This is a combined conventions and library stabilization RFC. The goal is to establish a set of naming and signature conventions for std::collections.

The major components of the RFC include:

  • Removing most of the traits in collections.

  • A general proposal for solving the “equiv” problem, as well as improving MaybeOwned.

  • Patterns for overloading on by-need values and predicates.

  • Initial, forwards-compatible steps toward Iterable.

  • A coherent set of API conventions across the full variety of collections.

A big thank-you to @Gankro, who helped collect API information and worked through an initial pass of some of the proposals here.


This RFC aims to improve the design of the std::collections module in preparation for API stabilization. There are a number of problems that need to be addressed, as spelled out in the subsections below.

Collection traits

The collections module defines several traits:

  • Collection
  • Mutable
  • MutableSeq
  • Deque
  • Map, MutableMap
  • Set, MutableSet

There are several problems with the current trait design:

  • Most important: the traits do not provide iterator methods like iter. It is not possible to do so in a clean way without higher-kinded types, as the RFC explains in more detail below.

  • The split between mutable and immutable traits is not well-motivated by any of the existing collections.

  • The methods defined in these traits are somewhat anemic compared to the suite of methods provided on the concrete collections that implement them.

Divergent APIs

Despite the current collection traits, the APIs of various concrete collections has diverged; there is not a globally coherent design, and there are many inconsistencies.

One problem in particular is the lack of clear guiding principles for the API design. This RFC proposes a few along the way.

Providing slice APIs on Vec and String

The String and Vec types each provide a limited subset of the methods provides on string and vector slices, but there is not a clear reason to limit the API in this way. Today, one has to write things like some_str.as_slice().contains(...), which is not ergonomic or intuitive.

The Equiv problem

There is a more subtle problem related to slices. It’s common to use a HashMap with owned String keys, but then the natural API for things like lookup is not very usable:

fn find(&self, k: &K) -> Option<&V>

The problem is that, since K will be String, the find function requests a &String value – whereas one typically wants to work with the more flexible &str slices. In particular, using find with a literal string requires something like:

map.find(&"some literal".to_string())

which is unergonomic and requires an extra allocation just to get a borrow that, in some sense, was already available.

The current HashMap API works around this problem by providing an additional set of methods that uses a generic notion of “equivalence” of values that have different types:

pub trait Equiv<T> {
    fn equiv(&self, other: &T) -> bool;

impl Equiv<str> for String {
    fn equiv(&self, other: &str) -> bool {
        self.as_slice() == other

fn find_equiv<Q: Hash<S> + Equiv<K>>(&self, k: &Q) -> Option<&V>

There are a few downsides to this approach:

  • It requires a duplicated _equiv variant of each method taking a reference to the key. (This downside could likely be mitigated using multidispatch.)

  • Its correctness depends on equivalent values producing the same hash, which is not checked.

  • String-keyed hash maps are very common, so newcomers are likely to run headlong into the problem. First, find will fail to work in the expected way. But the signature of find_equiv is more difficult to understand than find, and it it’s not immediately obvious that it solves the problem.

  • It is the right API for HashMap, but not helpful for e.g. TreeMap, which would want an analog for Ord.

The TreeMap API currently deals with this problem in an entirely different way:

/// Returns the value for which f(key) returns Equal.
/// f is invoked with current key and guides tree navigation.
/// That means f should be aware of natural ordering of the tree.
fn find_with(&self, f: |&K| -> Ordering) -> Option<&V>

Besides being less convenient – you cannot write map.find_with("some literal") – this function navigates the tree according to an ordering that may have no relationship to the actual ordering of the tree.


Sometimes a function does not know in advance whether it will need or produce an owned copy of some data, or whether a borrow suffices. A typical example is the from_utf8_lossy function:

fn from_utf8_lossy<'a>(v: &'a [u8]) -> MaybeOwned<'a>

This function will return a string slice if the input was correctly utf8 encoded – without any allocation. But if the input has invalid utf8 characters, the function allocates a new String and inserts utf8 “replacement characters” instead. Hence, the return type is an enum:

pub enum MaybeOwned<'a> {
    Slice(&'a str),

This interface makes it possible to allocate only when necessary, but the MaybeOwned type (and connected machinery) are somewhat ad hoc – and specialized to String/str. It would be somewhat more palatable if there were a single “maybe owned” abstraction usable across a wide range of types.


A frequently-requested feature for the collections module is an Iterable trait for “values that can be iterated over”. There are two main motivations:

  • Abstraction. Today, you can write a function that takes a single Iterator, but you cannot write a function that takes a container and then iterates over it multiple times (perhaps with differing mutability levels). An Iterable trait could allow that.

  • Ergonomics. You’d be able to write

    for v in some_vec { ... }

    rather than

    for v in some_vec.iter() { ... }

    and consume_iter(some_vec) rather than consume_iter(some_vec.iter()).

Detailed design

The collections today

The concrete collections currently available in std fall into roughly three categories:

  • Sequences

    • Vec
    • String
    • Slices
    • Bitv
    • DList
    • RingBuf
    • PriorityQueue
  • Sets

    • HashSet
    • TreeSet
    • TrieSet
    • EnumSet
    • BitvSet
  • Maps

    • HashMap
    • TreeMap
    • TrieMap
    • LruCache
    • SmallIntMap

The primary goal of this RFC is to establish clean and consistent APIs that apply across each group of collections.

Before diving into the details, there is one high-level changes that should be made to these collections. The PriorityQueue collection should be renamed to BinaryHeap, following the convention that concrete collections are named according to their implementation strategy, not the abstract semantics they implement. We may eventually want PriorityQueue to be a trait that’s implemented by multiple concrete collections.

The LruCache could be renamed for a similar reason (it uses a HashMap in its implementation), However, the implementation is actually generic with respect to this underlying map, and so in the long run (with HKT and other language changes) LruCache should probably add a type parameter for the underlying map, defaulted to HashMap.

Design principles

  • Centering on Iterators. The Iterator trait is a strength of Rust’s collections library. Because so many APIs can produce iterators, adding an API that consumes one is very powerful – and conversely as well. Moreover, iterators are highly efficient, since you can chain several layers of modification without having to materialize intermediate results. Thus, whenever possible, collection APIs should strive to work with iterators.

