Questions tagged [rust]
Rust is a systems programming language without a garbage collector focused on three goals: safety, speed, and concurrency. Use this tag for questions about code written in Rust. Use an edition specific tag for questions that refer to code which requires a particular edition, like [rust-2018]. Use more specific tags for subtopics like [rust-cargo] and [rust-macros].
rust
41,881
questions
1030
votes
16
answers
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views
What are the differences between Rust's `String` and `str`?
Why does Rust have both String and str? What are the differences between them, and when should one be used over the other? Is one of them getting deprecated?
- 17.6k
634
votes
9
answers
196k
views
Why doesn't println! work in Rust unit tests?
I've implemented the following method and unit test:
use std::fs::File;
use std::path::Path;
use std::io::prelude::*;
fn read_file(path: &Path) {
let mut file = File::open(path).unwrap();
...
- 16k
594
votes
9
answers
497k
views
How do I concatenate strings?
How do I concatenate the following combinations of types:
str and str
String and str
String and String
- 6,198
528
votes
14
answers
277k
views
How do you disable dead code warnings at the crate level in Rust?
While tinkering in Rust, I repeatedly encountered a lot of dead code warnings that made it difficult to focus. I tried using the outer attribute #[allow(dead_code)], but it only silences one warning ...
- 24k
499
votes
9
answers
498k
views
Convert a String to int?
Note: this question contains deprecated pre-1.0 code! The answer is correct, though.
To convert a str to an int in Rust, I can do this:
let my_int = from_str::<int>(my_str);
The only way I ...
- 9,089
490
votes
6
answers
151k
views
What is the difference between iter and into_iter?
I am doing the Rust by Example tutorial, which has this code snippet:
// Vec example
let vec1 = vec![1, 2, 3];
let vec2 = vec![4, 5, 6];
// `iter()` for vecs yields `&i32`. Destructure to `i32`.
...
- 8,910
475
votes
18
answers
229k
views
How do I print in Rust the type of a variable?
I have the following:
let mut my_number = 32.90;
How do I print the type of my_number?
Using type and type_of did not work. Is there another way I can print the number's type?
- 4,867
435
votes
4
answers
64k
views
Why can't I store a value and a reference to that value in the same struct?
I have a value and I want to store that value and a reference to
something inside that value in my own type:
struct Thing {
count: u32,
}
struct Combined<'a>(Thing, &'a u32);
fn ...
- 419k
432
votes
10
answers
198k
views
How to match a String against string literals?
I'm trying to figure out how to match a String in Rust.
I initially tried matching like this, but I figured out Rust cannot implicitly cast from std::string::String to &str.
fn main() {
...
- 16.3k
416
votes
1
answer
53k
views
Why does the Rust compiler not optimize code assuming that two mutable references cannot alias?
As far as I know, reference/pointer aliasing can hinder the compiler's ability to generate optimized code, since they must ensure the generated binary behaves correctly in the case where the two ...
- 4,324
410
votes
4
answers
156k
views
Package with both a library and a binary?
I would like to make a Rust package that contains both a reusable library (where most of the program is implemented), and also an executable that uses it.
Assuming I have not confused any semantics ...
- 24k
382
votes
5
answers
211k
views
What is the syntax for a multiline string literal?
I'm having a hard time figuring out how string syntax works in Rust. Specifically, I'm trying to figure out how to make a multiple line string.
- 4,419
378
votes
7
answers
425k
views
How do I split a string in Rust?
From the documentation, it's not clear. In Java you could use the split method like so:
"some string 123 ffd".split("123");
- 33.9k
368
votes
7
answers
106k
views
Why are Rust executables so huge?
I find the approach and the way they define the language in the first two chapters of the documentation particularly interesting. So I decided to get my fingers wet and started out with "Hello, ...
- 11.6k
352
votes
9
answers
196k
views
How do I create a global, mutable singleton?
What is the best way to create and use a struct with only one instantiation in the system? Yes, this is necessary, it is the OpenGL subsystem, and making multiple copies of this and passing it around ...
- 4,311
346
votes
4
answers
58k
views
What are Rust's exact auto-dereferencing rules?
I'm learning/experimenting with Rust, and in all the elegance that I find in this language, there is one peculiarity that baffles me and seems totally out of place.
Rust automatically dereferences ...
- 5,803
326
votes
3
answers
196k
views
What's the de-facto way of reading and writing files in Rust 1.x?
With Rust being comparatively new, I've seen far too many ways of reading and writing files. Many are extremely messy snippets someone came up with for their blog, and 99% of the examples I've found (...
