I touched src/sage/structure/factory.pyx, and wanted to rebuild only the relevant part of the reference manual. This is I got:
$ sage --docbuild reference/structure html
[structure] inventory <
https://ipywidgets.readthedocs.io/en/stable/> contains multiple definitions for std:label:examples/Widget Layout.ipynb#display
[structure] WARNING: failed to reach any of the inventories with the following issues:
[structure] intersphinx inventory '/Users/kwankyu/GitHub/sage-dev/local/share/doc/sage/inventory/en/reference/objects.inv' not readable due to FileNotFoundError: [Errno 2] No such file or directory: '/Users/kwankyu/GitHub/sage-dev/local/share/doc/sage/inventory/en/reference/objects.inv'
[structure] WARNING: failed to reach any of the inventories with the following issues:
[structure] intersphinx inventory '/Users/kwankyu/GitHub/sage-dev/local/share/doc/sage/inventory/en/reference/structure/objects.inv' not readable due to FileNotFoundError: [Errno 2] No such file or directory: '/Users/kwankyu/GitHub/sage-dev/local/share/doc/sage/inventory/en/reference/structure/objects.inv'
[structure] no targets are out of date.
[structure] The HTML pages are in local/share/doc/sage/html/en/reference/structure.
Error building the documentation.
Traceback (most recent call last):
File "<frozen runpy>", line 198, in _run_module_as_main
File "<frozen runpy>", line 88, in _run_code
File "/Users/kwankyu/GitHub/sage-dev/src/sage_docbuild/__main__.py", line 552, in <module>
sys.exit(main())
~~~~^^
File "/Users/kwankyu/GitHub/sage-dev/src/sage_docbuild/__main__.py", line 548, in main
build()
~~~~~^^
File "/Users/kwankyu/GitHub/sage-dev/src/sage_docbuild/builders.py", line 669, in _wrapper
getattr(DocBuilder, build_type)(self, *args, **kwds)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
File "/Users/kwankyu/GitHub/sage-dev/src/sage_docbuild/builders.py", line 142, in f
runsphinx()
~~~~~~~~~^^
File "/Users/kwankyu/GitHub/sage-dev/src/sage_docbuild/sphinxbuild.py", line 324, in runsphinx
sys.stderr.raise_errors()
~~~~~~~~~~~~~~~~~~~~~~~^^
File "/Users/kwankyu/GitHub/sage-dev/src/sage_docbuild/sphinxbuild.py", line 255, in raise_errors
raise OSError(self._error)
OSError: WARNING: failed to reach any of the inventories with the following issues:
The command "sage --docbuild" is an essential tool for any developer working on sagelib. It should work.