lender 🙂

A lender, also called a lending iterator, is an iterator that lends mutable borrows to the items it returns. In particular, this means that the reference to an item is invalidated by the subsequent call to next. Niko Matsakis has an interesting blog post explaining a general view of giving vs. lending traits.

The typical example that cannot be written with standard Rust iterators, but is covered by lenders, is that of a lender returning mutable, overlapping windows of a slice or array.

But lenders are more general than that, as they might return items that depend on some mutable state stored in the iterator. For example, a lender might return references to the lines of a file reusing an internal buffer; also, starting from an iterator on pairs of integers lexicographically sorted, a lender might return iterators on pairs with the same first coordinate without any copying; clearly, in all these cases any call on next would invalidate the reference returned by the previous call.

This crate provides a lender trait and an associated library of utility methods, “utilizing” #84533 and #25860 to implement the lender design based on higher-rank trait bounds proposed by Sabrina Jewson.

Similarly to what happens with standard iterators, besides the fundamental Lender trait there is an IntoLender trait, and methods such as for_each.

Indeed, the crate implements for Lender all of the methods as Iterator, except partition_in_place and array_chunks (the latter being replaced by chunky), and most methods provide the same functionality as the equivalent Iterator method.

Notable differences in behavior include next_chunk providing a lender instead of an array and certain closures requiring usage of the hrc!, hrc_mut!, hrc_once! (higher-ranked closure) macros, which provide a stable replacement for the closure_lifetime_binder feature.

Turn a lender into an iterator with cloned where lend is Clone, copied where lend is Copy, owned where lend is ToOwned, or iter where the lender already satisfies the restrictions of Iterator.

Usage

The Rust for syntax for iterating over types implementing IntoIterator will not work with lenders. The idiomatic way of iterating over a lender is to use a while let loop, as in:

while let Some(item) = lender.next() {
    // Do something with item
}

Note that the expression after the equal sign cannot be a method call returning a lender, as you would iterate over the first element forever.

To simplify usage, we provide a function-like procedural macro for_! that makes it possible to use a for-like syntax with types implementing IntoLender:

for_!(item in into_lender {
    // Do something with item
});

Finally, you can use the for_each method, which takes a closure as argument, but managing lifetimes in closures can be challenging:

lender.for_each{
    hrc_mut!(for<'lend> |item: &'lend mut TYPE| {
        // do something with item of type TYPE 
    })
};

Caveats

Forewarning, before you go on with this crate, you should consider using a more seasoned crate, like lending-iterator, which, however, does not use directly higher-rank trait bounds, but rather relies on simulating them using macros.

Also, if a dyn Lender trait object is in your future, this crate definitely isn't going to work. This crate was not made to be used in any sort of production code, so please, use it at your own risk (Documentation be damned! Unsafe transmutes beware!).

Finally, note that, as a general rule, if you can avoid using lenders, you should. You should heed the counsel of Polonius: “Neither a borrower nor a lender be”.

Examples

Let us compute the Fibonacci numbers using mutable windows:

use ::lender::prelude::*;
use lender_derive::for_;

// Fibonacci sequence
let mut data = vec![0u32; 3 * 3];
data[1] = 1;

// Fibonacci sequence, most ergonomic usage: for_! procedural macro.
for_!(w in data.array_windows_mut::<3>() {
   w[2] = w[0] + w[1];
});
assert_eq!(data, [0, 1, 1, 2, 3, 5, 8, 13, 21]);

// You can use decostructing assignments with for_!.
for_!([a, b, c] in data.array_windows_mut::<3>() {
   *c = *a + *b;
});
assert_eq!(data, [0, 1, 1, 2, 3, 5, 8, 13, 21]);

// Fibonacci sequence, explicit while let loop: you MUST assign the lender to a variable.
let mut windows = data.array_windows_mut::<3>();
while let Some(w) = windows.next() {
   w[2] = w[0] + w[1];
}
assert_eq!(data, [0, 1, 1, 2, 3, 5, 8, 13, 21]);

// Fibonacci sequence, for_each with hrc_mut!
data.array_windows_mut::<3>()
    .for_each(hrc_mut!(for<'lend> |w: &'lend mut [u32; 3]| {
         w[2] = w[0] + w[1]
    }));
assert_eq!(data, [0, 1, 1, 2, 3, 5, 8, 13, 21]);

This is a quite contrived example, but it shows how lenders can be used to mutate a slice in place.

So, let's look at a slightly more interesting example, LinesStr, an io::Lines with an Item of &str instead of String. It's a good example of borrowing from the iterator itself.

use std::io;
use ::lender::prelude::*;

struct LinesStr<B> {
    buf: B,
    line: String,
}
impl<'lend, B: io::BufRead> Lending<'lend> for LinesStr<B> {
    type Lend = io::Result<&'lend str>;
}
impl<B: io::BufRead> Lender for LinesStr<B> {
    fn next<'lend>(&'lend mut self) -> Option<io::Result<&'lend str>> {
        self.line.clear();
        match self.buf.read_line(&mut self.line) {
            Err(e) => return Some(Err(e)),
            Ok(0) => return None,
            Ok(_nread) => (),
        };
        if self.line.ends_with('\n') {
            self.line.pop();
            if self.line.ends_with('\r') {
                self.line.pop();
            }
        }
        Some(Ok(&self.line))
    }
}

let buf = io::BufReader::with_capacity(10, "Hello\nWorld\n".as_bytes());
let mut lines = LinesStr { buf, line: String::new() };
assert_eq!(lines.next().unwrap().unwrap(), "Hello");
assert_eq!(lines.next().unwrap().unwrap(), "World");

Implementing Lender

To implement Lender first you'll need to implement the Lending trait for your type. This is the equivalent provider of Iterator::Item:

use ::lender::prelude::*;
struct StrRef<'a>(&'a str);
impl<'this, 'lend> Lending<'lend> for StrRef<'this> {
    type Lend = &'lend str;
}

The lifetime parameter 'lend describes the lifetime of the Lend. It works by using under the hood a default generic of &'lend Self which induces an implicit reference lifetime bound 'lend: 'this, which is necessary for usage of higher-ranked trait bounds with Lend.

