Update darling requirement from 0.14 to 0.20

[dependabot]

Updates the requirements on darling to permit the latest version.

Release notes

Sourced from darling's releases.

v0.20.1

• Add Clone impl for NestedMeta #230

Changelog

Sourced from darling's changelog.

v0.20.1 (May 2, 2023)

• Add Clone impl for NestedMeta #230

v0.20.0 (April 27, 2023)

• Bump syn to version 2, courtesy of @​jonasbb #227

Breaking Changes

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

• Replacement for the deprecated AttributeArgs:

// 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());
}
};

• In GenericParamExt, LifetimeDef is now LifetimeParam.
• In GenericParamExt, as_lifetime_def is renamed to as_lifetime_param.
• Flag and SpannedValue no longer implement syn::spanned::Spanned.
• The MSRV (minimum supported Rust version) is now 1.56, because of syn.

Deprecation Warnings

In previous versions of darling, arbitrary expressions were passed in attributes by wrapping them in quotation marks. v0.20.0 preserves this behavior for syn::Expr, but as a result a field expecting a syn::Expr cannot accept a string literal - it will incorrectly attempt to parse the contents. If this is an issue for you, please add a comment to #229.

v0.14.4 (March 9, 2023)

• Add support for child diagnostics when diagnostics feature enabled #224

v0.14.3 (February 3, 2023)

• Re-export syn from darling to avoid requiring that consuming crates have a syn dependency.
• Change <SpannedValue<T> as FromMeta> impl to more precisely capture the value span, as opposed to the span of the entire item.
• Add darling::util::{AsShape, Shape, ShapeSet} to improve "shape" validation for structs and variants. #222

v0.14.2 (October 26, 2022)

... (truncated)

Commits

• f6167fe Bump version to 0.20.1
• e4d22dd Allow Cloneing NestedMeta
• ffb9cc8 Update CHANGELOG
• b0bfdfd Bump version to 0.20.0
• 2c9ba69 Bump syn to 2.0.15
• 1d6f8df Replace Default returns with explicit panics for unknown enum variants
• 3fcf61a Add support for unquoted expressions in value position
• cbb7389 Update example in README which used the outdated AttributeArgs type
• c036162 Handle invisible groups in from_expr
• 8097397 Remove unnecessary clones
• Additional commits viewable in compare view

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