Struct HeaderMap 

pub struct HeaderMap<T = HeaderValue> {
    mask: u16,
    indices: Box<[Pos]>,
    entries: Vec<Bucket<T>>,
    extra_values: Vec<ExtraValue<T>>,
    danger: Danger,
}

A specialized multimap for header names and values.

Overview

HeaderMap is designed specifically for efficient manipulation of HTTP headers. It supports multiple values per header name and provides specialized APIs for insertion, retrieval, and iteration.

The internal implementation is optimized for common usage patterns in HTTP, and may change across versions. For example, the current implementation uses Robin Hood hashing to store entries compactly and enable high load factors with good performance. However, the collision resolution strategy and storage mechanism are not part of the public API and may be altered in future releases.

Iteration order

Unless otherwise specified, the order in which items are returned by iterators from HeaderMap methods is arbitrary; there is no guaranteed ordering among the elements yielded by such an iterator. Changes to the iteration order are not considered breaking changes, so users must not rely on any incidental order produced by such an iterator. However, for a given crate version, the iteration order will be consistent across all platforms.

Adaptive hashing

HeaderMap uses an adaptive strategy for hashing to maintain fast lookups while resisting hash collision attacks. The default hash function prioritizes performance. In scenarios where high collision rates are detected—typically indicative of denial-of-service attacks—the implementation switches to a more secure, collision-resistant hash function.

Limitations

A HeaderMap can store at most 32,768 entries (header name/value pairs). Attempting to exceed this limit will result in a panic.

Examples

Basic usage

let mut headers = HeaderMap::new();

headers.insert(HOST, "example.com".parse().unwrap());
headers.insert(CONTENT_LENGTH, "123".parse().unwrap());

assert!(headers.contains_key(HOST));
assert!(!headers.contains_key(LOCATION));

assert_eq!(headers[HOST], "example.com");

headers.remove(HOST);

assert!(!headers.contains_key(HOST));

Fields

mask: u16

indices: Box<[Pos]>

entries: Vec<Bucket<T>>

extra_values: Vec<ExtraValue<T>>

danger: Danger

Implementations

impl HeaderMap

pub fn new() -> HeaderMap

Create an empty HeaderMap.

The map will be created without any capacity. This function will not allocate.

Examples

let map = HeaderMap::new();

assert!(map.is_empty());
assert_eq!(0, map.capacity());

impl<T> HeaderMap<T>

pub fn with_capacity(capacity: usize) -> HeaderMap<T>

Create an empty HeaderMap with the specified capacity.

The returned map will allocate internal storage in order to hold about capacity elements without reallocating. However, this is a “best effort” as there are usage patterns that could cause additional allocations before capacity headers are stored in the map.

More capacity than requested may be allocated.

Panics

This method panics if capacity exceeds max HeaderMap capacity.

Examples

let map: HeaderMap<u32> = HeaderMap::with_capacity(10);

assert!(map.is_empty());
assert_eq!(12, map.capacity());

pub fn try_with_capacity( capacity: usize, ) -> Result<HeaderMap<T>, MaxSizeReached>

Create an empty HeaderMap with the specified capacity.

The returned map will allocate internal storage in order to hold about capacity elements without reallocating. However, this is a “best effort” as there are usage patterns that could cause additional allocations before capacity headers are stored in the map.

More capacity than requested may be allocated.

Errors

This function may return an error if HeaderMap exceeds max capacity

Examples

let map: HeaderMap<u32> = HeaderMap::try_with_capacity(10).unwrap();

assert!(map.is_empty());
assert_eq!(12, map.capacity());

pub fn len(&self) -> usize

Returns the number of headers stored in the map.

This number represents the total number of values stored in the map. This number can be greater than or equal to the number of keys stored given that a single key may have more than one associated value.

Examples

let mut map = HeaderMap::new();

assert_eq!(0, map.len());

map.insert(ACCEPT, "text/plain".parse().unwrap());
map.insert(HOST, "localhost".parse().unwrap());

assert_eq!(2, map.len());

map.append(ACCEPT, "text/html".parse().unwrap());

assert_eq!(3, map.len());

pub fn keys_len(&self) -> usize

Returns the number of keys stored in the map.

