While it's true that the go tool does modify go.mod I would like to propose that go.mod be excluded from being treated as a "generated" file since understanding changes to it are a crucial part of the review process.

For example, here's the default files view from a recent PR to a project that groomed some dependencies:

A reviewer must "load diff" on go.mod. And for PRs that have lots of changes across many files it is easy to gloss over the two collapsed files without actually expanding the contents of go.mod.

From a precedence standpoint dep (Gopkg.toml & Gopkg.lock) has similar properties in that dep modifies Gopkg.toml, but only Gopkg.lock is considered "generated".

Thanks!