Copy is a Rust marker trait that gives a type copy semantics. When ownership would otherwise move into another scope or function, Rust implicitly duplicates the value instead, leaving the original usable. section 4.1.2 section 4.5.3
Primitive values versus structs
An integer can be passed to `use_value(a)` and printed afterward because `i32` implements `Copy`. A `CubeSat` value does not behave that way automatically: passing it to an ownership-taking function moves it, so using it again produces a moved-value error. section 4.1.2 section 4.1.1
Listing 4.5 The copy semantics of Rust’s primitive types
fn use_value(_val: i32) { #1
}
fn main() {
let a = 123 ;
use_value(a);
println!("{}", a); #2
}
Compare the short function call with the later print to see how Copy permits the original integer to remain accessible. section 4.1.2
Use Copy with these distinctions in mind:
- Derive `Copy` and `Clone` for structs or enums whose fields and variants support Copy, as with a `CubeSat { id: u64 }` and a simple status enum. section 4.5.3
- Treat Copy as implicit, fast, cheap, identical duplication; treat Clone as explicit duplication through `.clone()`, which may be slower or define different behavior. section 4.5.3
- Avoid assuming every useful type should implement Copy: exact duplication can be costly for large values and unsafe for ownership-bearing references. section 4.5.3
- Use `Clone` or `Copy` pragmatically when duplication helps resolve an ownership or borrow-checker impasse. section 4.6
