RFC 2115: argument-lifetimes

lang (typesystem | lifetimes)

Summary

Eliminate the need for separately binding lifetime parameters in fn definitions and impl headers, so that instead of writing:

fn two_args<'b>(arg1: &Foo, arg2: &'b Bar) -> &'b Baz
fn two_lifetimes<'a, 'b>(arg1: &'a Foo, arg2: &'b Bar) -> &'a Quux<'b>

fn nested_lifetime<'inner>(arg: &&'inner Foo) -> &'inner Bar
fn outer_lifetime<'outer>(arg: &'outer &Foo) -> &'outer Bar

you can write:

fn two_args(arg1: &Foo, arg2: &'b Bar) -> &'b Baz
fn two_lifetimes(arg1: &'a Foo, arg2: &'b Bar) -> &'a Quux<'b>

fn nested_lifetime(arg: &&'inner Foo) -> &'inner Bar
fn outer_lifetime(arg: &'outer &Foo) -> &'outer Bar

Lint against leaving off lifetime parameters in structs (like Ref or Iter), instead nudging people to use explicit lifetimes in this case (but leveraging the other improvements to make it ergonomic to do so).

The changes, in summary, are:

This RFC does not introduce any breaking changes.

Motivation

Today's system of lifetime elision has a kind of "cliff". In cases where elision applies (because the necessary lifetimes are clear from the signature), you don't need to write anything:

fn one_arg(arg: &Foo) -> &Baz

But the moment that lifetimes need to be disambiguated, you suddenly have to introduce a named lifetime parameter and refer to it throughout, which generally requires changing three parts of the signature:

fn two_args<'a, 'b: 'a>(arg1: &'a Foo, arg2: &'b Bar) -> &'a Baz<'b>

These concerns are just a papercut for advanced Rust users, but they also present a cliff in the learning curve, one affecting the most novel and difficult to learn part of Rust. In particular, when first explaining borrowing, we can say that & means "borrowed" and that borrowed values coming out of a function must come from borrowed values in its input:

fn accessor(&self) -> &Foo

It's then not too surprising that when there are multiple input borrows, you need to disambiguate which one you're borrowing from. But to learn how to do so, you must learn not only lifetimes, but also the system of lifetime parameterization and the subtle way you use it to tie lifetimes together. In the next section, I'll show how this RFC provides a gentler learning curve around lifetimes and disambiguation.

Another point of confusion for newcomers and old hands alike is the fact that you can leave off lifetime parameters for types:

struct Iter<'a> { ... }

impl SomeType {
    // Iter here implicitly takes the lifetime from &self
    fn iter(&self) -> Iter { ... }

As detailed in the ergonomics initiative blog post, this bit of lifetime elision is considered a mistake: it makes it difficult to see at a glance that borrowing is occurring, especially if you're unfamiliar with the types involved. (The & types, by contrast, are universally known to involve borrowing.) This RFC proposes some steps to rectify this situation without regressing ergonomics significantly.

In short, this RFC seeks to improve the lifetime story for existing and new users by simultaneously improving clarity and ergonomics. In practice it should reduce the total occurrences of <, > and 'a in signatures, while increasing the overall clarity and explicitness of the lifetime system.

Guide-level explanation

Note: this is a sketch of what it might look like to teach someone lifetimes given this RFC*.

Introducing references and borrowing

Assume that ownership has already been introduced, but not yet borrowing.

While ownership is important in Rust, it's not very expressive or convenient by itself; it's quite common to want to "lend" a value to a function you're calling, without permanently relinquishing ownership of it.

Rust provides support for this kind of temporary lending through references &T, which signify a temporarily borrowed value of type T. So, for example, you can write:

fn print_vec(vec: &Vec<i32>) {
    for i in vec {
        println!("{}", i);
    }
}

and you designate lending by writing an & on the callee side:

print_vec(&my_vec)

This borrow of my_vec lasts only for the duration of the print_vec call.

Imagine more explanation here...

Functions that return borrowed data

So far we've only seen functions that consume borrowed data; what about producing it?

In general, borrowed data is always borrowed from something. And that thing must always be available for longer than the borrow is. When a function returns, its stack frame is destroyed, which means that any borrowed data it returns must come from outside of its stack frame.

The most typical case is producing new borrowed data from already-borrowed data. For example, consider a "getter" method:

struct MyStruct {
    field1: Foo,
    field2: Bar,
}

impl MyStruct {
    fn get_field1(&self) -> &Foo {
        &self.field1
    }
}

Here we're making what looks like a "fresh" borrow, it's "derived" from the existing borrow of self, and hence fine to return back to our caller; the actual MyStruct value must live outside our stack frame anyway.

