Please find the attached patches which enable fontification and
indentation in M-x shell and ielm. The main fontification and
indentation machinery is implemented in comint.el and should be general
enough for use by other comint derived modes. It works by making an
indirect buffer, putting it in a suitable major mode (sh-mode in
M-x shell buffers, for example) and fontifying and indenting user input
that buffer.

The fourth patch also implements highlighting of non-existent or
misspelled commands in M-x shell, which I find particularly useful to
immediately spot typos. This functionality is disabled by default
because it sacrifices a bit of performance.