_ can only be used in two contexts: in patterns, to match a value to ignore, and as a placeholder for a type. In array types, the length is not a type, but an expression, and _ cannot be used in expressions. What you can do, though, is append f32 to only one of the … Read more
Building off of E_net4’s answer, you can also create an enum for the three possibilities: #[derive(Debug)] enum Patch<T> { Missing, Null, Value(T), } impl<T> Default for Patch<T> { fn default() -> Self { Patch::Missing } } impl<T> From<Option<T>> for Patch<T> { fn from(opt: Option<T>) -> Patch<T> { match opt { Some(v) => Patch::Value(v), None => … Read more
Rust 1.26 Closures implement both Copy and Clone if all of the captured variables do. You can rewrite your code to use generics instead of a boxed trait object to be able to clone it: use std::thread; #[derive(Clone)] struct WithCall<F> { fp: F, } impl<F> WithCall<F> where F: Fn(i8, i8) -> i8, { pub fn … Read more
The problem is that stdin() returns an object of type Stdio and File::open(…).unwrap() returns an object of type File. In Rust, all arms of a match have to return values of the same type. In this case you probably wanted to return a common Read object. Unfortunately Read is a trait so you cannot pass … Read more
Returning a function Returning the actual function pointer requires heap allocation and a wrapper: use std::future::Future; use std::pin::Pin; pub async fn some_async_func(arg: &str) {} pub fn some_async_func_wrapper<‘a>(arg: &’a str) -> Pin<Box<dyn Future<Output=()> + ‘a>> { Box::pin(some_async_func(arg)) } pub fn higher_order_func<‘a>(action: &str) -> fn(&’a str) -> Pin<Box<dyn Future<Output=()> + ‘a>> { some_async_func_wrapper } Why boxing? higher_order_func … Read more
a is of type [i32; 3]; an array of three i32s. [i32; 3] does not implement an iter method, but it does dereference into &[i32]. &[i32] implements an iter method which produces an iterator. This iterator implements Iterator<Item=&i32>. It uses &i32 rather than i32 because the iterator has to work on arrays of any type, … Read more
Crates uploaded to crates.io (and thus to docs.rs) will show what feature flags exist. crates.io issue #465 suggests placing the feature list on the crate’s page as well. Beyond that, the only guaranteed way to see what features are available is to look at the Cargo.toml for the crate. This generally means that you need … Read more
Starting with Rust 1.47 (2020-10-08), this is no longer true! Almost all traits are now implemented for arrays of arbitrary length. So you can now print arrays of length 33! Old answer below for reference. Sadly, Rust does not support integers as generic parameters yet. Therefore it’s not easy to implement a trait (like Debug) … Read more
Since I was essentially trying to emulate C macros, I tried to solve the problem with Rust macros and succeeded: macro_rules! description { () => ( “my program” ) } macro_rules! version { () => ( env!(“CARGO_PKG_VERSION”) ) } macro_rules! version_string { () => ( concat!(description!(), ” v”, version!()) ) } It feels a bit … Read more
In Rust, unlike in C, enums are tagged unions. That is, the enum knows which value it holds. So 8 bytes wouldn’t be enough because there would be no room for the tag.