Pinpointing borrows with lifetimes

For Rust to guarantee safety, it needs to track the lifetime of each loan, which says for what portion of code the loan is valid.

In particular, each & type also has an associated lifetime---but you can usually leave it off. The reason is that a lot of code works like the getter example above, where you're returning borrowed data which could only have come from the borrowed data you took in. Thus, in get_field1 the lifetime for &self and for &Foo are assumed to be the same.

Rust is conservative about leaving lifetimes off, though: if there's any ambiguity, you need to say explicitly state the relationships between the loans. So for example, the following function signature is not accepted:

fn select(data: &Data, params: &Params) -> &Item;

Rust cannot tell how long the resulting borrow of Item is valid for; it can't deduce its lifetime. Instead, you need to connect it to one or both of the input borrows:

fn select(data: &'data Data, params: &Params) -> &'data Item;
fn select(data: &'both Data, params: &'both Params) -> &'both Item;

This notation lets you name the lifetime associated with a borrow and refer to it later:

structs and lifetimes

Sometimes you need to build data types that contain borrowed data. Since those types can then be used in many contexts, you can't say in advance what the lifetime of those borrows will be. Instead, you must take it as a parameter:

struct VecIter<'vec, T> {
    vec: &'vec Vec<T>,
    index: usize,
}

Here we're defining a type for iterating over a vector, without requiring ownership of that vector. To do so, we store a borrow of the vector. But because our new VecIter struct contains borrowed data, it needs to surface that fact, and the lifetime connected with it. It does so by taking an explicit 'vec parameter for the relevant lifetime, and using it within.

When using this struct, you can apply explicitly-named lifetimes as usual:

impl<T> Vec<T> {
    fn iter(&'vec self) -> VecIter<'vec, T> { ... }
}

However, in cases like this example, we would normally be able to leave off the lifetime with &, since there's only one source of data we could be borrowing from. We can do something similar with structs:

impl<T> Vec<T> {
    fn iter(&self) -> VecIter<'_, T> { ... }
}

The '_ marker makes clear to the reader that borrowing is happening, which might not otherwise be clear.

impl blocks and lifetimes

When writing an impl block for a structure that takes a lifetime parameter, you can give that parameter a name, which you should strive to make meaningful:

impl<T> VecIter<'vec, T> { ... }

This name can then be referred to in the body:

impl<T> VecIter<'vec, T> {
    fn foo(&self) -> &'vec T { ... }
    fn bar(&self, arg: &'a Bar) -> &'a Bar { ... }
}

If the type's lifetime is not relevant, you can leave it off using '_:

impl<T> VecIter<'_, T> { ... }

Reference-level explanation

Note: these changes are designed to not require a new edition. They do expand our naming style lint, however.

Lifetimes in impl headers

When writing an impl header, you can mention lifetimes without binding them in the generics list. Any lifetimes that are not already in scope (which, today, means any lifetime whatsoever) is treated as being bound as a parameter of the impl.

Thus, where today you would write:

impl<'a> Iterator for MyIter<'a> { ... }
impl<'a, 'b> SomeTrait<'a> for SomeType<'a, 'b> { ... }

tomorrow you would write:

impl Iterator for MyIter<'iter> { ... }
impl SomeTrait<'tcx, 'gcx> for SomeType<'tcx, 'gcx> { ... }

If any lifetime names are explicitly bound, they all must be.

This change goes hand-in-hand with a convention that lifetimes introduced in impl headers (and perhaps someday, modules) should be multiple characters, i.e. "meaningful" names, to reduce the chance of collision with typical 'a usage in functions.

Lifetimes in fn signatures

When writing a fn declaration, if a lifetime appears that is not already in scope, it is taken to be a new binding, i.e. treated as a parameter to the function.

Thus, where today you would write:

fn elided(&self) -> &str
fn two_args<'b>(arg1: &Foo, arg2: &'b Bar) -> &'b Baz
fn two_lifetimes<'a, 'b: 'a>(arg1: &'a Foo, arg2: &'b Bar) -> &'a Quux<'b>

impl<'a> MyStruct<'a> {
    fn foo(&self) -> &'a str
    fn bar<'b>(&self, arg: &'b str) -> &'b str
}

fn take_fn_simple(f: fn(&Foo) -> &Bar)
fn take_fn<'a>(x: &'a u32, y: for<'b> fn(&'a u32, &'b u32, &'b u32))

tomorrow you would write:

fn elided(&self) -> &str
fn two_args(arg1: &Foo, arg2: &'arg2 Bar) -> &'arg2 Baz
fn two_lifetimes(arg1: &'arg1 Foo, arg2: &'arg2 Bar) -> &'arg1 Quux<'arg2>

impl MyStruct<'A> {
    fn foo(&self) -> &'A str
    fn bar(&self, arg: &'b str) -> &'b str
}

fn take_fn_simple(f: fn(&Foo) -> &Bar)
fn take_fn(x: &'a u32, y: for<'b> fn(&'a u32, &'b u32, &'b u32))

If any lifetime names are explicitly bound, they all must be.