This number will be less than or equal to len() as each key may have more than one associated value.

Examples

let mut map = HeaderMap::new();

assert_eq!(0, map.keys_len());

map.insert(ACCEPT, "text/plain".parse().unwrap());
map.insert(HOST, "localhost".parse().unwrap());

assert_eq!(2, map.keys_len());

map.insert(ACCEPT, "text/html".parse().unwrap());

assert_eq!(2, map.keys_len());

pub fn is_empty(&self) -> bool

Returns true if the map contains no elements.

Examples

let mut map = HeaderMap::new();

assert!(map.is_empty());

map.insert(HOST, "hello.world".parse().unwrap());

assert!(!map.is_empty());

pub fn clear(&mut self)

Clears the map, removing all key-value pairs. Keeps the allocated memory for reuse.

Examples

let mut map = HeaderMap::new();
map.insert(HOST, "hello.world".parse().unwrap());

map.clear();
assert!(map.is_empty());
assert!(map.capacity() > 0);

pub fn capacity(&self) -> usize

Returns the number of headers the map can hold without reallocating.

This number is an approximation as certain usage patterns could cause additional allocations before the returned capacity is filled.

Examples

let mut map = HeaderMap::new();

assert_eq!(0, map.capacity());

map.insert(HOST, "hello.world".parse().unwrap());
assert_eq!(6, map.capacity());

pub fn reserve(&mut self, additional: usize)

Reserves capacity for at least additional more headers to be inserted into the HeaderMap.

The header map may reserve more space to avoid frequent reallocations. Like with with_capacity, this will be a “best effort” to avoid allocations until additional more headers are inserted. Certain usage patterns could cause additional allocations before the number is reached.

Panics

Panics if the new allocation size overflows HeaderMap MAX_SIZE.

Examples

let mut map = HeaderMap::new();
map.reserve(10);

pub fn try_reserve(&mut self, additional: usize) -> Result<(), MaxSizeReached>

Reserves capacity for at least additional more headers to be inserted into the HeaderMap.

The header map may reserve more space to avoid frequent reallocations. Like with with_capacity, this will be a “best effort” to avoid allocations until additional more headers are inserted. Certain usage patterns could cause additional allocations before the number is reached.

Errors

This method differs from reserve by returning an error instead of panicking if the value is too large.

Examples

let mut map = HeaderMap::new();
map.try_reserve(10).unwrap();

pub fn get<K>(&self, key: K) -> Option<&T>

where K: AsHeaderName,

Returns a reference to the value associated with the key.

If there are multiple values associated with the key, then the first one is returned. Use get_all to get all values associated with a given key. Returns None if there are no values associated with the key.

Examples

let mut map = HeaderMap::new();
assert!(map.get("host").is_none());

map.insert(HOST, "hello".parse().unwrap());
assert_eq!(map.get(HOST).unwrap(), &"hello");
assert_eq!(map.get("host").unwrap(), &"hello");

map.append(HOST, "world".parse().unwrap());
assert_eq!(map.get("host").unwrap(), &"hello");

pub fn get_mut<K>(&mut self, key: K) -> Option<&mut T>

where K: AsHeaderName,

Returns a mutable reference to the value associated with the key.

If there are multiple values associated with the key, then the first one is returned. Use entry to get all values associated with a given key. Returns None if there are no values associated with the key.

Examples

let mut map = HeaderMap::default();
map.insert(HOST, "hello".to_string());
map.get_mut("host").unwrap().push_str("-world");

assert_eq!(map.get(HOST).unwrap(), &"hello-world");

pub fn get_all<K>(&self, key: K) -> GetAll<'_, T>

where K: AsHeaderName,

Returns a view of all values associated with a key.

The returned view does not incur any allocations and allows iterating the values associated with the key. See GetAll for more details. Returns None if there are no values associated with the key.

Examples

let mut map = HeaderMap::new();

map.insert(HOST, "hello".parse().unwrap());
map.append(HOST, "goodbye".parse().unwrap());

let view = map.get_all("host");

let mut iter = view.iter();
assert_eq!(&"hello", iter.next().unwrap());
assert_eq!(&"goodbye", iter.next().unwrap());
assert!(iter.next().is_none());

pub fn contains_key<K>(&self, key: K) -> bool

where K: AsHeaderName,

Returns true if the map contains a value for the specified key.

