darling_core breaking changes

• v0.21.3 \u2192 v0.22.0breaking

  Add `#[darling(default = || expr(val))]` support, allowing a closure where a path was previously required [#380](https://github.com/TedDriggs/darling/pull/380)

• v0.21.3 \u2192 v0.22.0breaking

  Preserve span information for paths given to `darling::util::Callable` as literal strings

• v0.21.3 \u2192 v0.22.0breaking

  Fix some documentation typos

• v0.14.4 \u2192 v0.20.0breaking

  Replace all occurrences of syn::NestedMeta with darling::ast::NestedMeta.

• v0.14.4 \u2192 v0.20.0breaking

  Replacement for the deprecated AttributeArgs: ```rust // Before parse_macro_input!(args as AttributeArgs); // After match NestedMeta::parse_meta_list(args) { Ok(v) => v, Err(e) => { return TokenStream::from(Error::from(e).write_errors()); } }; ```

• v0.14.4 \u2192 v0.20.0breaking

  In GenericParamExt, `LifetimeDef` is now `LifetimeParam`.

• v0.14.4 \u2192 v0.20.0breaking

  In GenericParamExt, `as_lifetime_def` is renamed to `as_lifetime_param`.

• v0.14.4 \u2192 v0.20.0breaking

  Flag and SpannedValue no longer implement `syn::spanned::Spanned`.

• v0.13.4 \u2192 v0.14.0breaking

  Remove need for `#[darling(default)]` on `Option<T>` and `Flag` fields [#161](https://github.com/TedDriggs/darling/issues/161)

• v0.13.4 \u2192 v0.14.0breaking

  Improve validation of enum shapes [#178](https://github.com/TedDriggs/darling/pull/178)

• v0.13.4 \u2192 v0.14.0breaking

  Bump `proc-macro2` dependency to 1.0.37 [#180](https://github.com/TedDriggs/darling/pull/180)

• v0.13.4 \u2192 v0.14.0breaking

  Bump `quote` dependency to 1.0.18 [#180](https://github.com/TedDriggs/darling/pull/180)

• v0.13.4 \u2192 v0.14.0breaking

  Bump `syn` dependency to 1.0.91 [#180](https://github.com/TedDriggs/darling/pull/180)

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