For higher-ranked types (including cases like Fn syntax), elision works as it does today. However, it is an error to mention a lifetime in a higher-ranked type that hasn't been explicitly bound (either at the outer fn definition, or within an explicit for<>). These cases are extremely rare, and making them an error keeps our options open for providing an interpretation later on.

Similarly, if a fn definition is nested inside another fn definition, it is an error to mention lifetimes from that outer definition (without binding them explicitly). This is again intended for future-proofing and clarity, and is an edge case.

The wildcard lifetime

When referring to a type (other than &/&mut) that requires lifetime arguments, it is deprecated to leave off those parameters.

Instead, you can write a '_ for the parameters, rather than giving a lifetime name, which will have identical behavior to leaving them off today.

Thus, where today you would write:

fn foo(&self) -> Ref<SomeType>
fn iter(&self) -> Iter<T>

tomorrow you would write:

fn foo(&self) -> Ref<'_, SomeType>
fn iter(&self) -> Iter<'_, T>

Additional lints

Beyond the change to the style lint for impl header lifetimes, two more lints are provided:

Drawbacks

The style lint for impl headers could introduce some amount of churn. This could be mitigated by only applying that lint for lifetimes not bound in the generics list.

The fact that lifetime parameters are not bound in an out-of-band way is somewhat unusual and might be confusing---but then, so are lifetime parameters! Putting the bindings out of band buys us very little, as argued in the next section.

It's possible that the inconsistency with type parameters, which must always be bound explicitly, will be confusing. In particular, lifetime parameters for struct definitions appear side-by-side with parameter lists, but elsewhere are bound differently. However, users are virtually certain to encounter type generics prior to explicit lifetime generics, and if they try to follow the same style -- by binding lifetime parameters explicitly -- that will work just fine (but may be linted in Clippy as unnecessary).

Requiring a '_ rather than being able to leave off lifetimes altogether may be a slight decrease in ergonomics in some cases. In particular, SomeType<'_> is pretty sigil-heavy.

Cases where you could write fn foo<'a, 'b: 'a>(...) now need the 'b: 'a to be given in a where clause, which might be slightly more verbose. These are relatively rare, though, due to our type well-formedness rule.

Otherwise, it's a bit hard to see drawbacks here: nothings is made less explicit or harder to determine, since the binding structure continues to be completely unambiguous; ergonomics and, arguably, learnability both improve. And signatures become less noisy and easier to read.

Rationale and Alternatives

Core rationale

The key insight of the proposed design is that out-of-band bindings for lifetime parameters is buying us very little today:

While this might change if we ever allow modules to be parameterized by lifetimes, it won't change in any essential way: the point is that there are generally going to be very few in-scope lifetimes when writing a function signature. So the premise is that we can use naming conventions to distinguish between the impl header (or eventual module headers) and fn bindings.

Alternatively, we could instead distinguish these cases at the use-site, for example by writing outer('a) or some such to refer to the impl block bindings.

Possible extension or alternative: "backreferences"

A different approach would be refering to elided lifetimes through their parameter name, like so:

fn scramble(&self, arg: &Foo) -> &'self Bar

The idea is that each parameter that involves a single, elided lifetime will be understood to bind a lifetime using that parameter's name.

Earlier iterations of this RFC combined these "backreferences" with the rest of the proposal, but this was deemed too confusing and error-prone, and in particular harmed readability by requiring you to scan both lifetime mentions and parameter names.

We could consider only allowing "backreferences" (i.e. references to argument names), and otherwise keeping binding as-is. However, this has a few downsides:

Alternatives

We could consider using this as an opportunity to eliminate ' altogether, by tying these improvements to a new way of providing lifetimes, e.g. &ref(x) T.

The internals thread on this topic covers a wide array of syntactic options for leaving off a struct lifetime (which is '_ in this RFC), including: _, &, ref. The choice of '_ was driven by two factors: it's short, and it's self-explanatory, given our use of wildcards elsewhere. On the other hand, the syntax is pretty clunky.

As mentioned above, we could consider alternatives to the case distinction in lifetime variables, instead using something like outer('a) to refer to lifetimes from an impl header.

Unresolved questions

Copyright 2022 Nick Cameron and the RFC authors.