Examples

let mut map = HeaderMap::new();
assert!(!map.contains_key(HOST));

map.insert(HOST, "world".parse().unwrap());
assert!(map.contains_key("host"));

pub fn iter(&self) -> Iter<'_, T>

An iterator visiting all key-value pairs.

The iteration order is arbitrary, but consistent across platforms for the same crate version. Each key will be yielded once per associated value. So, if a key has 3 associated values, it will be yielded 3 times.

Examples

let mut map = HeaderMap::new();

map.insert(HOST, "hello".parse().unwrap());
map.append(HOST, "goodbye".parse().unwrap());
map.insert(CONTENT_LENGTH, "123".parse().unwrap());

for (key, value) in map.iter() {
    println!("{:?}: {:?}", key, value);
}

pub fn iter_mut(&mut self) -> IterMut<'_, T>

An iterator visiting all key-value pairs, with mutable value references.

The iterator order is arbitrary, but consistent across platforms for the same crate version. Each key will be yielded once per associated value, so if a key has 3 associated values, it will be yielded 3 times.

Examples

let mut map = HeaderMap::default();

map.insert(HOST, "hello".to_string());
map.append(HOST, "goodbye".to_string());
map.insert(CONTENT_LENGTH, "123".to_string());

for (key, value) in map.iter_mut() {
    value.push_str("-boop");
}

pub fn keys(&self) -> Keys<'_, T>

An iterator visiting all keys.

The iteration order is arbitrary, but consistent across platforms for the same crate version. Each key will be yielded only once even if it has multiple associated values.

Examples

let mut map = HeaderMap::new();

map.insert(HOST, "hello".parse().unwrap());
map.append(HOST, "goodbye".parse().unwrap());
map.insert(CONTENT_LENGTH, "123".parse().unwrap());

for key in map.keys() {
    println!("{:?}", key);
}

pub fn values(&self) -> Values<'_, T>

An iterator visiting all values.

The iteration order is arbitrary, but consistent across platforms for the same crate version.

Examples

let mut map = HeaderMap::new();

map.insert(HOST, "hello".parse().unwrap());
map.append(HOST, "goodbye".parse().unwrap());
map.insert(CONTENT_LENGTH, "123".parse().unwrap());

for value in map.values() {
    println!("{:?}", value);
}

pub fn values_mut(&mut self) -> ValuesMut<'_, T>

An iterator visiting all values mutably.

The iteration order is arbitrary, but consistent across platforms for the same crate version.

Examples

let mut map = HeaderMap::default();

map.insert(HOST, "hello".to_string());
map.append(HOST, "goodbye".to_string());
map.insert(CONTENT_LENGTH, "123".to_string());

for value in map.values_mut() {
    value.push_str("-boop");
}

pub fn drain(&mut self) -> Drain<'_, T>

Clears the map, returning all entries as an iterator.

The internal memory is kept for reuse.

For each yielded item that has None provided for the HeaderName, then the associated header name is the same as that of the previously yielded item. The first yielded item will have HeaderName set.

Examples

let mut map = HeaderMap::new();

map.insert(HOST, "hello".parse().unwrap());
map.append(HOST, "goodbye".parse().unwrap());
map.insert(CONTENT_LENGTH, "123".parse().unwrap());

let mut drain = map.drain();


assert_eq!(drain.next(), Some((Some(HOST), "hello".parse().unwrap())));
assert_eq!(drain.next(), Some((None, "goodbye".parse().unwrap())));

assert_eq!(drain.next(), Some((Some(CONTENT_LENGTH), "123".parse().unwrap())));

assert_eq!(drain.next(), None);

pub fn entry<K>(&mut self, key: K) -> Entry<'_, T>

where K: IntoHeaderName,

Gets the given key’s corresponding entry in the map for in-place manipulation.