- 4,600
325
votes
0
answers
9k
views
Managing the lifetimes of garbage-collected objects [closed]
I am making a simplistic mark-and-compact garbage collector. Without going too much into details, the API it exposes is like this:
/// Describes the internal structure of a managed object.
pub struct ...
- 19.6k
320
votes
6
answers
269k
views
How can I include a module from another file from the same project?
By following this guide I created a Cargo project.
src/main.rs
fn main() {
hello::print_hello();
}
mod hello {
pub fn print_hello() {
println!("Hello, world!");
}
}
...
- 19k
288
votes
4
answers
108k
views
What is this question mark operator about?
I'm reading the documentation for File:
//..
let mut file = File::create("foo.txt")?;
//..
What is the ? in this line? I do not recall seeing it in the Rust Book before.
- 21k
287
votes
24
answers
160k
views
Cargo build hangs with " Blocking waiting for file lock on the registry index" after building parity from source
I followed the readme instructions for building Parity from source and then executed:
cargo build --release
~/.cargo/bin/cargo build --release
as instructed, both of which returned the following ...
- 6,923
287
votes
3
answers
226k
views
What is the equivalent of the join operator over a vector of Strings?
I wasn't able to find the Rust equivalent for the "join" operator over a vector of Strings. I have a Vec<String> and I'd like to join them as a single String:
let string_list = vec
- 5,878
285
votes
6
answers
286k
views
How do I convert a Vector of bytes (u8) to a string?
I am trying to write simple TCP/IP client in Rust and I need to print out the buffer I got from the server.
How do I convert a Vec<u8> (or a &[u8]) to a String?
- 4,191
284
votes
5
answers
59k
views
How can a Rust program access metadata from its Cargo package?
How do you access a Cargo package's metadata (e.g. version) from the Rust code in the package? In my case, I am building a command line tool that I'd like to have a standard --version flag, and I'd ...
- 36.6k
281
votes
11
answers
35k
views
Why are explicit lifetimes needed in Rust?
I was reading the lifetimes chapter of the Rust book, and I came across this example for a named/explicit lifetime:
struct Foo<'a> {
x: &'a i32,
}
fn main() {
let x; ...
- 32k
278
votes
6
answers
92k
views
What is the difference between Copy and Clone?
This issue seems to imply it's just an implementation detail (memcpy vs ???), but I can't find any explicit description of the differences.
- 7,023
268
votes
11
answers
167k
views
How do I fix the Rust error "linker 'cc' not found" for Debian on Windows 10?
I'm running Debian on Windows 10 (Windows Subsystem for Linux) and installed Rust using the command:
curl https://sh.rustup.rs -sSf | sh
There were no errors in the install, but when I tried to ...
- 9,468
267
votes
10
answers
156k
views
Default function arguments in Rust
Is it possible to create a function with a default argument?
fn add(a: int = 1, b: int = 2) { a + b }
- 16.3k
260
votes
3
answers
114k
views
How can I build multiple binaries with Cargo?
I'd like to make a project with a daemon and a client, connecting through a unix socket.
A client and a daemon requires two binaries, so how do I tell Cargo to build two targets from two different ...
- 2,793
259
votes
2
answers
23k
views
Why is there a large performance impact when looping over an array with 240 or more elements?
When running a sum loop over an array in Rust, I noticed a huge performance drop when CAPACITY >= 240. CAPACITY = 239 is about 80 times faster.
Is there special compilation optimization Rust is ...
- 9,408
258
votes
4
answers
27k
views
Why is it discouraged to accept a reference &String, &Vec, or &Box as a function argument?
I wrote some Rust code that takes a &String as an argument:
fn awesome_greeting(name: &String) {
println!("Wow, you are awesome, {}!", name);
}
I've also written code that takes in a ...
- 419k
250
votes
5
answers
186k
views
Is there a command to update Cargo to the latest official release?
I seem to have diverging versions of rustc and cargo (I think),
$ rustc -V
rustc 1.9.0 (e4e8b6668 2016-05-18)
$ cargo -V
cargo 0.10.0-nightly (10ddd7d 2016-04-08)
Is there a command akin to
pip ...
- 4,721
244
votes
2
answers
97k
views
How to use a local unpublished crate?
I've made a library:
cargo new my_lib
and I want to use that library in a different program:
cargo new my_program --bin
extern crate my_lib;
fn main {
println!("Hello, World!");
}
what do I ...
