Update syn requirement from 2.0 to 3.0 in the dependencies group

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Updates the requirements on syn to permit the latest version.
Updates syn to 3.0.0

Release notes

Sourced from syn's releases.

3.0.0

This release contains adjustments to the syntax tree to account for ongoing Rust language development from the 3 years since syn 2.0.0 and to anticipate some in-flight Rust language RFCs.

These include: default values in fields, pinned type sugar, raw lifetimes, generator blocks and functions, unnamed enum variants, attributes in tuple types and tuple patterns, named arguments in parenthesized generic argument lists, lightweight clones, const traits, const function pointers, mutability restricted fields, supertrait auto implementation, final associated functions, trait implementability restrictions, const blocks in path arguments, item-level const blocks, return type notation, never patterns, function delegation, mutable by-reference bindings, in-place initialization, field projections, explicitly dyn-compatible traits, view types, file-level frontmatter, generic const arguments, guard patterns, lazy type aliases, explicitly safe foreign items, super let, unsafe fields, pattern types, heterogeneous try-blocks, function contracts, async function trait bounds, static closure coroutine syntax, unsafe binder types, move expressions, for-await loops, and postfix keywords.

Breaking changes

Modifiers

To reserve more room for language evolution, there are 10 new non-exhaustive structs in the syntax tree having the following commonality:

• Name ending in Modifiers. {BlockModifiers, ClosureModifiers, ConstModifiers, FieldModifiers, FnModifiers, ImplModifiers, LocalModifiers, TraitBoundModifiers, TraitModifiers, TypeModifiers}

• Each implements Default. The default value is guaranteed to comprise no tokens.

• Non-exhaustive. Can only be instantiated by Syn's parser or by creating and then mutating ▁▁Modifiers::default().

• Does not implement Parse. When parsing, they are parsed by the enclosing syntax tree node.

• Does not implement ToTokens. In some cases the syntax that these nodes might hold in the future is not necessarily contiguous tokens.

• Provides .require_empty() -> Result<()> which returns a meaningfully spanned error if the modifiers are different from the empty default. This enables a caller to reject syntax it does not recognize without knowing what that syntax may be.

Types

• Type::BareFn has been renamed to Type::FnPtr to mirror the compiler's terminology. Together with this, BareVariadic is renamed to FnPtrVariadic.

• The mutually exclusive const_token and mutability fields of Type::Ptr have been unified into an enum of type PointerMutability, which was already previously used by Expr::RawAddr.

• Every Type variant now holds attributes, which can represent the attributes of element types inside a tuple type, or attributes for a function return type.

• BareFnArg is renamed to NamedArg and is used in ParenthesizedGenericArguments, in addition to the existing use in Type::FnPtr.

Expressions

• In Expr::Closure, the fields or1_token and or2_token have been renamed to inputs_begin and inputs_end to indicate the beginning and ending | token of the closure inputs.

Statements

• Attributes are now preserved on all expression kinds in statement position. Previously, parsing would silently lose attributes on some expression statements without a dedicated Expr variant, such as tail-call expressions.

Patterns

• The guard field of Arm is replaced by a new Pat::Guard variant held in the arm's pat.

Items

... (truncated)

Commits

• 51d6959 Release 3.0.0
• d63e2e1 More concise dependency version requirements
• deb8c8a Merge pull request #2054 from dtolnay/fielddefault
• f2e874d Parse field default values
• 6718d21 Merge pull request #2053 from dtolnay/kwlifetime
• 4dcc5d7 Permit keyword lifetimes in most positions
• 063f2ef Update test suite to nightly-2026-07-18
• efdbb0f Use new safe keyword type in peek_signature
• 480ddca Merge pull request #2051 from dtolnay/optiontuple
• 6ebb4c7 Align GenericParam defaults with option-of-tuple convention
• Additional commits viewable in compare view

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