This is a minimal example. I have a function foo that takes a reference of a vector and returns a brand new vector. Then, there's another function bar that iteratively calls foo and update its state. But I cannot get it to compile.
fn foo(input: &Vec<i32>) -> Vec<i32> {
let len = input.len();
return input[0..len-1].to_vec();
}
fn bar() {
let input = vec![1,2,3,4,5];
let mut out = &input;
for _ in 0..2 {
out = foo(out);
}
}
Gives this error:
78 | out = foo(out);
| ^^^^^^^^
| |
| expected `&Vec<i32>`, found struct `Vec`
| help: consider borrowing here: `&foo(out)`
So that makes sense since out has a mismatched type of &Vec<i32>. Now, if I add an ampersand in front of the foo(out) call, then I got a different error:
78 | out = &foo(out);
| ^^^^---^- temporary value is freed at the end of this statement
| | |
| | borrow later used here
| creates a temporary which is freed while still in use
What is the right way to fix this?
The clean way, if possible, is to consistently hold onto the actual value instead of a reference:
fn bar() {
let input = vec![1,2,3,4,5];
let mut out = input;
for _ in 0..2 {
out = foo(&out);
}
}
or, equivalently,
fn bar() {
let input = vec![1,2,3,4,5];
let out = (0..2).fold(input, |prev, _| foo(&prev));
}
But if you need to continue using input afterwards, then you’ll have to find some other way to make the return value of foo live longer than the loop, e.g.
fn bar() {
let input = vec![1,2,3,4,5];
let mut out_storage;
let mut out = &input;
for _ in 0..2 {
out_storage = foo(out);
out = &out_storage;
}
}
Related
I have a function like the following:
pub fn process_options<T>(in_refs:& mut Vec<T>,ivals:Vec<Option<T>>) -> i32 {
let mut nones = 0;
for int in 0..ivals.len() {
if let None = ivals[int] {
nones += 1;
} else {
in_refs.push(ivals[int].unwrap());
}
}
return nones;
}
What this function does is that it:
It pushes all the some(T) from one vector to another as T. And it also records the number of none values in the first vector. As written above the code fails to compile and gives me:
error[E0507]: cannot move out of index of `Vec<Option<T>>`
--> src/functions.rs:40:26
|
40 | in_refs.push(ivals[int].unwrap());
| ^^^^^^^^^^ -------- value moved due to this method call
| |
| help: consider calling `.as_ref()` or `.as_mut()` to borrow the type's contents
| move occurs because value has type `Option<T>`, which does not implement the `Copy` trait
|
note: this function takes ownership of the receiver `self`, which moves value
--> /Users/yunfeichen/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/src/rust/library/core/src/option.rs:772:25
|
772 | pub const fn unwrap(self) -> T {
Which I frankly do not understand can you explain this to me?
Edit:
This question has been solved:
It turns out I just need to use a different style of for loop, Not sure why this for loop works but it does:
pub fn process_options<T>(in_refs:& mut Vec<T>,ivals:Vec<Option<T>>) -> i32 {
let mut nones = 0;
for element in ivals{
if let None = element {
nones += 1;
} else {
in_refs.push(element.unwrap());
}
}
return nones;
}
I am trying to convert a yaml file to xml using Rust and I am not able to figure out how to fix this error regarding the use of moved value. I think I understand why this error is coming, but haven't got a clue about what to do next.
