f7c48297ce
Check for duplicate attributes. This adds some checks for duplicate attributes. In many cases, the duplicates were being ignored without error or warning. This adds several kinds of checks (see `AttributeDuplicates` enum). The motivation here is to issue unused warnings with similar reasoning for any unused lint, and to error for cases where there are conflicts. This also adds a check for empty attribute lists in a few attributes where this causes the attribute to be ignored. Closes #55112.
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.