Now, when code needs to look up attributes on an item, it fetches the complete list of attributes and sequentially looks for the interesting one. This could be made more efficient by making the attribute list a SortedIndexMultiMap.

Steps:

• replace &[ast::Attribute] by SortedIndexMultiMap<u32, Symbol, &ast::Attribute> in rustc_hir::hir, rustc_middle::query and rustc_middle::ty;
• build the SortedIndexMultiMap in rustc_ast_lowering and rustc_metadata::decoder using Attribute::name_or_empty as key;
• adapt the use sites to fetch the attributes using attrs.get_by_key(my_name) instead of attrs.iter().filter(|attr| attr.has_name(my_name);
• evaluate performance impact.

Extra:

• implement item_attrs query for the local crate, and replace calls to tcx.get_attrs by calls to tcx.item_attrs.

I am available on zulip for more detailed information.