    In particular, some existing convenience methods avoid iterators for either performance or ergonomic reasons. We should instead improve the ergonomics and performance of iterators, so that these extra convenience methods are not necessary and so that all collections can benefit.

  • Minimizing method variants. One problem with some of the current collection APIs is the proliferation of method variants. For example, HashMap include seven methods that begin with the name find! While each method has a motivation, the API as a whole can be bewildering, especially to newcomers.

    When possible, we should leverage the trait system, or find other abstractions, to reduce the need for method variants while retaining their ergonomics and power.

  • Conservatism. It is easier to add APIs than to take them away. This RFC takes a fairly conservative stance on what should be included in the collections APIs. In general, APIs should be very clearly motivated by a wide variety of use cases, either for expressiveness, performance, or ergonomics.

Removing the traits

This RFC proposes a somewhat radical step for the collections traits: rather than reform them, we should eliminate them altogether – for now.

Unlike inherent methods, which can easily be added and deprecated over time, a trait is “forever”: there are very few backwards-compatible modifications to traits. Thus, for something as fundamental as collections, it is prudent to take our time to get the traits right.

Lack of iterator methods

In particular, there is one way in which the current traits are clearly wrong: they do not provide standard methods like iter, despite these being fundamental to working with collections in Rust. Sadly, this gap is due to inexpressiveness in the language, which makes directly defining iterator methods in a trait impossible:

trait Iter {
    type A;
    type I: Iterator<&'a A>;    // what is the lifetime here?
    fn iter<'a>(&'a self) -> I; // and how to connect it to self?

The problem is that, when implementing this trait, the return type I of iter should depend on the lifetime of self. For example, the corresponding method in Vec looks like the following:

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

This means that, given a Vec<T>, there isn’t a single type Items<T> for iteration – rather, there is a family of types, one for each input lifetime. In other words, the associated type I in the Iter needs to be “higher-kinded”: not just a single type, but rather a family:

trait Iter {
    type A;
    type I<'a>: Iterator<&'a A>;
    fn iter<'a>(&self) -> I<'a>;

In this case, I is parameterized by a lifetime, but in other cases (like map) an associated type needs to be parameterized by a type.

In general, such higher-kinded types (HKTs) are a much-requested feature for Rust. But the design and implementation of higher-kinded types is, by itself, a significant investment.

HKT would also allow for parameterization over smart pointer types, which has many potential use cases in the context of collections.

Thus, the goal in this RFC is to do the best we can without HKT for now, while allowing a graceful migration if or when HKT is added.

Persistent/immutable collections

Another problem with the current collection traits is the split between immutable and mutable versions. In the long run, we will probably want to provide persistent collections (which allow non-destructive “updates” that create new collections that share most of their data with the old ones).

However, persistent collection APIs have not been thoroughly explored in Rust; it would be hasty to standardize on a set of traits until we have more experience.

Downsides of removal

There are two main downsides to removing the traits without a replacement:

  1. It becomes impossible to write code using generics over a “kind” of collection (like Map).

  2. It becomes more difficult to ensure that the collections share a common API.

For point (1), first, if the APIs are sufficiently consistent it should be possible to transition code from e.g. a TreeMap to a HashMap by changing very few lines of code. Second, generic programming is currently quite limited, given the inability to iterate. Finally, generic programming over collections is a large design space (with much precedent in C++, for example), and we should take our time and gain more experience with a variety of concrete collections before settling on a design.

For point (2), first, the current traits have failed to keep the APIs in line, as we will see below. Second, this RFC is the antidote: we establish a clear set of conventions and APIs for concrete collections up front, and stabilize on those, which should make it easy to add traits later on.

Why not leave the traits as “experimental”?

An alternative to removal would be to leave the traits intact, but marked as experimental, with the intent to radically change them later.

Such a strategy doesn’t buy much relative to removal (given the arguments above), but risks the traits becoming “de facto” stable if people begin using them en masse.

Solving the _equiv and MaybeOwned problems

The basic problem that leads to _equiv methods is that:

  • &String and &str are not the same type.
  • The &str type is more flexible and hence more widely used.
  • Code written for a generic type T that takes a reference &T will therefore not be suitable when T is instantiated with String.

A similar story plays out for &Vec<T> and &[T], and with DST and custom slice types the same problem will arise elsewhere.

The Borrow trait

This RFC proposes to use a trait, Borrow to connect borrowed and owned data in a generic fashion:

/// A trait for borrowing.
trait Borrow<Sized? B> {
    /// Immutably borrow from an owned value.
    fn borrow(&self) -> &B;

    /// Mutably borrow from an owned value.
    fn borrow_mut(&mut self) -> &mut B;

// The Sized bound means that this impl does not overlap with the impls below.
impl<T: Sized> Borrow<T> for T {
    fn borrow(a: &T) -> &T {
    fn borrow_mut(a: &mut T) -> &mut T {

impl Borrow<str> for String {
    fn borrow(s: &String) -> &str {
    fn borrow_mut(s: &mut String) -> &mut str {

impl<T> Borrow<[T]> for Vec<T> {
    fn borrow(s: &Vec<T>) -> &[T] {
    fn borrow_mut(s: &mut Vec<T>) -> &mut [T] {

(Note: thanks to @epdtry for suggesting this variation! The original proposal is listed in the Alternatives.)

A primary goal of the design is allowing a blanket impl for non-sliceable types (the first impl above). This blanket impl ensures that all new sized, cloneable types are automatically borrowable; new impls are required only for new unsized types, which are rare. The Sized bound on the initial impl means that we can freely add impls for unsized types (like str and [T]) without running afoul of coherence.

Because of the blanket impl, the Borrow trait can largely be ignored except when it is actually used – which we describe next.

Using Borrow to replace _equiv methods

With the Borrow trait in place, we can eliminate the _equiv method variants by asking map keys to be Borrow:

impl<K,V> HashMap<K,V> where K: Hash + Eq {
    fn find<Q>(&self, k: &Q) -> &V where K: Borrow<Q>, Q: Hash + Eq { ... }
    fn contains_key<Q>(&self, k: &Q) -> bool where K: Borrow<Q>, Q: Hash + Eq { ... }
    fn insert(&mut self, k: K, v: V) -> Option<V> { ... }


The benefits of this approach over _equiv are:

  • The Borrow trait captures the borrowing relationship between an owned data structure and both references to it and slices from it – once and for all. This means that it can be used anywhere we need to program generically over “borrowed” data. In particular, the single trait works for both HashMap and TreeMap, and should work for other kinds of data structures as well. It also helps generalize MaybeOwned, for similar reasons (see below.)

