# Fields in syn - Rust
Data stored within an enum variant or struct.

syn

# Enum Fields

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    pub enum Fields {
        Named(FieldsNamed),
        Unnamed(FieldsUnnamed),
        Unit,
    }

Expand description

Data stored within an enum variant or struct.

## §Syntax tree enum

This type is a syntax tree enum.

## Variants§

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### Named(FieldsNamed)

Named fields of a struct or struct variant such as `Point { x: f64, y: f64 }`.

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### Unnamed(FieldsUnnamed)

Unnamed fields of a tuple struct or tuple variant such as `Some(T)`.

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### Unit

Unit struct or unit variant such as `None`.

## Implementations§

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### impl Fields

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#### pub fn iter(&self) -> Iter<'_, Field> ⓘ

Get an iterator over the borrowed `Field` items in this object. This iterator can be used to iterate over a named or unnamed struct or variant’s fields uniformly.

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#### pub fn iter_mut(&mut self) -> IterMut<'_, Field> ⓘ

Get an iterator over the mutably borrowed `Field` items in this object. This iterator can be used to iterate over a named or unnamed struct or variant’s fields uniformly.

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#### pub fn len(&self) -> usize

Returns the number of fields.

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#### pub fn is_empty(&self) -> bool

Returns `true` if there are zero fields.

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#### pub fn members(&self) -> Members<'_>

Get an iterator over the fields of a struct or variant as `Member`s. This iterator can be used to iterate over a named or unnamed struct or variant’s fields uniformly.

##### §Example

The following is a simplistic `Clone` derive for structs. (A more complete implementation would additionally want to infer trait bounds on the generic type parameters.)

    fn derive_clone(input: &syn::ItemStruct) -> proc_macro2::TokenStream {
        let ident = &input.ident;
        let members = input.fields.members();
        let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
        quote! {
            impl #impl_generics Clone for #ident #ty_generics #where_clause {
                fn clone(&self) -> Self {
                    Self {
                        #(#members: self.#members.clone()),*
                    }
                }
            }
        }
    }

For structs with named fields, it produces an expression like `Self { a: self.a.clone() }`. For structs with unnamed fields, `Self { 0: self.0.clone() }`. And for unit structs, `Self {}`.

## Trait Implementations§

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### impl Clone for Fields

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#### fn clone(&self) -> Self

Returns a copy of the value. Read more
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#### fn clone_from(&mut self, source: &Self)

Performs copy-assignment from `source`. Read more
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### impl Debug for Fields

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

Formats the value using the given formatter. Read more
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### impl From<FieldsNamed> for Fields

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#### fn from(e: FieldsNamed) -> Fields

Converts to this type from the input type.
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### impl From<FieldsUnnamed> for Fields

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#### fn from(e: FieldsUnnamed) -> Fields

Converts to this type from the input type.
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### impl Hash for Fields

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#### fn hash<H>(&self, state: &mut H)
where H: Hasher,

Feeds this value into the given `Hasher`. Read more
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#### fn hash_slice<H>(data: &[Self], state: &mut H)
where H: Hasher, Self: Sized,

Feeds a slice of this type into the given `Hasher`. Read more
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### impl<'a> IntoIterator for &'a Fields

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#### type Item = &'a Field

The type of the elements being iterated over.
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#### type IntoIter = Iter<'a, Field>

Which kind of iterator are we turning this into?
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#### fn into_iter(self) -> Self::IntoIter

Creates an iterator from a value. Read more
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### impl<'a> IntoIterator for &'a mut Fields

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#### type Item = &'a mut Field

The type of the elements being iterated over.
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#### type IntoIter = IterMut<'a, Field>

Which kind of iterator are we turning this into?
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#### fn into_iter(self) -> Self::IntoIter

Creates an iterator from a value. Read more
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### impl IntoIterator for Fields

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#### type Item = Field

The type of the elements being iterated over.
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#### type IntoIter = IntoIter<Field>

Which kind of iterator are we turning this into?
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#### fn into_iter(self) -> Self::IntoIter

Creates an iterator from a value. Read more
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### impl PartialEq for Fields

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

Tests for `self` and `other` values to be equal, and is used by `==`.
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#### fn ne(&self, other: &Rhs) -> bool

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

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#### fn to_tokens(&self, tokens: &mut TokenStream)

Write `self` to the given `TokenStream`. Read more
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#### fn to_token_stream(&self) -> TokenStream

Convert `self` directly into a `TokenStream` object. Read more
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#### fn into_token_stream(self) -> TokenStream
where Self: Sized,

Convert `self` directly into a `TokenStream` object. Read more
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### impl Eq for Fields

## Auto Trait Implementations§

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### impl Freeze for Fields

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### impl RefUnwindSafe for Fields

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### impl !Send for Fields

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### impl !Sync for Fields

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### impl Unpin for Fields

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### impl UnwindSafe for Fields

## Blanket Implementations§

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### impl<T> Any for T
where T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> Borrow<T> for T
where T: ?Sized,

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for T
where T: ?Sized,

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#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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### impl<T> CloneToUninit for T
where T: Clone,

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#### unsafe fn clone_to_uninit(&self, dst: *mut u8)

🔬This is a nightly-only experimental API. (`clone_to_uninit`)
Performs copy-assignment from `self` to `dst`. Read more
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### impl<T> From<T> for T

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#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T, U> Into<U> for T
where U: From<T>,

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#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

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### impl<T> Spanned for T
where T: Spanned + ?Sized,

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#### fn span(&self) -> Span

Returns a `Span` covering the complete contents of this syntax tree node, or `Span::call_site()` if this node is empty.
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### impl<T> ToOwned for T
where T: Clone,

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#### type Owned = T

The resulting type after obtaining ownership.
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#### fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
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#### fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
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### impl<T, U> TryFrom<U> for T
where U: Into<T>,

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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### impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

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#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.