Here's the code:
struct Element {
element_name: String,
indentation_count: i16,
}
struct Attribute<'a> {
attribute_name: &'a str,
attribute_value: &'a str,
}
fn convert_yaml_to_xml(content: String, indentation_count: i16) -> String {
let mut xml_elements: Vec<Element> = vec![];
let mut attributes: Vec<Attribute> = vec![];
xml_elements.push(Element {element_name: "xmlRoot".to_string(), indentation_count: -1});
let mut target: Vec<u8> = Vec::new();
let mut xml_data_writer = EmitterConfig::new().perform_indent(true).create_writer(&mut target);
let mut attribute_written_flag = false;
let mut xml_event;
xml_event = XmlEvent::start_element("xmlRoot");
for line in content.lines() {
let current_line = line.trim();
let caps = indentation_count_regex.captures(current_line).unwrap();
let current_indentation_count = caps.get(1).unwrap().as_str().to_string().len() as i16;
if ELEMENT_REGEX.is_match(current_line) {
loop {
let current_attribute_option = attributes.pop();
match current_attribute_option {
Some(current_attribute_option) => {
xml_event.attr(current_attribute_option.attribute_name, current_attribute_option.attribute_value)
},
None => {
break;
},
};
}
xml_data_writer.write(xml_event);
// Checking if the line is an element
let caps = ELEMENT_REGEX.captures(current_line).unwrap();
let element_name = caps.get(2);
let xml_element_struct = Element {
indentation_count: current_indentation_count,
element_name: element_name.unwrap().as_str().to_string(),
};
xml_elements.push(xml_element_struct);
xml_event = XmlEvent::start_element(element_name.unwrap().as_str());
attribute_written_flag = false;
} else if ATTR_REGEX.is_match(current_line) {
// Checking if the line is an attribute
let caps = ATTR_REGEX.captures(current_line).unwrap();
let attr_name = caps.get(2);
let attr_value = caps.get(3);
// Saving attributes to a stack
attributes.push(Attribute{ attribute_name: attr_name.unwrap().as_str(), attribute_value: attr_value.unwrap().as_str() });
// xml_event.attr(attr_name.unwrap().as_str(), attr_value.unwrap().as_str());
}/* else if NEW_ATTR_SET_REGEX.is_match(current_line) {
let caps = NEW_ATTR_SET_REGEX.captures(current_line).unwrap();
let new_attr_set_name = caps.get(2);
let new_attr_set_value = caps.get(3);
current_xml_hash.insert("name".to_string(), new_attr_set_name.unwrap().as_str().to_string());
current_xml_hash.insert("value".to_string(), new_attr_set_value.unwrap().as_str().to_string());
} */
}
if attribute_written_flag {
xml_data_writer.write(xml_event);
}
for item in xml_elements.iter() {
let event = XmlEvent::end_element();
let event_name = item.element_name.to_string();
xml_data_writer.write(event.name(event_name.as_str()));
}
println!("OUTPUT");
println!("{:?}", target);
return "".to_string();
}
And here's the error:
error[E0382]: use of moved value: `xml_event`
--> src/main.rs:77:25
|
65 | let mut xml_event;
| ------------- move occurs because `xml_event` has type `StartElementBuilder<'_>`, which does not implement the `Copy` trait
...
77 | xml_event.attr(current_attribute_option.attribute_name, current_attribute_option.attribute_value)
| ^^^^^^^^^ --------------------------------------------------------------------------------------- `xml_event` moved due to this method call, in previous iteration of loop
|
note: this function takes ownership of the receiver `self`, which moves `xml_event`
--> /Users/defiant/.cargo/registry/src/github.com-1ecc6299db9ec823/xml-rs-0.8.4/src/writer/events.rs:193:24
|
193 | pub fn attr<N>(mut self, name: N, value: &'a str) -> StartElementBuilder<'a>
| ^^^^
From XmlEvent::start_element() documentation we see that it produces a StartElementBuilder<'a>.
From StartElementBuilder<'a>::attr() documentation we see that it consumes the StartElementBuilder<'a> (the first parameter is self, not &mut self) and produces a new StartElementBuilder<'a> (which is probably similar to self but considers the expected effect of .attr()).
This approach is known as the consuming builder pattern, which is used in Rust (for example std::thread::Builder).
The typical usage of such an approach consists in chaining the function calls: something.a().b().c().d() such as something is consumed by a(), its result is consumed by b(), the same about c() and finally d() does something useful with the last result.
The alternative would be to use mutable borrows in order to modify in place something but dealing with mutable borrows is known as difficult in some situations.
In your case, you can just reassign the result of .attr() to xml_event because otherwise the .attr() function would have no effect (its result is discarded) and xml_event would become unusable because it is consumed; reassigning it makes it usable again afterwards (at least i guess, i didn't try).