    A very important consequence is that the map methods using Borrow can potentially be put into a common Map trait that’s implemented by HashMap, TreeMap, and others. While we do not propose to do so now, we definitely want to do so later on.

  • When using a HashMap<String, T>, all of the basic methods like find, contains_key and insert “just work”, without forcing you to think about &String vs &str.

  • We don’t need separate _equiv variants of methods. (However, this could probably be addressed with multidispatch by providing a blanket Equiv implementation.)

On the other hand, this approach retains some of the downsides of _equiv:

  • The signature for methods like find and contains_key is more complex than their current signatures. There are two counterpoints. First, over time the Borrow trait is likely to become a well-known concept, so the signature will not appear completely alien. Second, what is perhaps more important than the signature is that, when using find on HashMap<String, T>, various method arguments just work as expected.

  • The API does not guarantee “coherence”: the Hash and Eq (or Ord, for TreeMap) implementations for the owned and borrowed keys might differ, breaking key invariants of the data structure. This is already the case with _equiv.

The Alternatives section includes a variant of Borrow that doesn’t suffer from these downsides, but has some downsides of its own.

Clone-on-write (Cow) pointers

A side-benefit of the Borrow trait is that we can give a more general version of the MaybeOwned as a “clone-on-write” smart pointer:

/// A generalization of Clone.
trait FromBorrow<Sized? B>: Borrow<B> {
    fn from_borrow(b: &B) -> Self;

/// A clone-on-write smart pointer
pub enum Cow<'a, T, B> where T: FromBorrow<B> {
    Shared(&'a B),

impl<'a, T, B> Cow<'a, T, B> where T: FromBorrow<B> {
    pub fn new(shared: &'a B) -> Cow<'a, T, B> {

    pub fn new_owned(owned: T) -> Cow<'static, T, B> {

    pub fn is_owned(&self) -> bool {
        match *self {
            Owned(_) => true,
            Shared(_) => false

    pub fn to_owned_mut(&mut self) -> &mut T {
        match *self {
            Shared(shared) => {
                *self = Owned(FromBorrow::from_borrow(shared));
            Owned(ref mut owned) => owned

    pub fn into_owned(self) -> T {
        match self {
            Shared(shared) => FromBorrow::from_borrow(shared),
            Owned(owned) => owned

impl<'a, T, B> Deref<B> for Cow<'a, T, B> where T: FromBorrow<B>  {
    fn deref(&self) -> &B {
        match *self {
            Shared(shared) => shared,
            Owned(ref owned) => owned.borrow()

impl<'a, T, B> DerefMut<B> for Cow<'a, T, B> where T: FromBorrow<B> {
    fn deref_mut(&mut self) -> &mut B {

The type Cow<'a, String, str> is roughly equivalent to today’s MaybeOwned<'a> (and Cow<'a, Vec<T>, [T]> to MaybeOwnedVector<'a, T>).

By implementing Deref and DerefMut, the Cow type acts as a smart pointer – but in particular, the mut variant actually clones if the pointed-to value is not currently owned. Hence “clone on write”.

One slight gotcha with the design is that &mut str is not very useful, while &mut String is (since it allows extending the string, for example). On the other hand, Deref and DerefMut must deref to the same underlying type, and for Deref to not require cloning, it must yield a &str value.

Thus, the Cow pointer offers a separate to_owned_mut method that yields a mutable reference to the owned version of the type.

Note that, by not using into_owned, the Cow pointer itself may be owned by some other data structure (perhaps as part of a collection) and will internally track whether an owned copy is available.

Altogether, this RFC proposes to introduce Borrow and Cow as above, and to deprecate MaybeOwned and MaybeOwnedVector. The API changes for the collections are discussed below.

IntoIterator (and Iterable)

As discussed in earlier, some form of an Iterable trait is desirable for both expressiveness and ergonomics. Unfortunately, a full treatment of Iterable requires HKT for similar reasons to the collection traits. However, it’s possible to get some of the way there in a forwards-compatible fashion.

In particular, the following two traits work fine (with associated items):

trait Iterator {
    type A;
    fn next(&mut self) -> Option<A>;

trait IntoIterator {
    type A;
    type I: Iterator<A = A>;

    fn into_iter(self) -> I;

Because IntoIterator consumes self, lifetimes are not an issue.

It’s tempting to also define a trait like:

trait Iterable<'a> {
    type A;
    type I: Iterator<&'a A>;

    fn iter(&'a self) -> I;

(along the lines of those proposed by an earlier RFC).

The problem with Iterable as defined above is that it’s locked to a particular lifetime up front. But in many cases, the needed lifetime is not even nameable in advance:

fn iter_through_rc<I>(c: Rc<I>) where I: Iterable<?> {
    // the lifetime of the borrow is established here,
    // so cannot even be named in the function signature
    for x in c.iter() {
        // ...

To make this kind of example work, you’d need to be able to say something like:

where <'a> I: Iterable<'a>

that is, that I implements Iterable for every lifetime 'a. While such a feature is feasible to add to where clauses, the HKT solution is undoubtedly cleaner.

Fortunately, we can have our cake and eat it too. This RFC proposes the IntoIterator trait above, together with the following blanket impl:

impl<I: Iterator> IntoIterator for I {
    type A = I::A;
    type I = I;
    fn into_iter(self) -> I {

which means that taking IntoIterator is strictly more flexible than taking Iterator. Note that in other languages (like Java), iterators are not iterable because the latter implies an unlimited number of iterations. But because IntoIterator consumes self, it yields only a single iteration, so all is good.

