feat(lsp): add relatedInformation to diagnostics message

runtime/doc/news.txt

@@ -264,6 +264,7 @@ The following new APIs and features were added.
     respective capability can be unset.
   • |vim.lsp.start()| accepts a "silent" option for suppressing messages
     if an LSP server failed to start.
+  • Add support for `relatedInformation` field in LSP diagnostics.
 
 • Treesitter
   • Bundled parsers and queries (highlight, folds) for Markdown, Python, and

runtime/lua/vim/lsp/diagnostic.lua

@@ -110,6 +110,14 @@ local function diagnostic_lsp_to_vim(diagnostics, bufnr, client_id)
   return vim.tbl_map(function(diagnostic)
     local start = diagnostic.range.start
     local _end = diagnostic.range['end']
+    for _, relatedInfo in pairs(diagnostic.relatedInformation or {}) do
+      if relatedInfo.location then
+        local filename = vim.fn.fnamemodify(vim.uri_to_fname(relatedInfo.location.uri), ':t')
+        diagnostic.message = string.format("%s\n%s(%d, %d): %s", diagnostic.message, filename,
+          relatedInfo.location.range.start.line + 1, relatedInfo.location.range.start.character + 1,
+          relatedInfo.message)
+      end
+    end
     --- @type vim.Diagnostic
     return {
       lnum = start.line,