Proposal: to better structure symbol details add LSPArray languageSpecificDetails property to callHierarchy and documentSymbol etc.

Why

Useful attributes for a C++ symbol displayed in call hierarchy or just searched for would probably include (but not limited to) signature, scope, return value, access specifier.
Similar set of useful attributes for other languages will likely differ but at least for C, C++ and Java this set already has more than two entries. Currently implementations are probably trying to fit all of these data items into single detail field of CallHierarchyItem, DocumentSymbol etc. using funny encoding which needs to be parsed by hand.

How

To improve this, let's add another property languageSpecificDetails of type LSPArray to CallHierarchyItem, DocumentSymbol etc.
This new property would be used by language server to communicate useful language-specific attributes as key/value pairs, easing life of client and allowing for more attributes to be passed.

Further steps

Some useful attributes like method signature appear to be common to many different languages and servers may choose to unify corresponding key names across supported languages, allowing for better unified structured UI implementations. Well-known keys shoud probably be not a fixed set but just a list of strings (with descriptions?) allowing for any extension.

Originally discussed in https://github.com/clangd/clangd/discussions/1940 regarding extra details for callHierarchy.

Start by reviewing the protocol definitions for CallHierarchyItem and DocumentSymbol, including the existing detail fields and LSPArray type. Determine which additional symbol-bearing structures should receive languageSpecificDetails and how key names should be specified. Done means the proposal has an agreed property shape, scope, and conventions for language-specific key/value attributes.