Is there a rule of thumb regarding the use of の when using one noun to modify another?


I wrote 担当先生 in my composition today. I was not confident about it, so I
searched BCCWJ 中納言 and found out that 担任の先生 is the most popular way of saying
it, and that 担当の先生 is also acceptable to a less degree. But it seems that 担当先生
and 担任先生 are not entirely unacceptable, with respectively 1 hit yielded in the
corpus.

[This
post](https://www.reddit.com/r/japanese/comments/fs8zeb/combining_nouns/)
suggests that maybe there is not a consistent standard for using の between two
nouns, but the range of cases it covers is limited. No similar question is
found (on my part) on Japanese SE.

