How does Rust provide move semantics?

Question

The Rust language website claims move semantics as one of the features of the language. But I can\'t see how move semantics is implemented in Rust.

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Answer 1

Rust supports move semantics with features like these:

• All types are moveable.

• Sending a value somewhere is a move, by default, throughout the language. For non-Copy types, like Vec, the following are all moves in Rust: passing an argument by value, returning a value, assignment, pattern-matching by value.

  You don't have std::move in Rust because it's the default. You're really using moves all the time.

• Rust knows that moved values must not be used. If you have a value x: String and do channel.send(x), sending the value to another thread, the compiler knows that x has been moved. Trying to use it after the move is a compile-time error, "use of moved value". And you can't move a value if anyone has a reference to it (a dangling pointer).

• Rust knows not to call destructors on moved values. Moving a value transfers ownership, including responsibility for cleanup. Types don't have to be able to represent a special "value was moved" state.

• Moves are cheap and the performance is predictable. It's basically memcpy. Returning a huge Vec is always fast—you're just copying three words.

• The Rust standard library uses and supports moves everywhere. I already mentioned channels, which use move semantics to safely transfer ownership of values across threads. Other nice touches: all types support copy-free std::mem::swap() in Rust; the Into and From standard conversion traits are by-value; Vec and other collections have .drain() and .into_iter() methods so you can smash one data structure, move all the values out of it, and use those values to build a new one.

Rust doesn't have move references, but moves are a powerful and central concept in Rust, providing a lot of the same performance benefits as in C++, and some other benefits as well.