I'm new to Rust and I'm struggle with the concept of lifetimes. I want to make a struct that iterates through a file a character at a time, but I'm running into issues where I need lifetimes. I've tried to add them where I thought they should be but the compiler isn't happy. Here's my code:
struct Advancer<'a> {
line_iter: Lines<BufReader<File>>,
char_iter: Chars<'a>,
current: Option<char>,
peek: Option<char>,
}
impl<'a> Advancer<'a> {
pub fn new(file: BufReader<File>) -> Result<Self, Error> {
let mut line_iter = file.lines();
if let Some(Ok(line)) = line_iter.next() {
let char_iter = line.chars();
let mut advancer = Advancer {
line_iter,
char_iter,
current: None,
peek: None,
};
// Prime the pump. Populate peek so the next call to advance returns the first char
let _ = advancer.next();
Ok(advancer)
} else {
Err(anyhow!("Failed reading an empty file."))
}
}
pub fn next(&mut self) -> Option<char> {
self.current = self.peek;
if let Some(char) = self.char_iter.next() {
self.peek = Some(char);
} else {
if let Some(Ok(line)) = self.line_iter.next() {
self.char_iter = line.chars();
self.peek = Some('\n');
} else {
self.peek = None;
}
}
self.current
}
pub fn current(&self) -> Option<char> {
self.current
}
pub fn peek(&self) -> Option<char> {
self.peek
}
}
fn main() -> Result<(), Error> {
let file = File::open("input_file.txt")?;
let file_buf = BufReader::new(file);
let mut advancer = Advancer::new(file_buf)?;
while let Some(char) = advancer.next() {
print!("{}", char);
}
Ok(())
}
And here's what the compiler is telling me:
error[E0515]: cannot return value referencing local variable `line`
--> src/main.rs:37:13
|
25 | let char_iter = line.chars();
| ---- `line` is borrowed here
...
37 | Ok(advancer)
| ^^^^^^^^^^^^ returns a value referencing data owned by the current function
error[E0597]: `line` does not live long enough
--> src/main.rs:49:34
|
21 | impl<'a> Advancer<'a> {
| -- lifetime `'a` defined here
...
49 | self.char_iter = line.chars();
| -----------------^^^^--------
| | |
| | borrowed value does not live long enough
| assignment requires that `line` is borrowed for `'a`
50 | self.peek = Some('\n');
51 | } else {
| - `line` dropped here while still borrowed
error: aborting due to 2 previous errors
Some errors have detailed explanations: E0515, E0597.
For more information about an error, try `rustc --explain E0515`.
error: could not compile `advancer`.
Some notes:
The Chars iterator borrows from the String it was created from. So you can't drop the String while the iterator is alive. But that's what happens in your new() method, the line variable owning the String disappears while the iterator referencing it is stored in the struct.
You could also try storing the current line in the struct, then it would live long enough, but that's not an option – a struct cannot hold a reference to itself.
Can you make a char iterator on a String that doesn't store a reference into the String? Yes, probably, for instance by storing the current position in the string as an integer – it shouldn't be the index of the char, because chars can be more than one byte long, so you'd need to deal with the underlying bytes yourself (using e.g. is_char_boundary() to take the next bunch of bytes starting from your current index that form a char).
Is there an easier way? Yes, if performance is not of highest importance, one solution is to make use of Vec's IntoIterator instance (which uses unsafe magic to create an object that hands out parts of itself) :
let char_iter = file_buf.lines().flat_map(|line_res| {
let line = line_res.unwrap_or(String::new());
line.chars().collect::<Vec<_>>()
});
Note that just returning line.chars() would have the same problem as the first point.
You might think that String should have a similar IntoIterator instance, and I wouldn't disagree.
Given this:
fn main() {
let variable = [0; 15];
}
The Rust compiler produces this warning:
= note: #[warn(unused_variables)] on by default
= note: to avoid this warning, consider using `_variable` instead
What's the difference between variable and _variable?
The difference is an underscore at the front, which causes the Rust compiler to allow it to be unused. It is kind of a named version of the bare underscore _ which can be used to ignore a value.
However, _name acts differently than _. The plain underscore drops the value immediately while _name acts like any other variable and drops the value at the end of the scope.