For individual collections, one can then implement IntoIterator on both the collection and borrows of it:

impl<T> IntoIterator for Vec<T> {
    type A = T;
    type I = MoveItems<T>;
    fn into_iter(self) -> MoveItems<T> { ... }

impl<'a, T> IntoIterator for &'a Vec<T> {
    type A = &'a T;
    type I = Items<'a, T>;
    fn into_iter(self) -> Items<'a, T> { ... }

impl<'a, T> IntoIterator for &'a mut Vec<T> {
    type A = &'a mut T;
    type I = ItemsMut<'a, T>;
    fn into_iter(self) -> ItemsMut<'a, T> { ... }

If/when HKT is added later on, we can add an Iterable trait and a blanket impl like the following:

// the HKT version
trait Iterable {
    type A;
    type I<'a>: Iterator<&'a A>;
    fn iter<'a>(&'a self) -> I<'a>;

impl<'a, C: Iterable> IntoIterator for &'a C {
    type A = &'a C::A;
    type I = C::I<'a>;
    fn into_iter(self) -> I {

This gives a clean migration path: once Vec implements Iterable, it can drop the IntoIterator impls for borrowed vectors, since they will be covered by the blanket implementation. No code should break.

Likewise, if we add a feature like the “universal” where clause mentioned above, it can be used to deal with embedded lifetimes as in the iter_through_rc example; and if the HKT version of Iterable is later added, thanks to the suggested blanket impl for IntoIterator that where clause could be changed to use Iterable instead, again without breakage.

Benefits of IntoIterator

What do we gain by incorporating IntoIterator today?

This RFC proposes that for loops should use IntoIterator rather than Iterator. With the blanket impl of IntoIterator for any Iterator, this is not a breaking change. However, given the IntoIterator impls for Vec above, we would be able to write:

let v: Vec<Foo> = ...

for x in &v { ... }     // iterate over &Foo
for x in &mut v { ... } // iterate over &mut Foo
for x in v { ... }      // iterate over Foo

Similarly, methods that currently take slices or iterators can be changed to take IntoIterator instead, immediately becoming more general and more ergonomic.

In general, IntoIterator will allow us to move toward more Iterator-centric APIs today, in a way that’s compatible with HKT tomorrow.

Additional methods

Another typical desire for an Iterable trait is to offer defaulted versions of methods that basically re-export iterator methods on containers (see the earlier RFC). Usually these methods would go through a reference iterator (i.e. the iter method) rather than a moving iterator.

It is possible to add such methods using the design proposed above, but there are some drawbacks. For example, should Vec::map produce an iterator, or a new vector? It would be possible to do the latter generically, but only with HKT. (See this discussion.)

This RFC only proposes to add the following method via IntoIterator, as a convenience for a common pattern:

trait IterCloned {
    type A;
    type I: Iterator<A>;
    fn iter_cloned(self) -> I;

impl<'a, T, I: IntoIterator> IterCloned for I where I::A = &'a T {
    type A = T;
    type I = ClonedItems<I>;
    fn into_iter(self) -> I { ... }

(The iter_cloned method will help reduce the number of method variants in general for collections, as we will see below).

We will leave to later RFCs the incorporation of additional methods. Notice, in particular, that such methods can wait until we introduce an Iterable trait via HKT without breaking backwards compatibility.

Minimizing variants: ByNeed and Predicate traits

There are several kinds of methods that, in their most general form take closures, but for which convenience variants taking simpler data are common:

  • Taking values by need. For example, consider the unwrap_or and unwrap_or_else methods in Option:

    fn unwrap_or(self, def: T) -> T
    fn unwrap_or_else(self, f: || -> T) -> T

    The unwrap_or_else method is the most general: it invokes the closure to compute a default value only when self is None. When the default value is expensive to compute, this by-need approach helps. But often the default value is cheap, and closures are somewhat annoying to write, so unwrap_or provides a convenience wrapper.

  • Taking predicates. For example, a method like contains often shows up (inconsistently!) in two variants:

    fn contains(&self, elem: &T) -> bool; // where T: PartialEq
    fn contains_fn(&self, pred: |&T| -> bool) -> bool;

    Again, the contains_fn version is the more general, but it’s convenient to provide a specialized variant when the element type can be compared for equality, to avoid writing explicit closures.

As it turns out, with multidispatch) it is possible to use a trait to express these variants through overloading:

trait ByNeed<T> {
    fn compute(self) -> T;

impl<T> ByNeed<T> for T {
    fn compute(self) -> T {

// Due to multidispatch, this impl does NOT overlap with the above one
impl<T> ByNeed<T> for || -> T {
    fn compute(self) -> T {

impl<T> Option<T> {
    fn unwrap_or<U>(self, def: U) where U: ByNeed<T> { ... }
trait Predicate<T> {
    fn check(&self, &T) -> bool;

impl<T: Eq> Predicate<T> for &T {
    fn check(&self, t: &T) -> bool {
        *self == t

impl<T> Predicate<T> for |&T| -> bool {
    fn check(&self, t: &T) -> bool {

impl<T> Vec<T> {
    fn contains<P>(&self, pred: P) where P: Predicate<T> { ... }

Since these two patterns are particularly common throughout std, this RFC proposes adding both of the above traits, and using them to cut down on the number of method variants.

In particular, some methods on string slices currently work with CharEq, which is similar to Predicate<char>:

pub trait CharEq {
    fn matches(&mut self, char) -> bool;
    fn only_ascii(&self) -> bool;

The difference is the only_ascii method, which is used to optimize certain operations when the predicate only holds for characters in the ASCII range.

To keep these optimizations intact while connecting to Predicate, this RFC proposes the following restructuring of CharEq:

pub trait CharPredicate: Predicate<char> {
    fn only_ascii(&self) -> bool {

Why not leverage unboxed closures?

A natural question is: why not use the traits for unboxed closures to achieve a similar effect? For example, you could imagine writing a blanket impl for Fn(&T) -> bool for any T: PartialEq, which would allow PartialEq values to be used anywhere a predicate-like closure was requested.

The problem is that these blanket impls will often conflict. In particular, any type T could implement Fn() -> T, and that single blanket impl would preclude any others (at least, assuming that unboxed closure traits treat the argument and return types as associated (output) types).

In addition, the explicit use of traits like Predicate makes the intended semantics more clear, and the overloading less surprising.

The APIs

Now we’ll delve into the detailed APIs for the various concrete collections. These APIs will often be given in tabular form, grouping together common APIs across multiple collections. When writing these function signatures:

  • We will assume a type parameter T for Vec, BinaryHeap, DList and RingBuf; we will also use this parameter for APIs on String, where it should be understood as char.

  • We will assume type parameters K: Borrow and V for HashMap and TreeMap; for TrieMap and SmallIntMap the K is assumed to be uint

  • We will assume a type parameter K: Borrow for HashSet and TreeSet; for BitvSet it is assumed to be uint.

