Add todo post.

blog/rust_todo.md

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+---
+title: "Rust: todo! macro"
+author: "James Pace"
+date: "2024/01/21"
+---
+
+One of the features of Rust that I've found really helpful when getting started
+on a project is the `todo!` macro.
+
+The `todo!` macro tells the compiler that you know that some section of code
+isn't complete yet and that it should compile the rest of it anyway.
+
+An example of where I've used this is when I get started on a project I like protoyping
+out the signatures for the primary functions I think I'm going to need before I start
+implemententing them.
+For example, if I'm working on functions that do math operations on a custom type,
+I'll start by thinking about what functions I need and what their signatures are
+going to be, type that out, and then fill them in.
+With `todo!`, that means I may type out something like:
+```rust
+struct Point {
+   pub x: f32,
+   pub y: f32,
+   pub z: f32,
+}
+impl Point {
+    pub fn add(&self, other: &Point) -> Point {
+        todo!()
+    }
+
+    pub fn subtract(&self, other: &Point) -> Point {
+        todo!()
+    }
+}
+```
+and that compiles, as long as there are not other mistakes.
+
+The reason that's useful is let's say I made a mistake and type `Pint`
+instead of `Point` somewhere.
+I can build the code, have the compiler point that out, and fix it
+before needing to implement everything.
+
+The official documentation about `todo!` can be found
+[here](https://doc.rust-lang.org/std/macro.todo.html).