Trait darling::FromMeta

source · [−]
pub trait FromMeta {
    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_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

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.

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)].

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.

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.

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

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.

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

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

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

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

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

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

Implementors

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