Something that I've found confusing in Rust: the fact that the return type of a function call seems to depend on how the returned value is used. This thread [0] has the following example, where the statements:

  let v: Vec<u8> = b"3q2+7w==".iter().cloned().collect();

and

  let v: HashSet<u8> = b"3q2+7w==".iter().cloned().collect();

are both valid, even though both statements have the same expression on the right hand side. In a language like Java you could assign a value to different types based on a sub-type / super-type relationship (Object foo = Arrays.asList(...)), but that's not the case here, nor is it the case that Vec<u8> and HashSet<u8> have the same layout in memory. Instead, according to the thread, the result from collect can be interpreted as any type that implements the FromIterator trait.

I don't know what the name for this sort of behavior is, and I haven't seen it in other languages. Perhaps it's just a property of Rust's trait system? Regardless, it feels intuitive to me - like the compiler is taking type information and using it to make implicit casts throughout the program.

Not a huge impediment - just something I found confusing that isn't related to the borrow checker.

[0] https://users.rust-lang.org/t/why-cant-the-compiler-the-type...