fix(lsp): clean the diagnostic cache when buffer delete (#19449)

Co-authored-by: Gregory Anders <greg@gpanders.com>

runtime/lua/vim/diagnostic.lua

@@ -45,18 +45,24 @@ local bufnr_and_namespace_cacher_mt = {
   end,
 }
 
-local diagnostic_cache = setmetatable({}, {
-  __index = function(t, bufnr)
-    assert(bufnr > 0, 'Invalid buffer number')
-    vim.api.nvim_buf_attach(bufnr, false, {
-      on_detach = function()
-        rawset(t, bufnr, nil) -- clear cache
-      end,
-    })
-    t[bufnr] = {}
-    return t[bufnr]
-  end,
-})
+local diagnostic_cache
+do
+  local group = vim.api.nvim_create_augroup('DiagnosticBufDelete', {})
+  diagnostic_cache = setmetatable({}, {
+    __index = function(t, bufnr)
+      assert(bufnr > 0, 'Invalid buffer number')
+      vim.api.nvim_create_autocmd('BufDelete', {
+        group = group,
+        buffer = bufnr,
+        callback = function()
+          rawset(t, bufnr, nil)
+        end,
+      })
+      t[bufnr] = {}
+      return t[bufnr]
+    end,
+  })
+end
 
 local diagnostic_cache_extmarks = setmetatable({}, bufnr_and_namespace_cacher_mt)
 local diagnostic_attached_buffers = {}

test/functional/lua/diagnostic_spec.lua

@@ -128,6 +128,28 @@ describe('vim.diagnostic', function()
     eq('Diagnostic #1', result[1].message)
   end)
 
+  it('removes diagnostics from the cache when a buffer is removed', function()
+    eq(2, exec_lua [[
+      vim.api.nvim_win_set_buf(0, diagnostic_bufnr)
+      local other_bufnr = vim.fn.bufadd('test | test')
+      local lines = vim.api.nvim_buf_get_lines(diagnostic_bufnr, 0, -1, true)
+      vim.api.nvim_buf_set_lines(other_bufnr, 0, 1, false, lines)
+      vim.cmd('bunload! ' .. other_bufnr)
+
+      vim.diagnostic.set(diagnostic_ns, diagnostic_bufnr, {
+        make_error('Diagnostic #1', 1, 1, 1, 1),
+        make_error('Diagnostic #2', 2, 1, 2, 1),
+      })
+      vim.diagnostic.set(diagnostic_ns, other_bufnr, {
+        make_error('Diagnostic #3', 3, 1, 3, 1),
+      })
+      vim.api.nvim_set_current_buf(other_bufnr)
+      vim.opt_local.buflisted = true
+      vim.cmd('bwipeout!')
+      return #vim.diagnostic.get()
+    ]])
+  end)
+
   it('resolves buffer number 0 to the current buffer', function()
     eq(2, exec_lua [[
       vim.api.nvim_set_current_buf(diagnostic_bufnr)