Panics

This method panics if capacity exceeds max HeaderMap capacity

Examples

let mut map: HeaderMap<u32> = HeaderMap::default();

let headers = &[
    "content-length",
    "x-hello",
    "Content-Length",
    "x-world",
];

for &header in headers {
    let counter = map.entry(header).or_insert(0);
    *counter += 1;
}

assert_eq!(map["content-length"], 2);
assert_eq!(map["x-hello"], 1);

pub fn try_entry<K>( &mut self, key: K, ) -> Result<Entry<'_, T>, InvalidHeaderName>

where K: AsHeaderName,

Gets the given key’s corresponding entry in the map for in-place manipulation.

Errors

This method differs from entry by allowing types that may not be valid HeaderNames to passed as the key (such as String). If they do not parse as a valid HeaderName, this returns an InvalidHeaderName error.

If reserving space goes over the maximum, this will also return an error. However, to prevent breaking changes to the return type, the error will still say InvalidHeaderName, unlike other try_* methods which return a MaxSizeReached error.

pub fn insert<K>(&mut self, key: K, val: T) -> Option<T>

where K: IntoHeaderName,

Inserts a key-value pair into the map.

If the map did not previously have this key present, then None is returned.

If the map did have this key present, the new value is associated with the key and all previous values are removed. Note that only a single one of the previous values is returned. If there are multiple values that have been previously associated with the key, then the first one is returned. See insert_mult on OccupiedEntry for an API that returns all values.

The key is not updated, though; this matters for types that can be == without being identical.

Panics

This method panics if capacity exceeds max HeaderMap capacity

Examples

let mut map = HeaderMap::new();
assert!(map.insert(HOST, "world".parse().unwrap()).is_none());
assert!(!map.is_empty());

let mut prev = map.insert(HOST, "earth".parse().unwrap()).unwrap();
assert_eq!("world", prev);

pub fn try_insert<K>( &mut self, key: K, val: T, ) -> Result<Option<T>, MaxSizeReached>

where K: IntoHeaderName,

Inserts a key-value pair into the map.

If the map did not previously have this key present, then None is returned.

If the map did have this key present, the new value is associated with the key and all previous values are removed. Note that only a single one of the previous values is returned. If there are multiple values that have been previously associated with the key, then the first one is returned. See insert_mult on OccupiedEntry for an API that returns all values.

The key is not updated, though; this matters for types that can be == without being identical.

Errors

This function may return an error if HeaderMap exceeds max capacity

Examples

let mut map = HeaderMap::new();
assert!(map.try_insert(HOST, "world".parse().unwrap()).unwrap().is_none());
assert!(!map.is_empty());

let mut prev = map.try_insert(HOST, "earth".parse().unwrap()).unwrap().unwrap();
assert_eq!("world", prev);

pub fn append<K>(&mut self, key: K, value: T) -> bool

where K: IntoHeaderName,

Inserts a key-value pair into the map.

If the map did not previously have this key present, then false is returned.

If the map did have this key present, the new value is pushed to the end of the list of values currently associated with the key. The key is not updated, though; this matters for types that can be == without being identical.

Panics

This method panics if capacity exceeds max HeaderMap capacity

Examples

let mut map = HeaderMap::new();
assert!(map.insert(HOST, "world".parse().unwrap()).is_none());
assert!(!map.is_empty());

map.append(HOST, "earth".parse().unwrap());

let values = map.get_all("host");
let mut i = values.iter();
assert_eq!("world", *i.next().unwrap());
assert_eq!("earth", *i.next().unwrap());

pub fn try_append<K>( &mut self, key: K, value: T, ) -> Result<bool, MaxSizeReached>

where K: IntoHeaderName,

Inserts a key-value pair into the map.

If the map did not previously have this key present, then false is returned.

If the map did have this key present, the new value is pushed to the end of the list of values currently associated with the key. The key is not updated, though; this matters for types that can be == without being identical.

Errors

This function may return an error if HeaderMap exceeds max capacity

Examples

let mut map = HeaderMap::new();
assert!(map.try_insert(HOST, "world".parse().unwrap()).unwrap().is_none());
assert!(!map.is_empty());

map.try_append(HOST, "earth".parse().unwrap()).unwrap();

let values = map.get_all("host");
let mut i = values.iter();
assert_eq!("world", *i.next().unwrap());
assert_eq!("earth", *i.next().unwrap());

pub fn remove<K>(&mut self, key: K) -> Option<T>

where K: AsHeaderName,

Removes a key from the map, returning the value associated with the key.

