fix(clangd): set offset_encoding correctly #3887

Problem:
Neovim's native Client:initialize implementation does not support
clangd's utf8 offsets extension, resulting in an inconsistency in
offsetEncoding between clangd and Neovim when using vim.lsp.config.

Solution:
Add a custom on_init to set the offset_encoding correctly.

Author: someoneinjd

lsp/clangd.lua

@@ -57,6 +57,9 @@ local function symbol_info()
   end, bufnr)
 end
 
+---@class ClangdInitializeResult: lsp.InitializeResult
+---@field offsetEncoding? string
+
 return {
   cmd = { 'clangd' },
   filetypes = { 'c', 'cpp', 'objc', 'objcpp', 'cuda', 'proto' },
@@ -77,6 +80,13 @@ return {
     },
     offsetEncoding = { 'utf-8', 'utf-16' },
   },
+  ---@param client vim.lsp.Client
+  ---@param init_result ClangdInitializeResult
+  on_init = function(client, init_result)
+    if init_result.offsetEncoding then
+      client.offset_encoding = init_result.offsetEncoding
+    end
+  end,
   on_attach = function()
     vim.api.nvim_buf_create_user_command(0, 'LspClangdSwitchSourceHeader', function()
       switch_source_header(0)