rustdoc: deprecate #![doc(no_default_passes)], #![doc(passes "...")], #![doc(plugins = "...")]

[QuietMisdreavus]

While digging around in librustdoc, i found this little nugget:

rust/src/librustdoc/lib.rs

Lines 593 to 610 in 16362c7

	for attr in krate.module.as_ref().unwrap().attrs.lists("doc") {
	let name = attr.name().map(|s| s.as_str());
	let name = name.as_ref().map(|s| &s[..]);
	if attr.is_word() {
	if name == Some("no_default_passes") {
	default_passes = false;
	}
	} else if let Some(value) = attr.value_str() {
	let sink = match name {
	Some("passes") => &mut passes,
	Some("plugins") => &mut plugins,
	_ => continue,
	};
	for p in value.as_str().split_whitespace() {
	sink.push(p.to_string());
	}
	}
	}

What this does is handle the #![doc(no_default_passes)], #![doc(passes "...")], and #![doc(plugins = "...")] attributes the same way as the --no-defaults, --passes, and --plugins CLI flags. It would be useful to provide an analogue to --document-private-items and emit deprecation warnings for these attributes, as part of deprecating passes and plugins generally.