We will begin by outlining the most widespread APIs in tables, making it easy to compare names and signatures across different kinds of collections. Then we will focus on some APIs specific to particular classes of collections – e.g. sets and maps. Finally, we will briefly discuss APIs that are specific to a single concrete collection.


All of the collections should support a static function:

fn new() -> Self

that creates an empty version of the collection; the constructor may take arguments needed to set up the collection, e.g. the capacity for LruCache.

Several collections also support separate constructors for providing capacities in advance; these are discussed below.

The FromIterator trait

All of the collections should implement the FromIterator trait:

pub trait FromIterator {
    type A:
    fn from_iter<T>(T) -> Self where T: IntoIterator<A = A>;

Note that this varies from today’s FromIterator by consuming an IntoIterator rather than Iterator. As explained above, this choice is strictly more general and will not break any existing code.

This constructor initializes the collection with the contents of the iterator. For maps, the iterator is over key/value pairs, and the semantics is equivalent to inserting those pairs in order; if keys are repeated, the last value is the one left in the map.


The table below gives methods for inserting items into various concrete collections:

fn push(&mut self, T)Vec, BinaryHeap, String
fn push_front(&mut self, T)DList, RingBuf
fn push_back(&mut self, T)DList, RingBuf
fn insert(&mut self, uint, T)Vec, RingBuf, String
fn insert(&mut self, K::Owned) -> boolHashSet, TreeSet, TrieSet, BitvSet
fn insert(&mut self, K::Owned, V) -> Option<V>HashMap, TreeMap, TrieMap, SmallIntMap
fn append(&mut self, Self)DList
fn prepend(&mut self, Self)DList

There are a few changes here from the current state of affairs:

  • The DList and RingBuf data structures no longer provide push, but rather push_front and push_back. This change is based on (1) viewing them as deques and (2) not giving priority to the “front” or the “back”.

  • The insert method on maps returns the value previously associated with the key, if any. Previously, this functionality was provided by a swap method, which has been dropped (consolidating needless method variants.)

Aside from these changes, a number of insertion methods will be deprecated (e.g. the append and append_one methods on Vec). These are discussed further in the section on “specialized operations” below.

The Extend trait (was: Extendable)

In addition to the standard insertion operations above, all collections will implement the Extend trait. This trait was previously called Extendable, but in general we prefer to avoid -able suffixes and instead name the trait using a verb (or, especially, the key method offered by the trait.)

The Extend trait allows data from an arbitrary iterator to be inserted into a collection, and will be defined as follows:

pub trait Extend: FromIterator {
    fn extend<T>(&mut self, T) where T: IntoIterator<A = Self::A>;

As with FromIterator, this trait has been modified to take an IntoIterator value.


The table below gives methods for removing items into various concrete collections:

fn clear(&mut self)all
fn pop(&mut self) -> Option<T>Vec, BinaryHeap, String
fn pop_front(&mut self) -> Option<T>DList, RingBuf
fn pop_back(&mut self) -> Option<T>DList, RingBuf
fn remove(&mut self, uint) -> Option<T>Vec, RingBuf, String
fn remove(&mut self, &K) -> boolHashSet, TreeSet, TrieSet, BitvSet
fn remove(&mut self, &K) -> Option<V>HashMap, TreeMap, TrieMap, SmallIntMap
fn truncate(&mut self, len: uint)Vec, String, Bitv, DList, RingBuf
fn retain<P>(&mut self, f: P) where P: Predicate<T>Vec, DList, RingBuf
fn dedup(&mut self)Vec, DList, RingBuf where T: PartialEq

As with the insertion methods, there are some differences from today’s API:

  • The DList and RingBuf data structures no longer provide pop, but rather pop_front and pop_back – similarly to the push methods.

  • The remove method on maps returns the value previously associated with the key, if any. Previously, this functionality was provided by a separate pop method, which has been dropped (consolidating needless method variants.)

  • The retain method takes a Predicate.

  • The truncate, retain and dedup methods are offered more widely.

Again, some of the more specialized methods are not discussed here; see “specialized operations” below.


The next table gives methods for inspection and mutation of existing items in collections:

fn len(&self) -> uintall
fn is_empty(&self) -> boolall
fn get(&self, uint) -> Option<&T>[T], Vec, RingBuf
fn get_mut(&mut self, uint) -> Option<&mut T>[T], Vec, RingBuf
fn get(&self, &K) -> Option<&V>HashMap, TreeMap, TrieMap, SmallIntMap
fn get_mut(&mut self, &K) -> Option<&mut V>HashMap, TreeMap, TrieMap, SmallIntMap
fn contains<P>(&self, P) where P: Predicate<T>[T], str, Vec, String, DList, RingBuf, BinaryHeap
fn contains(&self, &K) -> boolHashSet, TreeSet, TrieSet, EnumSet
fn contains_key(&self, &K) -> boolHashMap, TreeMap, TrieMap, SmallIntMap

The biggest changes from the current APIs are:

  • The find and find_mut methods have been renamed to get and get_mut. Further, all get methods return Option values and do not invoke fail!. This is part of a general convention described in the next section (on the Index traits).

  • The contains method is offered more widely.

  • There is no longer an equivalent of find_copy (which should be called find_clone). Instead, we propose to add the following method to the Option<&'a T> type where T: Clone:

    fn cloned(self) -> Option<T> {|x| x.clone())

    so that some_map.find_copy(key) will instead be written some_map.find(key).cloned(). This method chain is slightly longer, but is more clear and allows us to drop the _copy variants. Moreover, all users of Option benefit from the new convenience method.

The Index trait

The Index and IndexMut traits provide indexing notation like v[0]:

pub trait Index {
    type Index;
    type Result;
    fn index(&'a self, index: &Index) -> &'a Result;

pub trait IndexMut {
    type Index;
    type Result;
    fn index_mut(&'a mut self, index: &Index) -> &'a mut Result;

These traits will be implemented for: [T], Vec, RingBuf, HashMap, TreeMap, TrieMap, SmallIntMap.

As a general convention, implementation of the Index traits will fail the task if the index is invalid (out of bounds or key not found); they will therefore return direct references to values. Any collection implementing Index (resp. IndexMut) should also provide a get method (resp. get_mut) as a non-failing variant that returns an Option value.

