

The doc in 1.46.0 has MaybeResult impl, while on nightly it doesn't have.

A way to fix this issue is ignoring this check when the compiling-crates
are being passed -Zforce-unstable-if-unmarked flag or the crate/module has rustc_private feature:

rust/src/librustdoc/clean/inline.rs

Lines 345 to 374 in c743fc4

	if let Some(stab) = tcx.lookup_stability(did) {
	if stab.level.is_unstable() && stab.feature == sym::rustc_private {
	return;
	}
	}
	}
	}
	let for_ = if let Some(did) = did.as_local() {
	let hir_id = tcx.hir().local_def_id_to_hir_id(did);
	match tcx.hir().expect_item(hir_id).kind {
	hir::ItemKind::Impl { self_ty, .. } => self_ty.clean(cx),
	_ => panic!("did given to build_impl was not an impl"),
	}
	} else {
	tcx.type_of(did).clean(cx)
	};
	// Only inline impl if the implementing type is
	// reachable in rustdoc generated documentation
	if !did.is_local() {
	if let Some(did) = for_.def_id() {
	if !cx.renderinfo.borrow().access_levels.is_public(did) {
	return;
	}
	if let Some(stab) = tcx.lookup_stability(did) {
	if stab.level.is_unstable() && stab.feature == sym::rustc_private {
	return;
	}

Meta

rustc --version --verbose: Version 1.48.0-nightly (9b41541 2020-09-14)

[jakevossen5]

Hey! I would like to check this out if that is okay

[camelid]

Is this actually a regression?