Next, you'll need to implement the Lender trait for your type, the lending equivalent of Iterator.

use ::lender::prelude::*;
struct StrRef<'a>(&'a str);
impl<'this, 'lend> Lending<'lend> for StrRef<'this> {
    type Lend = &'lend str;
}
impl<'this> Lender for StrRef<'this> {
    fn next<'lend>(&'lend mut self) -> Option<&'lend str> {
        Some(self.0)
    }
}

The Lend type alias can be used to avoid specifying twice the type of the lend; combined with lifetime elision, it can make your implementations more concise and less prone to errors:

use ::lender::prelude::*;
struct StrRef<'a>(&'a str);
impl<'this, 'lend> Lending<'lend> for StrRef<'this> {
    type Lend = &'lend str;
}
impl<'this> Lender for StrRef<'this> {
    fn next(&mut self) -> Option<Lend<'_, Self>> {
        Some(self.0)
    }
}

Type-inference problems

Due to the complex type dependencies and higher-kind trait bounds involved, the current Rust compiler cannot always infer the correct type of a lender and the items it returns. In general, when writing methods accepting a Lender restricting the returned item type with a type will work, as in:

use lender::*;

struct MockLender {}

impl<'lend> Lending<'lend> for MockLender {
    type Lend = &'lend str;
}

impl Lender for MockLender {
    fn next(&mut self) -> Option<Lend<'_, Self>> {
        None
    }
}

fn read_lender<L>(lender: L)
where
    L: Lender + for<'lend> Lending<'lend, Lend = &'lend str>,
{}

fn test_mock_lender(m: MockLender) {
    read_lender(m);
}

However, the following code, which restricts the returned items using a trait bound, does not compile as of Rust 1.74.1:

use lender::*;

struct MockLender {}

impl<'lend> Lending<'lend> for MockLender {
    type Lend = &'lend str;
}

impl Lender for MockLender {
    fn next(&mut self) -> Option<Lend<'_, Self>> {
        None
    }
}

fn read_lender<L>(lender: L)
where
    L: Lender,
    for<'lend> Lend<'lend, L>: AsRef<str>,
{}

fn test_mock_lender(m: MockLender) {
    read_lender(m);
}

The workaround is to use an explicit type annotation:

use lender::*;

struct MockLender {}

impl<'lend> Lending<'lend> for MockLender {
    type Lend = &'lend str;
}

impl Lender for MockLender {
    fn next(&mut self) -> Option<Lend<'_, Self>> {
        None
    }
}

fn read_lender<L>(lender: L)
where
    L: Lender,
    for<'lend> Lend<'lend, L>: AsRef<str>,
{}

fn test_mock_lender(m: MockLender) {
    read_lender::<MockLender>(m);
}

Why Not GATs?

Generic associated types (GATs) were introduced exactly having lending iterators as a use case in mind. With GATs, a lender trait could be easily defined as

pub trait Lender {
    type Lend<'lend>: where Self: 'lend;
    fn next(&mut self) -> Option<Self::Lend<'_>>;
}

This looks all nice and cozy, and you can even write a full-fledged library around it. But you will hit a wall when trying to specify trait bounds on the lend type, something that can be done only using higher-rank trait bounds:

pub trait Lender {
    type Lend<'lend>: where Self: 'lend;
    fn next(&mut self) -> Option<Self::Lend<'_>>;
}

fn read_lender<L: Lender>(lender: L) 
    where for<'lend> L::Lend<'lend>: AsRef<str> {}

Again, this will compile without problems, but as you try to use read_lender with a type implementing Lender, since the where clause specifies that that trait bound must hold for all lifetimes, that means it must be valid for 'static, and since the lender must outlive the lend, also the lender must be 'static. Thus, until there is some syntax that makes it possible to restrict the lifetime variable that appears in a higher-rank trait bound, GAT-based lending iterators are of little practical use.

Resources

Please check out the great resources below that helped me and many others learn about Rust and the lending iterator problem. Thank you to everyone!

• Sabrina Jewson's Blog for her awesome blog post on why lifetime GATs are not (yet) the solution to this problem, I highly recommend reading it.
• The awesome people on the Rust Users Forum in helping me understand the borrow checker and HRTBs better and being patient with me and other aspiring rustaceans as we try to learn more about Rust.
• Daniel Henry-Mantilla for writing lending-iterator and many other great crates and sharing their great work.
• Everyone who's contributed to Rust for making such a great language and iterator library.

Unsafe & Transmutes Beware!!!

Many patterns in lenders require polonius-emulating unsafe code, but if you see any unsafe code that can be made safe, please let me know!