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blog(aoc-2022): add post mortems to the first two weeks
Signed-off-by: Matej Focko <me@mfocko.xyz>
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@ -468,6 +468,33 @@ the same time as parsing. That could be done by adding additional fields to the
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nodes which would allow storing such information and updating it as we construct
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the filesystem.
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## Post Mortem
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Things that have been brought up in the discussion later on.
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### `Rc<T>` vs `Rc<RefCell<T>>`
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It has been brought up that I have a contradicting statement regarding the
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dynamically allocated memory. Specifically:
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- You can imagine `Rc<T>` as an `std::shared_ptr<T>` (in C++)
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- When you want an equivalent of `std::shared_ptr<T>`, you want to use
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`Rc<RefCell<T>>`
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Now, in Rust it is a bit more complicated, because the type that represents the
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„shared pointer“ is `Rc<T>`. What `RefCell<T>` does is making sure that there is
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only one „owner“ of a mutable reference at a time (and dynamically, as opposed
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to the `Cell<T>`).
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Therefore to be precise and correct about the equivalents of `std::shared_ptr<T>`
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in Rust, we can say that
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- `Rc<T>` is an equivalent of a `const std::shared_ptr<T>`,
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- and `Rc<RefCell<T>>` is an equivalent of a `std::shared_ptr<T>`.
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You can easily see that they only differ in the mutability. (And even that is not
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as simple as it seems, because there is also `Cell<T>`)
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[_Advent of Code_]: https://adventofcode.com
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[GitLab]: https://gitlab.com/mfocko/advent-of-code-2022
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[`/src/bin/`]: https://gitlab.com/mfocko/advent-of-code-2022/-/tree/main/src/bin
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@ -657,5 +657,217 @@ it up just a bit.
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Tedious.
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## Post Mortem
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### Indexing
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I was asked about the indexing after publishing the blog. And truly it is rather
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complicated topic, especially after releasing `SliceIndex<I>` trait. I couldn't
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leave it be, so I tried to implement the `Index` and `IndexMut` trait.
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:::note
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I have also mentioned that the `SliceIndex` trait is `unsafe`, but truth be told,
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only _unsafe_ part are the 2 methods that are named `*unchecked*`. Anyways, I will
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be implementing the `Index*` traits for now, rather than the `SliceIndex`.
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:::
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It's relatively straightforward…
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```rust
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impl<I, C> Index<Vector2D<I>> for [C]
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where
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I: Copy + TryInto<usize>,
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<I as TryInto<usize>>::Error: Debug,
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C: Index<usize>,
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{
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type Output = C::Output;
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fn index(&self, index: Vector2D<I>) -> &Self::Output {
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let (x, y): (usize, usize) =
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(index.x.try_into().unwrap(), index.y.try_into().unwrap());
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&self[y][x]
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}
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}
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impl<I, C> IndexMut<Vector2D<I>> for [C]
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where
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I: Copy + TryInto<usize>,
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<I as TryInto<usize>>::Error: Debug,
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C: IndexMut<usize>,
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{
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fn index_mut(&mut self, index: Vector2D<I>) -> &mut Self::Output {
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let (x, y): (usize, usize) =
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(index.x.try_into().unwrap(), index.y.try_into().unwrap());
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&mut self[y][x]
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}
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}
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```
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We can see a lot of similarities to the implementation of `index` and `index_mut`
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functions. In the end, they are 1:1, just wrapped in the trait that provides a
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syntax sugar for `container[idx]`.
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:::note
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I have also switched from using the `TryFrom` to `TryInto` trait, since it better
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matches what we are using, the `.try_into` rather than `usize::try_from`.