Returns None if the map does not contain the key. If there are multiple values associated with the key, then the first one is returned. See remove_entry_mult on OccupiedEntry for an API that yields all values.

Examples

let mut map = HeaderMap::new();
map.insert(HOST, "hello.world".parse().unwrap());

let prev = map.remove(HOST).unwrap();
assert_eq!("hello.world", prev);

assert!(map.remove(HOST).is_none());

Trait Implementations

impl<T> Clone for HeaderMap<T>

where T: Clone,

fn clone(&self) -> HeaderMap<T>

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<T> Debug for HeaderMap<T>

where T: Debug,

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl<T> Default for HeaderMap<T>

fn default() -> HeaderMap<T>

Returns the “default value” for a type.

impl<T> Extend<(HeaderName, T)> for HeaderMap<T>

fn extend<I>(&mut self, iter: I)

where I: IntoIterator<Item = (HeaderName, T)>,

Extends a collection with the contents of an iterator.

fn extend_one(&mut self, item: A)

🔬This is a nightly-only experimental API. (extend_one)

Extends a collection with exactly one element.

fn extend_reserve(&mut self, additional: usize)

🔬This is a nightly-only experimental API. (extend_one)

Reserves capacity in a collection for the given number of additional elements.

impl<T> Extend<(Option<HeaderName>, T)> for HeaderMap<T>

fn extend<I>(&mut self, iter: I)

where I: IntoIterator<Item = (Option<HeaderName>, T)>,

Extend a HeaderMap with the contents of another HeaderMap.

This function expects the yielded items to follow the same structure as IntoIter.

Panics

This panics if the first yielded item does not have a HeaderName.

Examples

let mut map = HeaderMap::new();

map.insert(ACCEPT, "text/plain".parse().unwrap());
map.insert(HOST, "hello.world".parse().unwrap());

let mut extra = HeaderMap::new();

extra.insert(HOST, "foo.bar".parse().unwrap());
extra.insert(COOKIE, "hello".parse().unwrap());
extra.append(COOKIE, "world".parse().unwrap());

map.extend(extra);

assert_eq!(map["host"], "foo.bar");
assert_eq!(map["accept"], "text/plain");
assert_eq!(map["cookie"], "hello");

let v = map.get_all("host");
assert_eq!(1, v.iter().count());

let v = map.get_all("cookie");
assert_eq!(2, v.iter().count());

fn extend_one(&mut self, item: A)

🔬This is a nightly-only experimental API. (extend_one)

Extends a collection with exactly one element.

fn extend_reserve(&mut self, additional: usize)

🔬This is a nightly-only experimental API. (extend_one)

Reserves capacity in a collection for the given number of additional elements.

impl<T> FromIterator<(HeaderName, T)> for HeaderMap<T>

fn from_iter<I>(iter: I) -> HeaderMap<T>

where I: IntoIterator<Item = (HeaderName, T)>,

Creates a value from an iterator.

impl<S> FromRequestParts<S> for HeaderMap

where S: Send + Sync,

Clone the headers from the request.

Prefer using TypedHeader to extract only the headers you need.

type Rejection = Infallible

If the extractor fails it’ll use this “rejection” type. A rejection is a kind of error that can be converted into a response.

async fn from_request_parts( parts: &mut Parts, _: &S, ) -> Result<HeaderMap, <HeaderMap as FromRequestParts<S>>::Rejection>

Perform the extraction.

impl<K, T> Index<K> for HeaderMap<T>

where K: AsHeaderName,

fn index(&self, index: K) -> &T

Panics

Using the index operator will cause a panic if the header you’re querying isn’t set.

type Output = T

The returned type after indexing.

impl<'a, T> IntoIterator for &'a HeaderMap<T>

type Item = (&'a HeaderName, &'a T)

The type of the elements being iterated over.

type IntoIter = Iter<'a, T>

Which kind of iterator are we turning this into?

fn into_iter(self) -> Iter<'a, T>

Creates an iterator from a value.

impl<'a, T> IntoIterator for &'a mut HeaderMap<T>

type Item = (&'a HeaderName, &'a mut T)

The type of the elements being iterated over.

type IntoIter = IterMut<'a, T>

Which kind of iterator are we turning this into?

fn into_iter(self) -> IterMut<'a, T>

Creates an iterator from a value.

impl<T> IntoIterator for HeaderMap<T>

fn into_iter(self) -> IntoIter<T>

Creates a consuming iterator, that is, one that moves keys and values out of the map in arbitrary order. The map cannot be used after calling this.

