shrewd lantern
Why does the borrow checker allow the following expression:
struct A([i32;3]);
fn main() {
let var_498 = A([1,2,3]);
let x = {var_498.0}[{
var_498;
1
}];
println!("{}", x)
}
but disallow
struct A([i32;3]);
fn main() {
let var_498 = A([1,2,3]);
let x = var_498.0[{
var_498;
1
}];
println!("{}", x)
}
--> run_874_manual.rs:4:13
|
3 | let var_498 = A([1,2,3]);
| ------- move occurs because `var_498` has type `A`, which does not implement the `Copy` trait
4 | let x = var_498.0[{
| _____________^
5 | | var_498;
| | ------- value moved here
6 | | 1
7 | | }];
| |______^ value used here after move
The code is just in an individual .rs file and not in a package
solemn moss
Ah, I see.
In the former snippet, you make a copy of var_498.0. This is because it's the return value of a block, so it's moved, and its type is Copy, so its copied instead. Then, you run the block expression inside the [].
In the second snippet, you first borrow var_498.0 (implicitly, as part of how [] works), which borrows var_498. Then, you try to move var_498 into the inner block, while it's still borrowed.
shrewd lantern
I see, so the index expression works by first borrowing var_498 in the block statement within the index expression. Then it attempts to borrow var_498.0 if it's a place expression and if it's a block statement then it makes a copy. The index expression makes a copy the result into x and then var_498.0's ownership is returned?
visual wadi