Trait darling::FromMeta

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

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

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, Error>

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, Error>

Create an instance from a list of nested meta items.

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

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, Error>

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

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

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

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

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

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

Object Safety§

This trait is not object safe.

Implementations on Foreign Types§

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impl FromMeta for RenameRule

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impl FromMeta for Meta

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impl FromMeta for Expr

Support for arbitrary expressions as values in a meta item.

For backwards-compatibility to versions of darling based on syn 1, string literals will be “unwrapped” and their contents will be parsed as an expression.

See util::parse_expr for functions to provide alternate parsing modes for this type.

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impl FromMeta for Lit

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impl FromMeta for Visibility

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impl FromMeta for Type

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impl FromMeta for bool

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impl FromMeta for char

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impl FromMeta for f32

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impl FromMeta for f64

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impl FromMeta for i8

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impl FromMeta for i16

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impl FromMeta for i32

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impl FromMeta for i64

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impl FromMeta for i128

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impl FromMeta for isize

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impl FromMeta for u8

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impl FromMeta for u16

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impl FromMeta for u32

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impl FromMeta for u64

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impl FromMeta for u128

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impl FromMeta for ()

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impl FromMeta for usize

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impl FromMeta for String

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impl FromMeta for Vec<WherePredicate>

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impl FromMeta for Vec<u8>

Parsing an unsigned integer array, i.e. example = "[1, 2, 3, 4]".

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impl FromMeta for Vec<u16>

Parsing an unsigned integer array, i.e. example = "[1, 2, 3, 4]".

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impl FromMeta for Vec<u32>

Parsing an unsigned integer array, i.e. example = "[1, 2, 3, 4]".

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impl FromMeta for Vec<u64>

Parsing an unsigned integer array, i.e. example = "[1, 2, 3, 4]".

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impl FromMeta for Vec<usize>

Parsing an unsigned integer array, i.e. example = "[1, 2, 3, 4]".

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impl FromMeta for Vec<Literal>

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impl FromMeta for Vec<LitBool>

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impl FromMeta for Vec<LitByte>

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impl FromMeta for Vec<LitByteStr>

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impl FromMeta for Vec<LitChar>

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impl FromMeta for Vec<LitFloat>

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impl FromMeta for Vec<LitInt>

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impl FromMeta for Vec<LitStr>

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impl FromMeta for NonZero<i8>

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impl FromMeta for NonZero<i16>

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impl FromMeta for NonZero<i32>

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impl FromMeta for NonZero<i64>

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impl FromMeta for NonZero<i128>

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impl FromMeta for NonZero<isize>

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impl FromMeta for NonZero<u8>

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impl FromMeta for NonZero<u16>

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impl FromMeta for NonZero<u32>

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impl FromMeta for NonZero<u64>

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impl FromMeta for NonZero<u128>

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impl FromMeta for NonZero<usize>

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impl FromMeta for AtomicBool

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impl FromMeta for PathBuf

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impl FromMeta for Ident

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impl FromMeta for Literal

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impl FromMeta for ExprArray

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impl FromMeta for ExprPath

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impl FromMeta for TypeParam

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impl FromMeta for WhereClause

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impl FromMeta for LitBool

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impl FromMeta for LitByte

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impl FromMeta for LitByteStr

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impl FromMeta for LitChar

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impl FromMeta for LitFloat

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impl FromMeta for LitInt

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impl FromMeta for LitStr

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impl FromMeta for Path

Parser for paths that supports both quote-wrapped and bare values.

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impl FromMeta for TypeArray

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impl FromMeta for TypeBareFn

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impl FromMeta for TypeGroup

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impl FromMeta for TypeImplTrait

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impl FromMeta for TypeInfer

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impl FromMeta for TypeMacro

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impl FromMeta for TypeNever

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impl FromMeta for TypeParen

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impl FromMeta for TypePath

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impl FromMeta for TypePtr

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impl FromMeta for TypeReference

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impl FromMeta for TypeSlice

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impl FromMeta for TypeTraitObject

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impl FromMeta for TypeTuple

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impl<T> FromMeta for Option<T>
where T: FromMeta,

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impl<T> FromMeta for Result<T, Meta>
where T: FromMeta,

Parses the meta-item, and in case of error preserves a copy of the input for later analysis.

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impl<T> FromMeta for Result<T, Error>
where T: FromMeta,

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impl<T> FromMeta for Box<T>
where T: FromMeta,

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impl<T> FromMeta for Rc<T>
where T: FromMeta,

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impl<T> FromMeta for Arc<T>
where T: FromMeta,

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impl<T> FromMeta for RefCell<T>
where T: FromMeta,

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impl<T, P> FromMeta for Punctuated<T, P>
where T: Parse, P: Parse,

Parsing support for punctuated. This attempts to preserve span information when available, but also supports parsing strings with the call site as the emitted span.

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impl<V, S> FromMeta for HashMap<String, V, S>
where V: FromMeta, S: BuildHasher + Default,

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impl<V, S> FromMeta for HashMap<Ident, V, S>
where V: FromMeta, S: BuildHasher + Default,

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impl<V, S> FromMeta for HashMap<Path, V, S>
where V: FromMeta, S: BuildHasher + Default,

Implementors§

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impl FromMeta for Flag

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impl FromMeta for IdentString

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impl FromMeta for Ignored

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impl FromMeta for PathList

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impl<T> FromMeta for Override<T>
where T: FromMeta,

Parses a Meta. A bare word will produce Override::Inherit, while any value will be forwarded to T::from_meta.

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impl<T> FromMeta for SpannedValue<T>
where T: FromMeta,

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impl<T> FromMeta for WithOriginal<T, Meta>
where T: FromMeta,