For each yielded item that has None provided for the HeaderName, then the associated header name is the same as that of the previously yielded item. The first yielded item will have HeaderName set.

Examples

Basic usage.

let mut map = HeaderMap::new();
map.insert(header::CONTENT_LENGTH, "123".parse().unwrap());
map.insert(header::CONTENT_TYPE, "json".parse().unwrap());

let mut iter = map.into_iter();
assert_eq!(iter.next(), Some((Some(header::CONTENT_LENGTH), "123".parse().unwrap())));
assert_eq!(iter.next(), Some((Some(header::CONTENT_TYPE), "json".parse().unwrap())));
assert!(iter.next().is_none());

Multiple values per key.

let mut map = HeaderMap::new();

map.append(header::CONTENT_LENGTH, "123".parse().unwrap());
map.append(header::CONTENT_LENGTH, "456".parse().unwrap());

map.append(header::CONTENT_TYPE, "json".parse().unwrap());
map.append(header::CONTENT_TYPE, "html".parse().unwrap());
map.append(header::CONTENT_TYPE, "xml".parse().unwrap());

let mut iter = map.into_iter();

assert_eq!(iter.next(), Some((Some(header::CONTENT_LENGTH), "123".parse().unwrap())));
assert_eq!(iter.next(), Some((None, "456".parse().unwrap())));

assert_eq!(iter.next(), Some((Some(header::CONTENT_TYPE), "json".parse().unwrap())));
assert_eq!(iter.next(), Some((None, "html".parse().unwrap())));
assert_eq!(iter.next(), Some((None, "xml".parse().unwrap())));
assert!(iter.next().is_none());

type Item = (Option<HeaderName>, T)

The type of the elements being iterated over.

type IntoIter = IntoIter<T>

Which kind of iterator are we turning this into?

impl IntoResponse for HeaderMap

fn into_response(self) -> Response<Body>

Create a response.

impl IntoResponseParts for HeaderMap

type Error = Infallible

The type returned in the event of an error.

fn into_response_parts( self, res: ResponseParts, ) -> Result<ResponseParts, <HeaderMap as IntoResponseParts>::Error>

Set parts of the response

impl<T> PartialEq for HeaderMap<T>

where T: PartialEq,

fn eq(&self, other: &HeaderMap<T>) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<'a, K, V, S, T> TryFrom<&'a HashMap<K, V, S>> for HeaderMap<T>

where K: Eq + Hash, HeaderName: TryFrom<&'a K>, <HeaderName as TryFrom<&'a K>>::Error: Into<Error>, T: TryFrom<&'a V>, <T as TryFrom<&'a V>>::Error: Into<Error>,

Try to convert a HashMap into a HeaderMap.

Examples

use std::collections::HashMap;
use std::convert::TryInto;
use http::HeaderMap;

let mut map = HashMap::new();
map.insert("X-Custom-Header".to_string(), "my value".to_string());

let headers: HeaderMap = (&map).try_into().expect("valid headers");
assert_eq!(headers["X-Custom-Header"], "my value");

type Error = Error

The type returned in the event of a conversion error.

fn try_from( c: &'a HashMap<K, V, S>, ) -> Result<HeaderMap<T>, <HeaderMap<T> as TryFrom<&'a HashMap<K, V, S>>>::Error>

Performs the conversion.

impl TryParse for HeaderMap

fn try_parse(buf: &[u8]) -> Result<Option<(usize, HeaderMap)>, Error>

Return Ok(None) if incomplete, Err on syntax error.

impl<T> Eq for HeaderMap<T>

where T: Eq,