This allows us to keep indexing notation maximally concise, while still providing convenient non-failing variants (which can be used to provide a check for index validity).


Every collection should provide the standard trio of iteration methods:

fn iter(&'a self) -> Items<'a>;
fn iter_mut(&'a mut self) -> ItemsMut<'a>;
fn into_iter(self) -> ItemsMove;

and in particular implement the IntoIterator trait on both the collection type and on (mutable) references to it.

Capacity management

many of the collections have some notion of “capacity”, which may be fixed, grow explicitly, or grow implicitly:

  • No capacity/fixed capacity: DList, TreeMap, TreeSet, TrieMap, TrieSet, slices, EnumSet
  • Explicit growth: LruCache
  • Implicit growth: Vec, RingBuf, HashMap, HashSet, BitvSet, BinaryHeap

Growable collections provide functions for capacity management, as follows.

Explicit growth

For explicitly-grown collections, the normal constructor (new) takes a capacity argument. Capacity can later be inspected or updated as follows:

fn capacity(&self) -> uint
fn set_capacity(&mut self, capacity: uint)

(Note, this renames LruCache::change_capacity to set_capacity, the prevailing style for setter method.)

Implicit growth

For implicitly-grown collections, the normal constructor (new) does not take a capacity, but there is an explicit with_capacity constructor, along with other functions to work with the capacity later on:

fn with_capacity(uint) -> Self
fn capacity(&self) -> uint
fn reserve(&mut self, additional: uint)
fn reserve_exact(&mut self, additional: uint)
fn shrink_to_fit(&mut self)

There are some important changes from the current APIs:

  • The reserve and reserve_exact methods now take as an argument the extra space to reserve, rather than the final desired capacity, as this usage is vastly more common. The reserve function may grow the capacity by a larger amount than requested, to ensure amortization, while reserve_exact will reserve exactly the requested additional capacity. The reserve_additional methods are deprecated.

  • The with_capacity constructor does not take any additional arguments, for uniformity with new. This change affects Bitv in particular.

Bounded iterators

Some of the maps (e.g. TreeMap) currently offer specialized iterators over their entries starting at a given key (called lower_bound) and above a given key (called upper_bound), along with _mut variants. While the functionality is worthwhile, the names are not very clear, so this RFC proposes the following replacement API (thanks to @Gankro for the suggestion):

Bound<T> {
    /// An inclusive bound

    /// An exclusive bound


/// Creates a double-ended iterator over a sub-range of the collection's items,
/// starting at min, and ending at max. If min is `Unbounded`, then it will
/// be treated as "negative infinity", and if max is `Unbounded`, then it will
/// be treated as "positive infinity". Thus range(Unbounded, Unbounded) will yield
/// the whole collection.
fn range(&self, min: Bound<&T>, max: Bound<&T>) -> RangedItems<'a, T>;

fn range_mut(&self, min: Bound<&T>, max: Bound<&T>) -> RangedItemsMut<'a, T>;

These iterators should be provided for any maps over ordered keys (TreeMap, TrieMap and SmallIntMap).

In addition, analogous methods should be provided for sets over ordered keys (TreeSet, TrieSet, BitvSet).

Set operations


All sets should offer the following methods, as they do today:

fn is_disjoint(&self, other: &Self) -> bool;
fn is_subset(&self, other: &Self) -> bool;
fn is_superset(&self, other: &Self) -> bool;


Sets can also be combined using the standard operations – union, intersection, difference and symmetric difference (exclusive or). Today’s APIs for doing so look like this:

fn union<'a>(&'a self, other: &'a Self) -> I;
fn intersection<'a>(&'a self, other: &'a Self) -> I;
fn difference<'a>(&'a self, other: &'a Self) -> I;
fn symmetric_difference<'a>(&'a self, other: &'a Self) -> I;

where the I type is an iterator over keys that varies by concrete set. Working with these iterators avoids materializing intermediate sets when they’re not needed; the collect method can be used to create sets when they are. This RFC proposes to keep these names intact, following the RFC on iterator conventions.

Sets should also implement the BitOr, BitAnd, BitXor and Sub traits from std::ops, allowing overloaded notation |, &, |^ and - to be used with sets. These are equivalent to invoking the corresponding iter_ method and then calling collect, but for some sets (notably BitvSet) a more efficient direct implementation is possible.

Unfortunately, we do not yet have a set of traits corresponding to operations |=, &=, etc, but again in some cases doing the update in place may be more efficient. Right now, BitvSet is the only concrete set offering such operations:

fn union_with(&mut self, other: &BitvSet)
fn intersect_with(&mut self, other: &BitvSet)
fn difference_with(&mut self, other: &BitvSet)
fn symmetric_difference_with(&mut self, other: &BitvSet)

This RFC punts on the question of naming here: it does not propose a new set of names. Ideally, we would add operations like |= in a separate RFC, and use those conventionally for sets. If not, we will choose fallback names during the stabilization of BitvSet.

Map operations

Combined methods

The HashMap type currently provides a somewhat bewildering set of find/insert variants:

fn find_or_insert(&mut self, k: K, v: V) -> &mut V
fn find_or_insert_with<'a>(&'a mut self, k: K, f: |&K| -> V) -> &'a mut V
fn insert_or_update_with<'a>(&'a mut self, k: K, v: V, f: |&K, &mut V|) -> &'a mut V
fn find_with_or_insert_with<'a, A>(&'a mut self, k: K, a: A, found: |&K, &mut V, A|, not_found: |&K, A| -> V) -> &'a mut V

These methods are used to couple together lookup and insertion/update operations, thereby avoiding an extra lookup step. However, the current set of method variants seems overly complex.

There is another RFC already in the queue addressing this problem in a very nice way, and this RFC defers to that one

Key and value iterators

In addition to the standard iterators, maps should provide by-reference convenience iterators over keys and values:

fn keys(&'a self) -> Keys<'a, K>
fn values(&'a self) -> Values<'a, V>

While these iterators are easy to define in terms of the main iter method, they are used often enough to warrant including convenience methods.

Specialized operations

Many concrete collections offer specialized operations beyond the ones given above. These will largely be addressed through the API stabilization process (which focuses on local API issues, as opposed to general conventions), but a few broad points are addressed below.

