feat(diagnostic): add default mappings for diagnostics (#16230)

runtime/lua/vim/_defaults.lua

@@ -127,7 +127,9 @@ do
     end, { desc = gx_desc })
   end
 
-  --- Default maps for built-in commenting
+  --- Default maps for built-in commenting.
+  ---
+  --- See |gc-default| and |gcc-default|.
   do
     local operator_rhs = function()
       return require('vim._comment').operator()
@@ -169,6 +171,35 @@ do
       vim.lsp.buf.signature_help()
     end, { desc = 'vim.lsp.buf.signature_help()' })
   end
+
+  --- Map [d and ]d to move to the previous/next diagnostic. Map <C-W>d to open a floating window
+  --- for the diagnostic under the cursor.
+  ---
+  --- See |[d-default|, |]d-default|, and |CTRL-W_d-default|.
+  do
+    vim.keymap.set('n', ']d', function()
+      vim.diagnostic.goto_next({ float = false })
+    end, {
+      desc = 'Jump to the next diagnostic with the highest severity',
+    })
+
+    vim.keymap.set('n', '[d', function()
+      vim.diagnostic.goto_prev({ float = false })
+    end, {
+      desc = 'Jump to the previous diagnostic with the highest severity',
+    })
+
+    vim.keymap.set('n', '<C-W>d', function()
+      vim.diagnostic.open_float({ border = 'rounded' })
+    end, {
+      desc = 'Open a floating window showing diagnostics under the cursor',
+    })
+
+    vim.keymap.set('n', '<C-W><C-D>', '<C-W>d', {
+      remap = true,
+      desc = 'Open a floating window showing diagnostics under the cursor',
+    })
+  end
 end
 
 --- Default menus