Struct Metadata 

pub struct Metadata(/* private fields */);

A cheaply cloneable map of key-value metadata, used by Field and Schema.

Cloning a Metadata is always cheap, as the underlying map is reference-counted. Mutating a shared Metadata (e.g. via Metadata::insert) will clone the underlying map if (and only if) it is shared (copy-on-write).

The entries are stored in a BTreeMap, so iteration order is deterministic (sorted by key).

Example

let mut metadata = Metadata::new();
metadata.insert("key", "value");

let clone = metadata.clone(); // cheap
assert_eq!(clone.get("key"), Some(&"value".to_string()));

// Mutating one does not affect the other:
metadata.insert("key2", "value2");
assert_eq!(metadata.len(), 2);
assert_eq!(clone.len(), 1);

Implementations

impl Metadata

pub const fn new() -> Metadata

Creates an empty Metadata.

This does not allocate.

pub fn len(&self) -> usize

Returns the number of entries.

pub fn is_empty(&self) -> bool

Returns true if there are no entries.

pub fn get(&self, key: &str) -> Option<&String>

Returns a reference to the value corresponding to the key.

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

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

pub fn insert( &mut self, key: impl Into<String>, value: impl Into<String>, ) -> Option<String>

Inserts a key-value pair, returning the old value of the key, if any.

If the map already contained this key, the value is replaced.

Clones the underlying map if (and only if) it is shared.

pub fn with(self, key: impl Into<String>, value: impl Into<String>) -> Metadata

Inserts a key-value pair and returns self, for builder-style chaining.

If the map already contained this key, the value is replaced.

Example

let metadata = Metadata::new().with("a", "1").with("b", "2");
assert_eq!(metadata.len(), 2);

pub fn remove(&mut self, key: &str) -> Option<String>

Removes a key from the map, returning the value at the key if the key was previously in the map.

Clones the underlying map if (and only if) it is shared and contains the key.

pub fn clear(&mut self)

Removes all entries.

pub fn as_arc(&self) -> Option<&Arc<BTreeMap<String, String>>>

Returns a reference to the underlying Arc, or None if the map is empty.

Useful for pointer-based deduplication (e.g. memory accounting) where callers need to detect clones that share the same allocation.

pub fn into_arc(self) -> Option<Arc<BTreeMap<String, String>>>

Consumes self and returns the underlying Arc, or None if the map is empty.

Useful for zero-copy conversion into wrapper types that also store an Arc<BTreeMap<String, String>> internally.

pub fn iter(&self) -> Flatten<IntoIter<Iter<'_, String, String>>>

Returns an iterator over the entries, sorted by key.

pub fn keys(&self) -> impl Iterator<Item = &String>

Returns an iterator over the keys, in sorted order.

pub fn values(&self) -> impl Iterator<Item = &String>

Returns an iterator over the values, sorted by key.

pub fn retain<F>(&mut self, f: F)

where F: FnMut(&String, &mut String) -> bool,

Retains only the entries for which f(&key, &mut value) returns true.

Entries for which f returns false are removed. Retained entries remain in their original (sorted) order.

Clones the underlying map if (and only if) it is shared.

Trait Implementations

impl Clone for Metadata

fn clone(&self) -> Metadata

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl Debug for Metadata

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

Formats the value using the given formatter.

impl Default for Metadata

fn default() -> Metadata

Returns the “default value” for a type.

impl Eq for Metadata

impl<K, V> Extend<(K, V)> for Metadata

where K: Into<String>, V: Into<String>,

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

where T: IntoIterator<Item = (K, V)>,

Extends a collection with the contents of an iterator.

fn extend_one(&mut self, item: T)

🔬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 From<Arc<BTreeMap<String, String>>> for Metadata

fn from(map: Arc<BTreeMap<String, String>>) -> Metadata

Converts to this type from the input type.

impl From<BTreeMap<String, String>> for Metadata

fn from(map: BTreeMap<String, String>) -> Metadata

Converts to this type from the input type.

impl From<HashMap<String, String>> for Metadata

fn from(map: HashMap<String, String>) -> Metadata

Converts to this type from the input type.

impl<K, V, const N: usize> From<[(K, V); N]> for Metadata

where K: Into<String>, V: Into<String>,

fn from(entries: [(K, V); N]) -> Metadata

Converts to this type from the input type.

impl<K, V> FromIterator<(K, V)> for Metadata

where K: Into<String>, V: Into<String>,

fn from_iter<T>(iter: T) -> Metadata

where T: IntoIterator<Item = (K, V)>,

Creates a value from an iterator.

impl Hash for Metadata

fn hash<__H>(&self, state: &mut __H)

where __H: Hasher,

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl Index<&str> for Metadata

type Output = String

The returned type after indexing.

fn index(&self, key: &str) -> &String

Performs the indexing (container[index]) operation.

impl<'a> IntoIterator for &'a Metadata

type Item = (&'a String, &'a String)

The type of the elements being iterated over.

type IntoIter = Flatten<IntoIter<Iter<'a, String, String>>>

Which kind of iterator are we turning this into?

fn into_iter(self) -> <&'a Metadata as IntoIterator>::IntoIter

Creates an iterator from a value.

impl IntoIterator for Metadata

type Item = (String, String)

The type of the elements being iterated over.

type IntoIter = IntoIter<String, String>

Which kind of iterator are we turning this into?

fn into_iter(self) -> <Metadata as IntoIterator>::IntoIter

Creates an iterator from a value.

impl Ord for Metadata

fn cmp(&self, other: &Metadata) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

where Self: Sized,

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

where Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

where Self: Sized,

Restrict a value to a certain interval.

fn clamp_to<R>(self, range: R) -> Self

where Self: Sized, R: ClampBounds<Self>,

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

Restrict a value to a certain range.

impl PartialEq for Metadata

fn eq(&self, other: &Metadata) -> bool

Equality operator ==.

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

Inequality operator !=.

impl PartialEq<BTreeMap<String, String>> for Metadata

fn eq(&self, other: &BTreeMap<String, String>) -> bool

Equality operator ==.

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

Inequality operator !=.

impl PartialEq<HashMap<String, String>> for Metadata

fn eq(&self, other: &HashMap<String, String>) -> bool

Equality operator ==.

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

Inequality operator !=.

impl PartialOrd for Metadata

fn partial_cmp(&self, other: &Metadata) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

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

Tests less than (for self and other) and is used by the < operator.

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

Tests less than or equal to (for self and other) and is used by the <= operator.

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

Tests greater than (for self and other) and is used by the > operator.

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

Tests greater than or equal to (for self and other) and is used by the >= operator.

impl StructuralPartialEq for Metadata