Relating Vec and String to slices

One goal of this RFC is to supply all of the methods on (mutable) slices on Vec and String. There are a few ways to achieve this, so concretely the proposal is for Vec<T> to implement Deref<[T]> and DerefMut<[T]>, and String to implement Deref<str>. This will automatically allow all slice methods to be invoked from vectors and strings, and will allow writing &*v rather than v.as_slice().

In this scheme, Vec and String are really “smart pointers” around the corresponding slice types. While counterintuitive at first, this perspective actually makes a fair amount of sense, especially with DST.

(Initially, it was unclear whether this strategy would play well with method resolution, but the planned resolution rules should work fine.)

String API

One of the key difficulties with the String API is that strings use utf8 encoding, and some operations are only efficient when working at the byte level (and thus taking this encoding into account).

As a general principle, we will move the API toward the following convention: index-related operations always work in terms of bytes, other operations deal with chars by default (but can have suffixed variants for working at other granularities when appropriate.)


The DList type offers a number of specialized methods:

swap_remove, insert_when, insert_ordered, merge, rotate_forward and rotate_backward

Prior to stabilizing the DList API, we will attempt to simplify its API surface, possibly by using idea from the collection views RFC.

Minimizing method variants via iterators

Partitioning via FromIterator

One place we can move toward iterators is functions like partition and partitioned on vectors and slices:

// on Vec<T>
fn partition(self, f: |&T| -> bool) -> (Vec<T>, Vec<T>);

// on [T] where T: Clone
fn partitioned(&self, f: |&T| -> bool) -> (Vec<T>, Vec<T>);

These two functions transform a vector/slice into a pair of vectors, based on a “partitioning” function that says which of the two vectors to place elements into. The partition variant works by moving elements of the vector, while partitioned clones elements.

There are a few unfortunate aspects of an API like this one:

  • It’s specific to vectors/slices, although in principle both the source and target containers could be more general.

  • The fact that two variants have to be exposed, for owned versus clones, is somewhat unfortunate.

This RFC proposes the following alternative design:

pub enum Either<T, U> {
    pub Left(T),
    pub Right(U),

impl<A, B> FromIterator for (A, B) where A: Extend, B: Extend {
    fn from_iter<I>(mut iter: I) -> (A, B) where I: IntoIterator<Either<T, U>> {
        let mut left: A = FromIterator::from_iter(None::<T>);
        let mut right: B = FromIterator::from_iter(None::<U>);

        for item in iter {
            match item {
                Left(t) => left.extend(Some(t)),
                Right(u) => right.extend(Some(u)),

        (left, right)

trait Iterator {
    fn partition(self, |&A| -> bool) -> Partitioned<A> { ... }

// where Partitioned<A>: Iterator<A = Either<A, A>>

This design drastically generalizes the partitioning functionality, allowing it be used with arbitrary collections and iterators, while removing the by-reference and by-value distinction.

Using this design, you have:

// The following two lines are equivalent:
let (u, w) = v.partition(f);
let (u, w): (Vec<T>, Vec<T>) = v.into_iter().partition(f).collect();

// The following two lines are equivalent:
let (u, w) = v.as_slice().partitioned(f);
let (u, w): (Vec<T>, Vec<T>) = v.iter_cloned().partition(f).collect();

There is some extra verbosity, mainly due to the type annotations for collect, but the API is much more flexible, since the partitioned data can now be collected into other collections (or even differing collections). In addition, partitioning is supported for any iterator.

Removing methods like from_elem, from_fn, grow, and grow_fn

Vectors and some other collections offer constructors and growth functions like the following:

fn from_elem(length: uint, value: T) -> Vec<T>
fn from_fn(length: uint, op: |uint| -> T) -> Vec<T>
fn grow(&mut self, n: uint, value: &T)
fn grow_fn(&mut self, n: uint, f: |uint| -> T)

These extra variants can easily be dropped in favor of iterators, and this RFC proposes to do so.

The iter module already contains a Repeat iterator; this RFC proposes to add a free function repeat to iter as a convenience for iter::Repeat::new.

With that in place, we have:

// Equivalent:
let v = Vec::from_elem(n, a);
let v = Vec::from_iter(repeat(a).take(n));

// Equivalent:
let v = Vec::from_fn(n, f);
let v = Vec::from_iter(range(0, n).map(f));

// Equivalent:
v.grow(n, a);

// Equivalent:
v.grow_fn(n, f);
v.extend(range(0, n).map(f));

While these replacements are slightly longer, an important aspect of ergonomics is memorability: by placing greater emphasis on iterators, programmers will quickly learn the iterator APIs and have those at their fingertips, while remembering ad hoc method variants like grow_fn is more difficult.

Long-term: removing push_all and push_all_move

The push_all and push_all_move methods on vectors are yet more API variants that could, in principle, go through iterators:

// The following are *semantically* equivalent

// The following are *semantically* equivalent

However, currently the push_all and push_all_move methods can rely on the exact size of the container being pushed, in order to elide bounds checks. We do not currently have a way to “trust” methods like len on iterators to elide bounds checks. A separate RFC will introduce the notion of a “trusted” method which should support such optimization and allow us to deprecate the push_all and push_all_move variants. (This is unlikely to happen before 1.0, so the methods will probably still be included with “experimental” status, and likely with different names.)


Borrow and the Equiv problem

Variants of Borrow

The original version of Borrow was somewhat more subtle:

/// A trait for borrowing.
/// If `T: Borrow` then `&T` represents data borrowed from `T::Owned`.
trait Borrow for Sized? {
    /// The type being borrowed from.
    type Owned;

    /// Immutably borrow from an owned value.
    fn borrow(&Owned) -> &Self;

    /// Mutably borrow from an owned value.
    fn borrow_mut(&mut Owned) -> &mut Self;

trait ToOwned: Borrow {
    /// Produce a new owned value, usually by cloning.
    fn to_owned(&self) -> Owned;

impl<A: Sized> Borrow for A {
    type Owned = A;
    fn borrow(a: &A) -> &A {
    fn borrow_mut(a: &mut A) -> &mut A {

impl<A: Clone> ToOwned for A {
    fn to_owned(&self) -> A {

impl Borrow for str {
    type Owned = String;
    fn borrow(s: &String) -> &str {
    fn borrow_mut(s: &mut String) -> &mut str {

impl ToOwned for str {
    fn to_owned(&self) -> String {

impl<T> Borrow for [T] {
    type Owned = Vec<T>;
    fn borrow(s: &Vec<T>) -> &[T] {
    fn borrow_mut(s: &mut Vec<T>) -> &mut [T] {

impl<T> ToOwned for [T] {
    fn to_owned(&self) -> Vec<T> {

impl<K,V> HashMap<K,V> where K: Borrow + Hash + Eq {
    fn find(&self, k: &K) -> &V { ... }
    fn insert(&mut self, k: K::Owned, v: V) -> Option<V> { ... }

pub enum Cow<'a, T> where T: ToOwned {
    Shared(&'a T),

This approach ties Borrow directly to the borrowed data, and uses an associated type to uniquely determine the corresponding owned data type.

