fix(deps): update rust crate darling to 0.21.0

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This PR contains the following updates:

Package	Type	Update	Change
darling	dependencies	minor	0.20.8 -> 0.21.0

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Release Notes

TedDriggs/darling (darling)

v0.21.3

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• Fix: Forward Override::<T>::from_expr to T::from_expr #​371

v0.21.2

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• Add #[darling(from_expr = ...)] when deriving FromMeta to support overriding the key-value form #​369
• Keep parsing the body and type params even if there are errors from parsing attributes. #​7
• Support #[darling(with = ...)] on the generics field when deriving FromDeriveInput.
• Return an error, rather than panicking, when doing shape validation on a union. #​365

v0.21.1

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• Track all alternate field names, and show them in error message if there aren't too many. #​325
• Track all alternate values for enum variants, and show them in error messages if there aren't too many. #​362

v0.21.0

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• Potentially breaking: Emit error when an attribute path is present in both attributes and forward_attrs. #​336
• Support parsing attributes which contain keywords #​238
• Add SpannedValue::into_inner #​342
• Add #[darling(derive_syn_parse)] to also impl syn::parse::Parse when deriving FromMeta #​285
• Make impl FromMeta for syn::TypePath support both quote-wrapped and bare values #​351
• Add util::PreservedStrExpr #​346
• Impl UsesTypeParams and UsesLifetimes for WithOriginal #​215
• Update error message emitted by <() as FromMeta>::from_list to allow use of () as a #[darling(flatten)] target #​353

v0.20.11

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• Support #[darling(with = ...)] on the data field when deriving FromDeriveInput. This allows the use of simpler receiver types, such as a Vec of enum variants.
• Bump version of proc-macro2 to 1.0.86.
• Accept closures for #[darling(with = ...)] on fields in FromDeriveInput, FromMeta, FromField, etc. #​309
• Add darling::util::Callable to accept a path or closure as a meta-item expression
• Add #[darling(from_word = ...)] and #[darling(from_none = ...)] to control shorthand and fallback behaviors for structs and enums deriving FromMeta #​320
• Add FromMeta impl for syn::ExprRange #​329

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⚠️ Artifact update problem

Renovate failed to update an artifact related to this branch. You probably do not want to merge this PR as-is.

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• any of the package files in this branch needs updating, or
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The artifact failure details are included below:

File name: spec/Cargo.lock

Command failed: cargo update --config net.git-fetch-with-cli=true --manifest-path spec/Cargo.toml --package darling@0.20.10 --precise 0.21.3
error: failed to acquire package cache lock

Caused by:
  failed to open: /home/ubuntu/.cargo/.package-cache

Caused by:
  failed to create directory `/home/ubuntu/.cargo`

Caused by:
  File exists (os error 17)