- 2,652
233
votes
5
answers
69k
views
How do I stop iteration and return an error when Iterator::map returns a Result::Err?
I have a function that returns a Result:
fn find(id: &Id) -> Result<Item, ItemError> {
// ...
}
Then another using it like this:
let parent_items: Vec<Item> = parent_ids.iter(...
- 29.6k
233
votes
3
answers
69k
views
When does a closure implement Fn, FnMut and FnOnce?
What are the specific conditions for a closure to implement the Fn, FnMut and FnOnce traits?
That is:
When does a closure not implement the FnOnce trait?
When does a closure not implement the FnMut ...
- 10.1k
227
votes
6
answers
206k
views
What is the best way to concatenate vectors in Rust?
Is it even possible to concatenate vectors in Rust? If so, is there an elegant way to do so? I have something like this:
let mut a = vec![1, 2, 3];
let b = vec![4, 5, 6];
for val in &b {
a....
- 6,167
223
votes
8
answers
229k
views
Is it possible to use global variables in Rust?
I know that in general, global-variables are to be avoided. Nevertheless, I think in a practical sense, it is sometimes desirable (in situations where the variable is integral to the program) to use ...
- 10.4k
221
votes
2
answers
78k
views
What is the correct way to return an Iterator (or any other trait)?
The following Rust code compiles and runs without any issues.
fn main() {
let text = "abc";
println!("{}", text.split(' ').take(2).count());
}
After that, I tried something like this .... ...
- 4,774
219
votes
2
answers
154k
views
Is there a faster/shorter way to initialize variables in a Rust struct?
In the following example, I would much prefer to assign a value to each field in the struct in the declaration of the fields. Alternatively, it effectively takes one additional statement for each ...
- 10.4k
219
votes
5
answers
140k
views
How do I iterate over a range with a custom step?
How can I iterate over a range in Rust with a step other than 1? I'm coming from a C++ background so I'd like to do something like
for(auto i = 0; i <= n; i+=2) {
//...
}
In Rust I need to use ...
- 19.7k
218
votes
5
answers
127k
views
How do I use a macro across module files?
I have two modules in separate files within the same crate, where the crate has macro_rules enabled. I want to use the macros defined in one module in another module.
// macros.rs
#[macro_export] // ...
- 5,290
211
votes
3
answers
32k
views
When is it appropriate to use an associated type versus a generic type?
In this question, an issue arose that could be solved by changing an attempt at using a generic type parameter into an associated type. That prompted the question "Why is an associated type more ...
- 419k
211
votes
3
answers
26k
views
What is the difference between traits in Rust and typeclasses in Haskell?
Traits in Rust seem at least superficially similar to typeclasses in Haskell, however I've seen people write that there are some differences between them. I was wondering exactly what these ...
- 4,382
206
votes
13
answers
74k
views
How can I access command line parameters in Rust?
The Rust tutorial does not explain how to take parameters from the command line. fn main() is only shown with an empty parameter list in all examples.
What is the correct way of accessing command line ...
- 6,736
205
votes
2
answers
52k
views
What's the difference between use and extern crate?
I think that use is used to import identifiers into the current scope and extern crate is used to declare an external module. But this understanding (maybe wrong) doesn't make any sense to me. Can ...
- 2,671
199
votes
2
answers
112k
views
Cannot move out of borrowed content / cannot move out of behind a shared reference
I don't understand the error cannot move out of borrowed content. I have received it many times and I have always solved it, but I've never understood why.
For example:
for line in self.xslg_file....
- 2,893
198
votes
4
answers
200k
views
How to convert a String into a &'static str
How do I convert a String into a &str? More specifically, I would like to convert it into a str with the static lifetime (&'static str).
- 27.4k
198
votes
3
answers
72k
views
How to lookup from and insert into a HashMap efficiently?
I'd like to do the following:
Lookup a Vec for a certain key, and store it for later use.
If it doesn't exist, create an empty Vec for the key, but still keep it in the variable.
How to do this ...
- 1,983
193
votes
4
answers
36k
views
How does Rust's 128-bit integer `i128` work on a 64-bit system?
Rust has 128-bit integers, these are denoted with the data type i128 (and u128 for unsigned ints):
let a: i128 = 170141183460469231731687303715884105727;
How does Rust make these i128 values work on ...
- 23.3k
187
votes
1
answer
203k
views
How do I convert between numeric types safely and idiomatically?
Editor's note: This question is from a version of Rust prior to 1.0 and references some items that are not present in Rust 1.0. The answers still contain valuable information.
What's the idiomatic ...
- 7,559