For string keys, we would use HashMap<str, V>. Then, the find method would take an &str key argument, while insert would take an owned String. On the other hand, for some other type Foo a HashMap<Foo, V> would take &Foo for find and Foo for insert. (More discussion on the choice of ownership is given in the alternatives section.

Benefits of this alternative:

  • Unlike the current _equiv or find_with methods, or the proposal in the RFC, this approach guarantees coherence about hashing or ordering. For example, HashMap above requires that K (the borrowed key type) is Hash, and will produce hashes from owned keys by first borrowing from them.

  • Unlike the proposal in this RFC, the signature of the methods for maps is very simple – essentially the same as the current find, insert, etc.

  • Like the proposal in this RFC, there is only a single Borrow trait, so it would be possible to standardize on a Map trait later on and include these APIs. The trait could be made somewhat simpler with this alternative form of Borrow, but can be provided in either case; see these comments for details.

  • The Cow data type is simpler than in the RFC’s proposal, since it does not need a type parameter for the owned data.

Drawbacks of this alternative:

  • It’s quite subtle that you want to use HashMap<str, T> rather than HashMap<String, T>. That is, if you try to use a map in the “obvious way” you will not be able to use string slices for lookup, which is part of what this RFC is trying to achieve. The same applies to Cow.

  • The design is somewhat less flexible than the one in the RFC, because (1) there is a fixed choice of owned type corresponding to each borrowed type and (2) you cannot use multiple borrow types for lookups at different types (e.g. using &String sometimes and &str other times). On the other hand, these restrictions guarantee coherence of hashing/equality/comparison.

  • This version of Borrow, mapping from borrowed to owned data, is somewhat less intuitive.

On the balance, the approach proposed in the RFC seems better, because using the map APIs in the obvious ways works by default.

The HashMapKey trait and friends

An earlier proposal for solving the _equiv problem was given in the associated items RFC):

trait HashMapKey : Clone + Hash + Eq {
    type Query: Hash = Self;
    fn compare(&self, other: &Query) -> bool { self == other }
    fn query_to_key(q: &Query) -> Self { q.clone() };

impl HashMapKey for String {
    type Query = str;
    fn compare(&self, other: &str) -> bool {
        self.as_slice() == other
    fn query_to_key(q: &str) -> String {

impl<K,V> HashMap<K,V> where K: HashMapKey {
    fn find(&self, q: &K::Query) -> &V { ... }

This solution has several drawbacks, however:

  • It requires a separate trait for different kinds of maps – one for HashMap, one for TreeMap, etc.

  • It requires that a trait be implemented on a given key without providing a blanket implementation. Since you also need different traits for different maps, it’s easy to imagine cases where a out-of-crate type you want to use as a key doesn’t implement the key trait, forcing you to newtype.

  • It doesn’t help with the MaybeOwned problem.

Daniel Micay’s hack

@strcat has a PR that makes it possible to, for example, coerce a &str to an &String value.

This provides some help for the _equiv problem, since the _equiv methods could potentially be dropped. However, there are a few downsides:

  • Using a map with string keys is still a bit more verbose:

    map.find("some static string".as_string()) // with the hack
    map.find("some static string")             // with this RFC
  • The solution is specialized to strings and vectors, and does not necessarily support user-defined unsized types or slices.

  • It doesn’t help with the MaybeOwned problem.

  • It exposes some representation interplay between slices and references to owned values, which we may not want to commit to or reveal.

For IntoIterator

Handling of for loops

The fact that for x in v moves elements from v, while for x in v.iter() yields references, may be a bit surprising. On the other hand, moving is the default almost everywhere in Rust, and with the proposed approach you get to use & and &mut to easily select other forms of iteration.

(See @huon’s comment for additional drawbacks.)

Unfortunately, it’s a bit tricky to make for use by-ref iterators instead. The problem is that an iterator is IntoIterator, but it is not Iterable (or whatever we call the by-reference trait). Why? Because IntoIterator gives you an iterator that can be used only once, while Iterable allows you to ask for iterators repeatedly.

If for demanded an Iterable, then for x in v.iter() and for x in v.iter_mut() would cease to work – we’d have to find some other approach. It might be doable, but it’s not obvious how to do it.

Input versus output type parameters

An important aspect of the IntoIterator design is that the element type is an associated type, not an input type.

This is a tradeoff:

  • Making it an associated type means that the for examples work, because the type of Self uniquely determines the element type for iteration, aiding type inference.

  • Making it an input type would forgo those benefits, but would allow some additional flexibility. For example, you could implement IntoIterator<A> for an iterator on &A when A is cloned, therefore implicitly cloning as needed to make the ownership work out (and obviating the need for iter_cloned). However, we have generally kept away from this kind of implicit magic, especially when it can involve hidden costs like cloning, so the more explicit design given in this RFC seems best.


Design tradeoffs were discussed inline.

Unresolved questions

Unresolved conventions/APIs

As mentioned above, this RFC does not resolve the question of what to call set operations that update the set in place.

It likewise does not settle the APIs that appear in only single concrete collections. These will largely be handled through the API stabilization process, unless radical changes are proposed.

Finally, additional methods provided via the IntoIterator API are left for future consideration.


Using the Borrow trait, it might be possible to safely add a coercion for auto-slicing:

  If T: Borrow:
    coerce  &'a T::Owned      to  &'a T
    coerce  &'a mut T::Owned  to  &'a mut T

For sized types, this coercion is forced to be trivial, so the only time it would involve running user code is for unsized values.

A general story about such coercions will be left to a follow-up RFC.