The collect_lines example won't even compile, it's not valid C++, but there's undefined behavior in one of the examples? I'm very surprised and would like to know what it is, that would be truly shocking.
The first example uses the int type. This is a signed integer type and in practice today it will usually be the 32-bit signed integer Rust calls i32 because that's cheap on almost any hardware you'd actually use for general purpose software.
In C++ this type has Undefined Behaviour if allowed to overflow. For the 32-bit signed integer that will happen once we see 2^31 identical lines.
In practice the observed behaviour will probably be that it treats 2^32 identical lines as equivalent to zero prior occurrences and I've verified that behaviour in a toy system.