I'm just asking why Rust decided to use &str for string literals instead of String. Isn't it possible for Rust to just automatically convert a string literal to a String and put it on the heap instead of putting it into the stack?
To understand the reasoning, consider that Rust wants to be a systems programming language. In general, this means that it needs to be (among other things) (a) as efficient as possible and (b) give the programmer full control over allocations and deallocations of heap memory. One use case for Rust is for embedded programming where memory is very limited.
Therefore, Rust does not want to allocate heap memory where this is not strictly necessary. String literals are known at compile time and can be written into the ro.data section of an executable/library, so they don't consume stack or heap space.
Now, given that Rust does not want to allocate the values on the heap, it is basically forced to treat string literals as &str: Strings own their values and can be moved and dropped, but how do you drop a value that is in ro.data? You can't really do that, so &str is the perfect fit.
Furthermore, treating string literals as &str (or, more accurately &'static str) has all the advantages and none of the disadvantages. They can be used in multiple places, can be shared without worrying about using heap memory and never have to be deleted. Also, they can be converted to owned Strings at will, so having them available as String is always possible, but you only pay the cost when you need to.