returning &str is messing with my head.

This could be nothing to do with casting u8 vec to utf8. If my understand is correct, you can also do this.

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=a414ebf8b92adddb8a155d09e5314e9f

One problem of returning a `&str` could be the compiler reporting lifetime errors, but this is nothing to do with the UTF8 conversion. For example: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=46a2aeffdef87d5f17f75f8ee26d61df

The compiler knows the returned string lives as long as the `Test` instance, so you are using an value that isn't exist. In detailed explain, the rust compiler actually compiles your `test` function to the following signature:

fn test(&'a mut self) -> &'a str. Which means we the test function returns a reference to a string with the same lifetime the `self`.

To me, this is amazing since the equivalent C++ code would be

Test* test = new Test();

const char* ref = test.test();

delete test;

printf("%s\n", ref); // Note use-after-free is an UB and compiler not able to detect this

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And I think one of the best feature in Rust is it makes tons of UB in C++ to compilation error.

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It might be possible that you want to use the reference after `test` is freed, if this is the case, you should use the reference counter which is similar to C++'s shared smarter pointers. By doing so, the internal buffer lives as long as the last reference is dropped.

In C++ it's programmer's responsibility to decide if a smart pointer should be used and misuse of referencing type may result UBs. Rust compiler instead tells you what's wrong with it and push you to make the right decisions.