LINQ

This section discusses LINQ within the context and for the purpose of querying or transforming sequences (IEnumerable/IEnumerable<T>) and typically collections like lists, sets and dictionaries.

IEnumerable<T>

The equivalent of IEnumerable<T> in Rust is IntoIterator. Just as an implementation of IEnumerable<T>.GetEnumerator() returns a IEnumerator<T> in .NET, an implementation of IntoIterator::into_iter returns an Iterator. However, when it's time to iterate over the items of a container advertising iteration support through the said types, both languages offer syntactic sugar in the form of looping constructs for iteratables. In C#, there is foreach:

using System;
using System.Text;

var values = new[] { 1, 2, 3, 4, 5 };
var output = new StringBuilder();

foreach (var value in values)
{
    if (output.Length > 0)
        output.Append(", ");
    output.Append(value);
}

Console.Write(output); // Prints: 1, 2, 3, 4, 5

In Rust, the equivalent is simply for:

use std::fmt::Write;

fn main() {
    let values = [1, 2, 3, 4, 5];
    let mut output = String::new();

    for value in values {
        if output.len() > 0 {
            output.push_str(", ");
        }
        // ! discard/ignore any write error
        _ = write!(output, "{value}");
    }

    println!("{output}");  // Prints: 1, 2, 3, 4, 5
}

The for loop over an iterable essentially gets desuraged to the following:

use std::fmt::Write;

fn main() {
    let values = [1, 2, 3, 4, 5];
    let mut output = String::new();

    let mut iter = values.into_iter();      // get iterator
    while let Some(value) = iter.next() {   // loop as long as there are more items
        if output.len() > 0 {
            output.push_str(", ");
        }
        _ = write!(output, "{value}");
    }

    println!("{output}");
}

Rust's ownership and data race condition rules apply to all instances and data, and iteration is no exception. So while looping over an array might look straightforward and very similar to C#, one has to be mindful about ownership when needing to iterate the same collection/iterable more than once. The following example iteraters the list of integers twice, once to print their sum and another time to determine and print the maximum integer:

fn main() {
    let values = vec![1, 2, 3, 4, 5];

    // sum all values

    let mut sum = 0;
    for value in values {
        sum += value;
    }
    println!("sum = {sum}");

    // determine maximum value

    let mut max = None;
    for value in values {
        if let Some(some_max) = max { // if max is defined
            if value > some_max {     // and value is greater
                max = Some(value)     // then note that new max
            }
        } else {                      // max is undefined when iteration starts
            max = Some(value)         // so set it to the first value
        }
    }
    println!("max = {max:?}");
}

However, the code above is rejected by the compiler due to a subtle difference: values has been changed from an array to a Vec<int>, a vector, which is Rust's type for growable arrays (like List<T> in .NET). The first iteration of values ends up consuming each value as the integers are summed up. In other words, the ownership of each item in the vector passes to the iteration variable of the loop: value. Since value goes out of scope at the end of each iteration of the loop, the instance it owns is dropped. Had values been a vector of heap-allocated data, the heap memory backing each item would get freed as the loop moved to the next item. To fix the problem, one has to request iteration over shared references via &values in the for loop. As a result, value ends up being a shared reference to an item as opposed to taking its ownership.

Below is the updated version of the previous example that compiles. The fix is to simply replace values with &values in each of the for loops.

fn main() {
    let values = vec![1, 2, 3, 4, 5];

    // sum all values

    let mut sum = 0;
    for value in &values {
        sum += value;
    }
    println!("sum = {sum}");

    // determine maximum value

    let mut max = None;
    for value in &values {
        if let Some(some_max) = max { // if max is defined
            if value > some_max {     // and value is greater
                max = Some(value)     // then note that new max
            }
        } else {                      // max is undefined when iteration starts
            max = Some(value)         // so set it to the first value
        }
    }
    println!("max = {max:?}");
}

The ownership and dropping can be seen in action even with values being an array instead of a vector. Consider just the summing loop from the above example over an array of a structure that wraps an integer:

struct Int(i32);

impl Drop for Int {
    fn drop(&mut self) {
        println!("{} dropped", self.0)
    }
}

fn main() {
    let values = [Int(1), Int(2), Int(3), Int(4), Int(5)];
    let mut sum = 0;

    for value in values {
        sum += value.0;
    }

    println!("sum = {sum}");
}

Int implements Drop so that a message is printed when an instance get dropped. Running the above code will print:

value = Int(1)
Int(1) dropped
value = Int(2)
Int(2) dropped
value = Int(3)
Int(3) dropped
value = Int(4)
Int(4) dropped
value = Int(5)
Int(5) dropped
sum = 15

