Struct Flag

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

Marker type equivalent to Option<()> for use in attribute parsing.

Implementations

impl Flag

pub fn present() -> Self

Creates a new Flag which corresponds to the presence of a value.

Methods from Deref<Target = Option<()>>

pub fn is_some(&self) -> bool

Returns true if the option is a Some value.

Examples

let x: Option<u32> = Some(2);
assert_eq!(x.is_some(), true);

let x: Option<u32> = None;
assert_eq!(x.is_some(), false);

pub fn is_none(&self) -> bool

Returns true if the option is a None value.

Examples

let x: Option<u32> = Some(2);
assert_eq!(x.is_none(), false);

let x: Option<u32> = None;
assert_eq!(x.is_none(), true);

pub fn as_ref(&self) -> Option<&T>

Converts from &Option<T> to Option<&T>.

Examples

Calculates the length of an Option<String> as an Option<usize> without moving the String. The map method takes the self argument by value, consuming the original, so this technique uses as_ref to first take an Option to a reference to the value inside the original.

let text: Option<String> = Some("Hello, world!".to_string());
// First, cast `Option<String>` to `Option<&String>` with `as_ref`,
// then consume *that* with `map`, leaving `text` on the stack.
let text_length: Option<usize> = text.as_ref().map(|s| s.len());
println!("still can print text: {text:?}");

pub fn as_pin_ref(self: Pin<&Option<T>>) -> Option<Pin<&T>>

Converts from Pin<&Option<T>> to Option<Pin<&T>>.

pub fn as_pin_mut(self: Pin<&mut Option<T>>) -> Option<Pin<&mut T>>

Converts from Pin<&mut Option<T>> to Option<Pin<&mut T>>.

pub fn as_slice(&self) -> &[T]

Returns a slice of the contained value, if any. If this is None, an empty slice is returned. This can be useful to have a single type of iterator over an Option or slice.

Note: Should you have an Option<&T> and wish to get a slice of T, you can unpack it via opt.map_or(&[], std::slice::from_ref).

Examples

assert_eq!(
    [Some(1234).as_slice(), None.as_slice()],
    [&[1234][..], &[][..]],
);

The inverse of this function is (discounting borrowing) [_]::first:

for i in [Some(1234_u16), None] {
    assert_eq!(i.as_ref(), i.as_slice().first());
}

pub fn as_deref(&self) -> Option<&<T as Deref>::Target>

where T: Deref,

Converts from Option<T> (or &Option<T>) to Option<&T::Target>.

Leaves the original Option in-place, creating a new one with a reference to the original one, additionally coercing the contents via Deref.

Examples

let x: Option<String> = Some("hey".to_owned());
assert_eq!(x.as_deref(), Some("hey"));

let x: Option<String> = None;
assert_eq!(x.as_deref(), None);

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

Returns an iterator over the possibly contained value.

Examples

let x = Some(4);
assert_eq!(x.iter().next(), Some(&4));

let x: Option<u32> = None;
assert_eq!(x.iter().next(), None);

Trait Implementations

impl BitAnd for Flag

type Output = Flag

The resulting type after applying the & operator.

fn bitand(self, rhs: Self) -> Self

Performs the & operation.

impl BitOr for Flag

type Output = Flag

The resulting type after applying the | operator.

fn bitor(self, rhs: Self) -> Self

Performs the | operation.

impl Clone for Flag

fn clone(&self) -> Flag

Returns a copy of the value.

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

Performs copy-assignment from source.

impl Debug for Flag

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

Formats the value using the given formatter.

impl Default for Flag

fn default() -> Flag

Returns the “default value” for a type.

impl Deref for Flag

type Target = Option<()>

The resulting type after dereferencing.

fn deref(&self) -> &Self::Target

Dereferences the value.

impl From<Flag> for bool

fn from(flag: Flag) -> Self

Converts to this type from the input type.

impl From<Option<()>> for Flag

fn from(v: Option<()>) -> Self

Converts to this type from the input type.

impl From<bool> for Flag

fn from(v: bool) -> Self

Converts to this type from the input type.

impl FromMeta for Flag

fn from_meta(mi: &Meta) -> Result<Self>

Create an instance from a syn::Meta by dispatching to the format-appropriate trait function. This generally should not be overridden by implementers.

fn from_nested_meta(item: &NestedMeta) -> Result<Self>

fn from_word() -> Result<Self>

Create an instance from the presence of the word in the attribute with no additional options specified.

fn from_list(items: &[NestedMeta]) -> Result<Self>

Create an instance from a list of nested meta items.

fn from_value(value: &Lit) -> Result<Self>

Create an instance from a literal value of either foo = "bar" or foo("bar"). This dispatches to the appropriate method based on the type of literal encountered, and generally should not be overridden by implementers.

fn from_char(value: char) -> Result<Self>

Create an instance from a char literal in a value position.

fn from_string(value: &str) -> Result<Self>

Create an instance from a string literal in a value position.

fn from_bool(value: bool) -> Result<Self>

Create an instance from a bool literal in a value position.

impl Not for Flag

type Output = Flag

The resulting type after applying the ! operator.

fn not(self) -> Self

Performs the unary ! operation.

impl PartialEq<Flag> for Option<()>

fn eq(&self, rhs: &Flag) -> 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 PartialEq<Option<()>> for Flag

fn eq(&self, rhs: &Option<()>) -> 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 PartialEq<bool> for Flag

fn eq(&self, rhs: &bool) -> 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 PartialEq for Flag

fn eq(&self, other: &Flag) -> 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 Copy for Flag

impl Eq for Flag

impl StructuralPartialEq for Flag