Rust tips @rust@octodon.social

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`#[deny(warnings)]` will sooner or later break your code.

#Rust keeps adding new warnings and refining behavior of existing ones. If you're not prepared to deal with potential new compilation errors every 6 weeks, don't use #[deny(warnings)].

Rustc has a cap-lints feature to disable this footgun in dependencies, but it doesn't help macro-generated code and top-level crates.

The `.collect()` method on #Rust iterators can construct many different types, and is extensible via FromIterator trait.

Here's the list of supported types in the standard library:

https://doc.rust-lang.org/std/iter/trait.FromIterator.html#implementors

You read it as FromIterator</* type of items in the iterator */> for /* type it makes */

If you want to rush a #Rust function to return quicker, write:

fn foo() -> i32 {
return return return return return !!!!!!!!!!111;
}

https://www.reddit.com/r/rust/comments/g5rsuh/show_me_the_most_illegal_rust_code_youve_ever/fo6onb9/

#Rust borrow checker won't let you call a method that takes &mut self and a reference to something else in the same struct:

self.mutate_item(&self.item) // :(

There are a few workarounds for this:

https://smallcultfollowing.com/babysteps/blog/2018/11/01/after-nll-interprocedural-conflicts/

If you're wondering why your #rustlang project is pulling in a certain dependency, you can find out with:

cargo tree -i dependency_name

You can also find duplicate (usually outdated) dependencies with:

cargo tree -d

#Rust constants create a new temporary value every time you mention them.

You can do with whatever you want with the temporary value from a constant, even mutate it. But it only affects each temporary copy, not the constant.

```
use std::cell::Cell;

const C: Cell<bool> = Cell::new(false);

fn main() {
C.set(true);
assert_eq!(false, C.get());
}
```

If your project depends on a Nightly version of the #Rust compiler, you can create `rust-toolchain` file with the required version. It will make Cargo automatically switch to the right compiler for the build.

https://rust-lang.github.io/rustup/overrides.html#the-toolchain-file

If you're defining an alias for your crate's Result type, instead of:

type Result<T> = std::result::Result<T, MyError>;

use:

type Result<T, E = MyError> = std::result::Result<T, E>;

This still works as `Result<()>`, but doesn't cause errors when someone accidentally shadows std's Result with it. It also makes Rustdoc display the error type explicitly in the documentation.

#rustlang

If you want to create an async stream, the easy solution is a generator macro:

https://lib.rs/async-stream

It lets you write the stream as a loop with `yield` instead of implementing traits and managing state manually.

#rustlang

Does your #rust crate have optional features? Are you sure all combinations of the feature flags work correctly? You can verify this with this cargo plugin: https://lib.rs/cargo-all-features

In #Rust you can cast concrete type to an abstract trait object (e.g. `&i32` as `&dyn Debug`), but you can't (yet) cast a trait object as another trait (e.g. `dyn Error` can't be cast as `dyn Debug`).

The workaround is to add a method to the trait that asks for a different dyn type (e.g. `as_debug(&self) -> &dyn Debug`). That allows trait implementers to cast their concrete type.

This workaround can be automated: https://stackoverflow.com/a/28664881/27009

async {} block in #Rustlang behaves like a closure. It does _not_ run any code automatically. It only creates an inactive object, which has to be spawned or awaited to start doing anything.

Example of incorrect usage:

fn main() {
async {
println!("closure");
return;
};
println!("surprise");
}

It prints "surprise", and nothing else!

Dropping of a #Rust #Future stops its execution. There's also a higher-level wrapper that makes Futures cancellable on demand:

https://docs.rs/futures/0.3.4/futures/future/struct.Abortable.html