Trait FromMeta

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pub trait FromMeta: Sized {
    // Provided methods
    fn from_nested_meta(item: &NestedMeta) -> Result<Self> { ... }
    fn from_meta(item: &Meta) -> Result<Self> { ... }
    fn from_none() -> Option<Self> { ... }
    fn from_word() -> Result<Self> { ... }
    fn from_list(items: &[NestedMeta]) -> Result<Self> { ... }
    fn from_value(value: &Lit) -> Result<Self> { ... }
    fn from_expr(expr: &Expr) -> Result<Self> { ... }
    fn from_char(value: char) -> Result<Self> { ... }
    fn from_string(value: &str) -> Result<Self> { ... }
    fn from_bool(value: bool) -> Result<Self> { ... }
}

Expand description

Create an instance from an item in an attribute declaration.

§Implementing `FromMeta`

Do not take a dependency on the ident of the passed-in meta item. The ident will be set by the field name of the containing struct.
Implement only the from_* methods that you intend to support. The default implementations will return useful errors.

§Provided Implementations

§bool

Word with no value specified - becomes true.
As a boolean literal, e.g. foo = true.
As a string literal, e.g. foo = "true".

§char

As a char literal, e.g. foo = '#'.
As a string literal consisting of a single character, e.g. foo = "#".

§String

As a string literal, e.g. foo = "hello".
As a raw string literal, e.g. foo = r#"hello "world""#.

§Number

As a string literal, e.g. foo = "-25".
As an unquoted positive value, e.g. foo = 404. Negative numbers must be in quotation marks.

§()

Word with no value specified, e.g. foo. This is best used with Option. See darling::util::Flag for a more strongly-typed alternative.

§Option

Any format produces Some.

§`Result<T, darling::Error>`

Allows for fallible parsing; will populate the target field with the result of the parse attempt.

Provided Methods§

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

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fn from_meta(item: &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.

§Error Spans

If this method is overridden and can introduce errors that weren’t passed up from other from_meta calls, the override must call with_span on the error using the item to make sure that the emitted diagnostic points to the correct location in source code.

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fn from_none() -> Option<Self>

When a field is omitted from a parent meta-item, from_none is used to attempt recovery before a missing field error is generated.

Most types should not override this method. darling already allows field-level missing-field recovery using #[darling(default)] and #[darling(default = "...")], and users who add a String field to their FromMeta-deriving struct would be surprised if they get back "" instead of a missing field error when that field is omitted.

The primary use-case for this is Option<T> fields gracefully handlling absence without needing #[darling(default)].

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fn from_word() -> Result<Self>

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

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

Create an instance from a list of nested meta items.

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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.

§Error Spans

If this method is overridden, the override must make sure to add value’s span information to the returned error by calling with_span(value) on the Error instance.

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fn from_expr(expr: &Expr) -> Result<Self>

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

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

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

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

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

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

Object Safety§

This trait is not object safe.

Trait FromMetaCopy item path

§Implementing FromMeta

§Provided Implementations

§bool

§char

§String

§Number

§()

§Option

§Result<T, darling::Error>

Provided Methods§

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

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

§Error Spans

fn from_none() -> Option<Self>

fn from_word() -> Result<Self>

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

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

§Error Spans

fn from_expr(expr: &Expr) -> Result<Self>

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

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

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

Object Safety§

Implementations on Foreign Types§

impl FromMeta for RenameRule

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

impl FromMeta for Meta

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

impl FromMeta for Expr

fn from_expr(expr: &Expr) -> Result<Self>

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

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

impl FromMeta for Lit

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

impl FromMeta for Visibility

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

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

impl FromMeta for Type

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

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

impl FromMeta for bool

fn from_word() -> Result<Self>

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

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

impl FromMeta for char

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

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

impl FromMeta for f32

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

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

impl FromMeta for f64

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

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

impl FromMeta for i8

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

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

impl FromMeta for i16

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

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

impl FromMeta for i32

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

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

impl FromMeta for i64

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

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

impl FromMeta for i128

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

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

impl FromMeta for isize

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

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

impl FromMeta for u8

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

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

impl FromMeta for u16

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

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

impl FromMeta for u32

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

Trait FromMeta

§Implementing `FromMeta`

§`Result<T, darling::Error>`