While naming a package name as a built-in identifier is not great, isn't this a breaking change?

In the case of a package with a name that conflicts with a predefined identifier, that is imported without a local renaming, and that doesn't conflict with any identifier used in the places it is imported, this could be a breaking change.

Testing the change inside Google's codebase isn't likely to turn up anything broken by this, because our local style is to always rename imported proto packages.

I think generating a local rename that conflicts with a predefined identifier (e.g., import string "...") is obviously wrong. We could keep the previous behavior for determining package names (package string is probably a bad idea, but it'll compile) and apply the new one for local renames only.

apply the new one for local renames only.

SGTM.

We could keep the previous behavior for determining package names (package string is probably a bad idea, but it'll compile)

I believe we should keep the existing logic for determining package names.

In most cases, the user should specify the go_package option, where they manually determine what the Go package path and name should be. If they really wanted to name a package the same as a built-in identifier, we should preserve that behavior. I find it odd that we already do magical name cleanups for a string that the user explicitly sets. In my opinion, if they put an invalid package name (perhaps a typo), we should have avoided magically cleaning it up for them, and let the Go compiler complain. I'm not sure adding more magical sanitization logic is a good idea.

Also, I can understand hardcoding the keywords since they cannot be used as identifiers and it is very unlikely that we'll add new keywords even in Go2 (at least the generics proposal seems to make a conscious effort to avoid a new keyword). However, it seems more plausible that new built-in identifiers may be added to Go over time, which would presumably be added to the backlist. The idea that package names can can change over time is concerning.