2021-04-26

Merge #8658

8658: Check more carefully for cases where a rename can't be done r=Veykril a=dzhu

Attempting to rename an element of a tuple field would previously replace the type with the new name, which doesn't make sense; now it fails instead.

The check is done in both `prepare_rename` and `rename` so that the case is caught before the user is prompted for a new name. Some other existing failure cases are also now additionally checked in `prepare_rename`.

Closes: #8592

(I threw in some doc edits for a relevant type; of course, I can remove those if the policy here is to be strict about scope of changes within a PR.)

Co-authored-by: Danny Zhu <[email protected]>