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Also implementing `TryFrom` automatically provides you with a `TryInto` trait,
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since it is relatively easy to implement. Just compare the following:
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```rust
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pub trait TryFrom<T>: Sized {
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type Error;
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fn try_from(value: T) -> Result<Self, Self::Error>;
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}
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pub trait TryInto<T>: Sized {
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type Error;
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fn try_into(self) -> Result<T, Self::Error>;
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}
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```
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:::
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OK, so we have our trait implemented, we should be able to use `container[index]`,
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right? Yes… but actually no :frowning:
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```
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error[E0277]: the type `[std::vec::Vec<i8>]` cannot be indexed by `aoc_2022::Vector2D<usize>`
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--> src/bin/day08.rs:26:18
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26 | if trees[pos] > tallest {
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| ^^^ slice indices are of type `usize` or ranges of `usize`
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= help: the trait `std::slice::SliceIndex<[std::vec::Vec<i8>]>` is not implemented for `aoc_2022::Vector2D<usize>`
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= note: required for `std::vec::Vec<std::vec::Vec<i8>>` to implement `std::ops::Index<aoc_2022::Vector2D<usize>>`
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error[E0277]: the type `[std::vec::Vec<i8>]` cannot be indexed by `aoc_2022::Vector2D<usize>`
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--> src/bin/day08.rs:30:28
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30 | max(tallest, trees[pos])
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| ^^^ slice indices are of type `usize` or ranges of `usize`
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= help: the trait `std::slice::SliceIndex<[std::vec::Vec<i8>]>` is not implemented for `aoc_2022::Vector2D<usize>`
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= note: required for `std::vec::Vec<std::vec::Vec<i8>>` to implement `std::ops::Index<aoc_2022::Vector2D<usize>>`
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error[E0277]: the type `[std::vec::Vec<i8>]` cannot be indexed by `aoc_2022::Vector2D<isize>`
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--> src/bin/day08.rs:52:28
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52 | let max_height = trees[position];
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| ^^^^^^^^ slice indices are of type `usize` or ranges of `usize`
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= help: the trait `std::slice::SliceIndex<[std::vec::Vec<i8>]>` is not implemented for `aoc_2022::Vector2D<isize>`
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= note: required for `std::vec::Vec<std::vec::Vec<i8>>` to implement `std::ops::Index<aoc_2022::Vector2D<isize>>`
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```
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Why? We have it implemented for the slices (`[C]`), why doesn't it work? Well,
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the fun part consists of the fact that in other place, where we were using it,
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we were passing the `&[Vec<T>]`, but this is coming from a helper functions that
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take `&Vec<Vec<T>>` instead. And… we don't implement `Index` and `IndexMut` for
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those. Just for the slices. :exploding_head: *What are we going to do about it?*
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We can either start copy-pasting or be smarter about it… I choose to be smarter,
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so let's implement a macro! The only difference across the implementations are
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the types of the outer containers. Implementation doesn't differ **at all**!
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Implementing the macro can be done in a following way:
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```rust
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macro_rules! generate_indices {
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($container:ty) => {
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impl<I, C> Index<Vector2D<I>> for $container
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where
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I: Copy + TryInto<usize>,
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<I as TryInto<usize>>::Error: Debug,
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C: Index<usize>,
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{
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type Output = C::Output;
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fn index(&self, index: Vector2D<I>) -> &Self::Output {
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let (x, y): (usize, usize) =
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(index.x.try_into().unwrap(), index.y.try_into().unwrap());
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&self[y][x]
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}
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}
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impl<I, C> IndexMut<Vector2D<I>> for $container
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where
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I: Copy + TryInto<usize>,
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<I as TryInto<usize>>::Error: Debug,
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C: IndexMut<usize>,
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{
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fn index_mut(&mut self, index: Vector2D<I>) -> &mut Self::Output {
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let (x, y): (usize, usize) =
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(index.x.try_into().unwrap(), index.y.try_into().unwrap());
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&mut self[y][x]
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}
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}
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};
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}
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```
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And now we can simply do
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```rust
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generate_indices!(VecDeque<C>);
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generate_indices!([C]);
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generate_indices!(Vec<C>);
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// generate_indices!([C; N], const N: usize);
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```
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The last type (I took the inspiration from the implementations of the `Index` and
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`IndexMut` traits) is a bit problematic, because of the `const N: usize` part,
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which I haven't managed to be able to parse. And that's how I got rid of the error.
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:::note
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If I were to use 2D-indexing over `[C; N]` slices, I'd probably just go with the
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copy-paste, cause the cost of this „monstrosity“ outweighs the benefits of no DRY.
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:::
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#### Cause of the problem
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This issue is relatively funny. If you don't use any type aliases, just the raw
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types, you'll get suggested certain changes by the _clippy_. For example if you
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consider the following piece of code
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```rust
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fn get_sum(nums: &Vec<i32>) -> i32 {
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nums.iter().sum()
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}
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fn main() {
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let nums = vec![1, 2, 3];
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println!("Sum: {}", get_sum(&nums));
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}
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```
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and you run _clippy_ on it, you will get
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```
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Checking playground v0.0.1 (/playground)
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warning: writing `&Vec` instead of `&[_]` involves a new object where a slice will do
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--> src/main.rs:1:18
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1 | fn get_sum(nums: &Vec<i32>) -> i32 {
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| ^^^^^^^^^ help: change this to: `&[i32]`
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= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_arg
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= note: `#[warn(clippy::ptr_arg)]` on by default
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warning: `playground` (bin "playground") generated 1 warning
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Finished dev [unoptimized + debuginfo] target(s) in 0.61s
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```
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However, if you introduce a type alias, such as
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```rust
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type Numbers = Vec<i32>;
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```
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Then _clippy_ won't say anything, cause there is literally nothing to suggest.
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However the outcome is not the same…
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[_Advent of Code_]: https://adventofcode.com
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[BFS above]: #day-12-hill-climbing-algorithm
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