An example of how it does not act exactly the same as a plain underscore:
struct Count(i32);
impl Drop for Count {
fn drop(&mut self) {
println!("dropping count {}", self.0);
}
}
fn main() {
{
let _a = Count(3);
let _ = Count(2);
let _c = Count(1);
}
{
let _a = Count(3);
let _b = Count(2);
let _c = Count(1);
}
}
prints the following (playground):
dropping count 2
dropping count 1
dropping count 3
dropping count 1
dropping count 2
dropping count 3
The key difference between _variable and variable is that first one tells compiler not to give any warnings if we do not use it in our code. Example:
// src/main.rs
fn main() {
let _x = 1;
let y = 2;
}
Compiling main.rs gives:
warning: unused variable: `y`
--> src/main.rs:3:9
|
3 | let y = 2;
| ^ help: if this is intentional, prefix it with an underscore: `_y`
|
= note: `#[warn(unused_variables)]` on by default
The more interesting case is when we are comparing _ with _variable.
Ignoring an Unused Variable by Starting Its Name with _:
The syntax _x still binds the value to the variable, whereas _ doesn’t bind at all.
Consider example:
// src/main.rs
fn main() {
let s = Some(String::from("Hello!"));
if let Some(_s) = s {
println!("found a string");
}
println!("{:?}", s);
}
When we try to compile main.rs we get error:
error[E0382]: borrow of moved value: `s`
--> src/main.rs:8:22
|
4 | if let Some(_s) = s {
| -- value moved here
...
8 | println!("{:?}", s);
| ^ value borrowed here after partial move
|
= note: move occurs because value has type `std::string::String`, which does not implement the `Copy` trait
help: borrow this field in the pattern to avoid moving `s.0`
|
4 | if let Some(ref _s) = s {
| ^^^
Aha! The syntax _x still binds the value to the variable, which means that we are moving the ownership of s to _s, thus, we can no longer access variable s anymore; which happens when we try to print value of s.
The correct way of doing the above is:
// src/main.rs
fn main() {
let s = Some(String::from("Hello!"));
if let Some(_) = s {
println!("found a string");
}
println!("{:?}", s);
}
Above code works just fine. s does not get moved into _, so we can still access it later.
Sometimes I use _ with iterators:
fn main() {
let v = vec![1, 2, 3];
let _ = v
.iter()
.map(|x| {
println!("{}", x);
})
.collect::<Vec<_>>();
}
Compiling gives result:
1
2
3
When doing more complex operations on iterable types above example acts as utility for me.
I'm trying to use the hyper library to make some requests. The Headers::get() method returns Option<&H>, where H is a tuple struct with one field. I can use if let Some() to destructure the Option. But how do we destructure the &H? Sure I could always access the field with .0, but I'm curious if Rust has a syntax to do this.
struct s(String);
fn f(input: &s) -> &s {
input
}
fn main() {
let my_struct1 = s("a".to_owned());
let s(foo) = my_struct1;
let my_struct2 = s("b".to_owned());
let &s(bar) = f(&my_struct2); // this does not work
let baz = &my_struct2.0; // this works
}
When you try to compile this, the Rust compiler will tell you how to fix the error with a nice message:
error[E0507]: cannot move out of borrowed content
--> <anon>:11:9
|
11 | let &s(bar) = f(&my_struct2); // this does not work
| ^^^---^
| | |
| | hint: to prevent move, use `ref bar` or `ref mut bar`
| cannot move out of borrowed content
This is needed to tell the compiler that you only want a reference to the field in the struct; the default matching will perform a move and the original struct value will no longer be valid.
Let's fix the example:
struct s(String);
fn f(input: &s) -> &s {
input
}
fn main() {
let my_struct1 = s("a".to_owned());
let s(foo) = my_struct1;
let my_struct2 = s("b".to_owned());
let &s(ref bar) = f(&my_struct2);
}
Another way is to dereference first and drop the &. I think this is preferred in Rust:
struct s(String);
fn f(input: &s) -> &s {
input
}
fn main() {
let my_struct1 = s("a".to_owned());
let s(foo) = my_struct1;
let my_struct2 = s("b".to_owned());
let s(ref bar) = *f(&my_struct2);
}