`TyCtxt::get_attr` should check that no duplicates are allowed

[lcnr]

get_attr currently happily ignores any additional attributes in case there are more than one. While this is fine for attributes which don't care about duplicates or emit a warning/error in case there are more than one, this isn't currently checked in the compiler, potentially causing bugs.

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

Lines 2282 to 2284 in a39bdb1

	pub fn get_attr(self, did: DefId, attr: Symbol) -> Option<&'tcx ast::Attribute> {
	self.get_attrs(did, attr).next()
	}

similar to the check in has_attr we should check that the attr declaration in rustc_feature/builtin_attrs.rs has an acceptable way of dealing with duplicates.

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

Lines 2287 to 2290 in a39bdb1

	pub fn has_attr(self, did: DefId, attr: Symbol) -> bool {
	if cfg!(debug_assertions) && !did.is_local() && rustc_feature::is_builtin_only_local(attr) {
	bug!("tried to access the `only_local` attribute `{}` from an extern crate", attr);
	} else {

in case there are any questions on how to implement this, feel free to open a thread on zulip and pinging me there.

[chenyukang]

@rustbot claim