Trait darling::FromMeta

source · 
pub trait FromMeta: Sized {
    fn from_nested_meta(item: &NestedMeta) -> Result<Self, Error> { ... }
    fn from_meta(item: &Meta) -> Result<Self, Error> { ... }
    fn from_word() -> Result<Self, Error> { ... }
    fn from_list(items: &[NestedMeta]) -> Result<Self, Error> { ... }
    fn from_value(value: &Lit) -> 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".

String

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

()

  • 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§

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

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

Create an instance from a list of nested meta items.

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.

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

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

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

Implementations on Foreign Types§

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

Implementors§

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