It's clear that each value is acquired and dropped while the loop is running. Once the loop is complete, the sum is printed. If values in the for loop is changed to &values instead, like this:

for value in &values {
    // ...
}

then the output of the program will change radically:

value = Int(1)
value = Int(2)
value = Int(3)
value = Int(4)
value = Int(5)
sum = 15
Int(1) dropped
Int(2) dropped
Int(3) dropped
Int(4) dropped
Int(5) dropped

This time, values are acquired but not dropped while looping because each item doesn't get owned by the interation loop's variable. The sum is printed ocne the loop is done. Finally, when the values array that still owns all the the Int instances goes out of scope at the end of main, its dropping in turn drops all the Int instances.

These examples demonstrate that while iterating collection types may seem to have a lot of parallels between Rust and C#, from the looping constructs to the iteration abstractions, there are still subtle differences with respect to ownership that can lead to the compiler rejecting the code in some instances.

See also:

• Iterator
• Iterating by reference

Operators

Operators in LINQ are implemented in the form of C# extension methods that can be chained together to form a set of operations, with the most common forming a query over some sort of data source. C# also offers a SQL-inspired query syntax with clauses like from, where, select, join and others that can serve as an alternative or a companion to method chaining. Many imperative loops can be re-written as much more expressive and composable queries in LINQ.

Rust does not offer anything like C#'s query syntax. It has methods, called adapters in Rust terms, over iterable types and therefore directly comparable to chaining of methods in C#. However, whlie rewriting an imperative loop as LINQ code in C# is often beneficial in expressivity, robustness and composability, there is a trade-off with performance. Compute-bound imperative loops usually run faster because they can be optimised by the JIT compiler and there are fewer virtual dispatches or indirect function invocations incurred. The surprising part in Rust is that there is no performance trade-off between choosing to use method chains on an abstraction like an iterator over writing an imperative loop by hand. It's therefore far more common to see the former in code.

The following table lists the most common LINQ methods and their approximate counterparts in Rust.

1. The Aggregate overload not accepting a seed value is equivalent to reduce, while the Aggregate overload accepting a seed value corresponds to fold.

2. collect in Rust generally works for any collectible type, which is defined as a type that can initialize itself from an iterator (see FromIterator). collect needs a target type, which the compiler sometimes has trouble inferring so the turbofish (::<>) is often used in conjunction with it, as in collect::<Vec<_>>(). This is why collect appears next to a number of LINQ extension methods that convert an enumerable/iterable source to some collection type instance.

The following example shows how similar transforming sequences in C# is to doing the same in Rust. First in C#:

var result =
    Enumerable.Range(0, 10)
              .Where(x => x % 2 == 0)
              .SelectMany(x => Enumerable.Range(0, x))
              .Aggregate(0, (acc, x) => acc + x);

Console.WriteLine(result); // 50

And in Rust:

let result = (0..10)
    .filter(|x| x % 2 == 0)
    .flat_map(|x| (0..x))
    .fold(0, |acc, x| acc + x);

println!("{result}"); // 50

Deferred execution (laziness)

Many operators in LINQ are designed to be lazy such that they only do work when absolutely required. This enables composition or chaining of several operations/methods without causing any side-effects. For example, a LINQ operator can return an IEnumerable<T> that is initialized, but does not produce, compute or materialize any items of T until iterated. The operator is said to have deferred execution semantics. If each T is computed as iteration reaches it (as opposed to when iteration begins) then the operator is said to stream the results.

Rust iterators have the same concept of laziness and streaming.

In both cases, this allows infinite sequences to be represented, where the underlying sequence is infinite, but the developer decides how the sequence should be terminated . The following example shows this in C#:

foreach (var x in InfiniteRange().Take(5))
    Console.Write($"{x} "); // Prints "0 1 2 3 4"

IEnumerable<int> InfiniteRange()
{
    for (var i = 0; ; ++i)
        yield return i;
}

Rust supports the same concept through infinite ranges:

// Generators and yield in Rust are unstable at the moment, so
// instead, this sample uses Range:
// https://doc.rust-lang.org/std/ops/struct.Range.html

for value in (0..).take(5) {
    print!("{value} "); // Prints "0 1 2 3 4"
}

Iterator Methods (yield)

C# has the yield keword that enables the developer to quickly write an iterator method. The return type of an iterator method can be an IEnumerable<T> or an IEnumerator<T>. The compiler then converts the body of the method into a concrete implementation of the return type, instead of the developer having to write a full-blown class each time. Coroutines, as they're called in Rust, are still considered an unstable feature at the time of this writing.