collect_failable

A set of traits for collecting values into containers that must uphold invariants during construction or extension. These traits let you propagate structured errors instead of panicking or silently discarding data. Examples include preventing duplicate keys in a HashMap or respecting capacity limits in types like ArrayVec.

Traits

This crate provides several complementary traits for failable collection:

• TryFromIterator – failably build a container from an IntoIterator.
• TryCollectEx – failably collect an IntoIterator into a container.
• TryExtend and TryExtendSafe – failably extend a container with an IntoIterator, with different error guarantees.
• TryExtendOne – failable extend a container with a single item.
• TryUnzip – failably unzip an IntoIterator of pairs into two containers (requires feature tuple, enabled by default).
• Capacity - expose capacity size hints for collection types (e.g., ArrayVec).

Additionally, several implementations are provided for common and popular containers. See the implementations section for more details.

Installation

It's on crates.io.

Features

This crate provides the following optional features:

• alloc (default) – Enables support for allocation-dependent types (BTreeMap, BTreeSet, Result, Rc, Vec, Box). Required for most functionality. When disabled, only the core traits are available.
• std (default) – Enables standard library support, including HashMap and HashSet implementations. When disabled, the crate operates in no_std mode with alloc.
• unsafe (default) – Enables TryFromIterator implementations for arrays using unsafe code.
• tuple (default) – Enables tuple extension (TryExtend for tuples) and TryUnzip trait, requiring a public dependency on the either crate (re-exported as collect_failable::Either).
• arrayvec – Enables TryFromIterator and TryExtend implementations for ArrayVec.
• hashbrown – Enables TryFromIterator and TryExtend implementations for hashbrown::HashMap and hashbrown::HashSet.
• indexmap – Enables TryFromIterator and TryExtend implementations for indexmap::IndexMap and indexmap::IndexSet.

No-std Support

This crate supports no_std environments when the std feature is disabled. The alloc feature provides allocation-dependent functionality (BTreeMap, BTreeSet, etc.) without requiring the full standard library.

Note: HashMap and HashSet require the std feature because they depend on the standard library's default hasher. For no_std environments, consider BTreeMap/BTreeSet (with alloc) or hashbrown/indexmap (with their respective features).

Usage

TryFromIterator and TryCollectEx

Construct a container from an iterator, with errors for invalid input. This behaves like FromIterator but returns Result<Self, E> instead of panicking or ignoring failures.

use std::collections::BTreeMap;
use collect_failable::TryFromIterator;

// try_from_iter is the core method - works on any TryFromIterator implementor
let map = BTreeMap::try_from_iter([(1, "a"), (2, "b")]).expect("no duplicates");
assert_eq!(map, BTreeMap::from([(1, "a"), (2, "b")]), "should contain all values");

// duplicate keys produce an error containing the colliding item
let err = BTreeMap::try_from_iter([(1, "a"), (2, "b"), (1, "c")]).expect_err("duplicate key");
assert_eq!(err.error.item, (1, "c"), "should contain the colliding item");

// errors contain all data needed to reconstruct the consumed iterator
// order is: rejected item, then collected items, then remaining iterator
let recovered: Vec<_> = err.into_iter().collect();
assert_eq!(recovered, [(1, "c"), (1, "a"), (2, "b")]);

TryCollectEx provides a more convenient alternative, similar to collect

use std::collections::HashMap;
use collect_failable::TryCollectEx;

let map: HashMap<_, _> = [(1, "a"), (2, "b")].into_iter().try_collect_ex().unwrap();
assert_eq!(map, HashMap::from([(1, "a"), (2, "b")]));

TryExtend and TryExtendSafe

Extend an existing container with items that may violate its invariants. Two different trait exposes two styles of error behavior:

• TryExtendSafe – strong guarantee on an error, the container must remain unchanged.
• TryExtend – basic guarantee the container may have partially ingested items, but must remain valid.

Use TryExtendSafe if you must avoid mutation on failure; otherwise, prefer the faster TryExtend.

use std::collections::HashMap;
use collect_failable::TryExtendSafe;

let mut map = HashMap::new();
map.try_extend_safe([(1, 2), (2, 3)]).expect("should be Ok");
assert_eq!(map, HashMap::from([(1, 2), (2, 3)]));

// on a failure, the container is not modified
map.try_extend_safe([(1, 3)]).expect_err("should be Err");
assert_eq!(map, HashMap::from([(1, 2), (2, 3)]));

TryExtendOne

Extend a collection with a single item. This trait always provides a strong guarantee: on failure, the collection remains unchanged. Implemented as a seperate trait with no default implementation due to limitations imposed by the trait definition.

TryUnzip

Fallible equivalent of Iterator::unzip. Given an iterator of (A, B) items, produce two collections that implement Default + TryExtend, stopping on the first failure.

use std::collections::{BTreeSet, HashSet};
use collect_failable::TryUnzip;

// Unzip into two different container types
let data = vec![(1, 'a'), (2, 'b'), (3, 'c')];
let (nums, chars): (BTreeSet<i32>, HashSet<char>) = data.into_iter().try_unzip().expect("should be ok");

assert_eq!(nums, BTreeSet::from([1, 2, 3]));
assert_eq!(chars, HashSet::from(['a', 'b', 'c']));

Implementations

Implementations for various containers are provided.

• HashMap, HashSet (feature std, enabled by default)
• BTreeMap, BTreeSet (feature alloc, enabled by default)
• Array (feature unsafe, enabled by default)
• ArrayVec (feature arrayvec)
• hashbrown::HashMap, hashbrown::HashSet (feature hashbrown)
• indexmap::IndexMap, indexmap::IndexSet (feature indexmap)

Tuple Implementations

Tuples of arity 2 implement TryExtend when their inner types do (requires feature tuple, enabled by default). For constructing tuple collections from IntoIterator TryUnzip is available.

Array Implementation

Arrays implement TryFromIterator for IntoIterator that yield exactly the right number of elements. This uses unsafe internally and is gated behind the unsafe feature (enabled by default).

Result Implementation

TryFromIterator is implemented for Result<C, E>, where C implements TryFromIterator<T>, similar to the FromIterator implementation for Result. This allows short-circuiting collection of failable values into a container whose construction is also failable.