From a BF investigation.

Testing on a standalone from a day-old version of master, I see this:

MongoDB Enterprise > db.adminCommand({renameCollection: 'Non-numeric data in input field which evaluates to an array should be ignored', to: 'test.b'})
{
 "ok" : 0,
 "errmsg" : "db name must be at most 63 characters, found: 18446744073709551615",
 "code" : 73,
 "codeName" : "InvalidNamespace"
}

Looking at the two locations that would produce this I see:

• database_name.h
  • StringData::size()
• namespace_string.h
  • std::string::find()

• • • There's some delicate error checking here, and std::string::npos (i.e. SIZE_MAX) gets printed.

SERVER-75529 appears to have touched a lot of this size handling.