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feat(move_semantics2): rewrite hint

This commit is contained in:
liv 2023-06-12 12:07:18 +02:00
parent 1ce9d93e94
commit 369ae2e63d
2 changed files with 19 additions and 20 deletions
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10
exercises/move_semantics/move_semantics2.rs
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@ -2,23 +2,21 @@
// Execute `rustlings hint move_semantics2` or use the `hint` watch subcommand for a hint.
// Expected output:
// vec0 has length 3 content `[22, 44, 66]`
// vec1 has length 4 content `[22, 44, 66, 88]`
// vec0 has length 3, with contents `[22, 44, 66]`
// vec1 has length 4, with contents `[22, 44, 66, 88]`
// I AM NOT DONE
fn main() {
let vec0 = Vec::new();
// Do not move the following line!
let mut vec1 = fill_vec(vec0);
// Do not change the following line!
println!("{} has length {} content `{:?}`", "vec0", vec0.len(), vec0);
println!("{} has length {}, with contents: `{:?}`", "vec0", vec0.len(), vec0);
vec1.push(88);
println!("{} has length {} content `{:?}`", "vec1", vec1.len(), vec1);
println!("{} has length {}, with contents `{:?}`", "vec1", vec1.len(), vec1);
}
fn fill_vec(vec: Vec<i32>) -> Vec<i32> {

29
info.toml
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@ -287,23 +287,24 @@ Also: Try accessing `vec0` after having called `fill_vec()`. See what happens!""
[[exercises]]
name = "move_semantics2"
path = "exercises/move_semantics/move_semantics2.rs"
mode = "compile"
mode = "test"
hint = """
So, `vec0` is passed into the `fill_vec` function as an argument. In Rust,
when an argument is passed to a function and it's not explicitly returned,
you can't use the original variable anymore. We call this "moving" a variable.
Variables that are moved into a function (or block scope) and aren't explicitly
returned get "dropped" at the end of that function. This is also what happens here.
There's a few ways to fix this, try them all if you want:
1. Make another, separate version of the data that's in `vec0` and pass that
When running this exercise for the first time, you'll notice an error about
"borrow of moved value". In Rust, when an argument is passed to a function and
it's not explicitly returned, you can't use the original variable anymore.
We call this "moving" a variable. When we pass `vec0` into `fill_vec`, it's being
"moved" into `vec1`, meaning we can't access `vec0` anymore after the fact.
Rust provides a couple of different ways to mitigate this issue, feel free to try them all:
1. You could make another, separate version of the data that's in `vec0` and pass that
to `fill_vec` instead.
2. Make `fill_vec` borrow its argument instead of taking ownership of it,
and then copy the data within the function in order to return an owned
`Vec<i32>`
3. Make `fill_vec` *mutably* borrow a reference to its argument (which will need to be
mutable), modify it directly, then not return anything. Then you can get rid
of `vec1` entirely -- note that this will change what gets printed by the
first `println!`"""
and then copy the data within the function (`vec.clone()`) in order to return an owned
`Vec<i32>`.
3. Or, you could make `fill_vec` *mutably* borrow a reference to its argument (which will need to be
mutable), modify it directly, then not return anything. This means that `vec0` will change over the
course of the function, and makes `vec1` redundant (make sure to change the parameters of the `println!`
statements if you go this route)
"""
[[exercises]]
name = "move_semantics3"