fix(deps): update rust crate syn to v3 (#808)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [syn](https://redirect.github.com/dtolnay/syn) | dependencies | major
| `2` → `3` |

---

### Release Notes

<details>
<summary>dtolnay/syn (syn)</summary>

###
[`v3.0.3`](https://redirect.github.com/dtolnay/syn/releases/tag/3.0.3)

[Compare
Source](https://redirect.github.com/dtolnay/syn/compare/3.0.2...3.0.3)

- Documentation improvements

###
[`v3.0.2`](https://redirect.github.com/dtolnay/syn/releases/tag/3.0.2)

[Compare
Source](https://redirect.github.com/dtolnay/syn/compare/3.0.1...3.0.2)

- Add [`Error::new_range(start..end,
"msg")`](https://docs.rs/syn/3/syn/struct.Error.html#method.new_range)
([#&#8203;2068](https://redirect.github.com/dtolnay/syn/issues/2068),
[#&#8203;2070](https://redirect.github.com/dtolnay/syn/issues/2070))
- Add
[`Cursor::prev_span`](https://docs.rs/syn/3/syn/buffer/struct.Cursor.html#method.prev_span)

###
[`v3.0.1`](https://redirect.github.com/dtolnay/syn/releases/tag/3.0.1)

[Compare
Source](https://redirect.github.com/dtolnay/syn/compare/3.0.0...3.0.1)

- Parse const traits
([#&#8203;2056](https://redirect.github.com/dtolnay/syn/issues/2056),
[#&#8203;2057](https://redirect.github.com/dtolnay/syn/issues/2057),
[#&#8203;2058](https://redirect.github.com/dtolnay/syn/issues/2058),
[#&#8203;2063](https://redirect.github.com/dtolnay/syn/issues/2063),
[#&#8203;2064](https://redirect.github.com/dtolnay/syn/issues/2064))
- Parse unsafe binder types
([#&#8203;2065](https://redirect.github.com/dtolnay/syn/issues/2065))
- Parse impl restrictions
([#&#8203;2066](https://redirect.github.com/dtolnay/syn/issues/2066))

###
[`v3.0.0`](https://redirect.github.com/dtolnay/syn/releases/tag/3.0.0)

[Compare
Source](https://redirect.github.com/dtolnay/syn/compare/2.0.119...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.

<br>

<p align="center"><a href="https://docs.rs/syn/3/syn">[API documentation
for 3.0]</a></p>

### 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

- The `unsafety` field of `Signature`, which represented the presence or
absence of `unsafe`, is replaced by a 3-way `Safety` enum which may be
safe, unsafe, or default. `ForeignItem::Static` also gets a `Safety`.

- Some of the fields of `Receiver` have been split to a non-exhaustive
`ReceiverKind` enum to create room for proposed new kinds of method
receivers, such as pinned.

- Type aliases now hold a `WhereClausePlacement` to distinguish between
early placement (which is the default for item-level type aliases and is
*deprecated* in associated type aliases) and late placement (which is
the default for associated type aliases and is *unstable* in item-level
type aliases). Parsing and printing a syntax tree will now preserve the
where-clause placement rather than rewriting it to the default placement
for the item kind.

#### Generics

- `WherePredicate::Lifetime` and `WherePredicate::Type` have a new field
holding the attributes on the where-predicates.

- `GenericParam::Type` and `GenericParam::Const` now match the rest of
the syntax tree in holding their optional default using a single Option
of tuple, rather than a pair of Option.

#### Literals

- The `Parse` implementation of `Lifetime` no longer permits keyword
lifetimes, matching a change in Rust 1.81 to deny such lifetimes
pre-expansion. `Lifetime::parse_any` may be used to parse lifetimes
permissive of keywords.

- `LitInt` and `LitFloat` no longer implement
`From<proc_macro2::Literal>`. Use `Lit::new(literal)` and then match
`Lit::Int` or `Lit::Float`.

- The `StrStyle` enum was unused and has been deleted.

#### More

- `File` contains a new `Option<Frontmatter>`, but does not yet parse
frontmatter in `syn::parse_file` until that language feature progresses
closer to stabilization.

- Some enums no longer provide `From` impls. Construct the variant by
name instead, such as `Expr::Array(e)` in place of `e.into()`.

- The `pop` method of `Punctuated<T, P>` now returns `Option<T>`
discarding the trailing punctuation if any, rather than `Option<Pair<T,
P>>`. A new method `pop_pair` provides the old behavior.

- Syntax tree traversal (`visit`, `visit_mut`, `fold`) no longer
provides a method for processing `Span`. This was already incompletely
supported and never walked most of the spans in a syntax tree.

- The `Speculative` and `AnyDelimiter` traits have been sealed and
cannot be implemented for types outside of Syn's `ParseStream`.

</details>

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Cargo.lock

@@ -22,7 +22,7 @@ proc-macro2 = "1"
 quote = "1"
 # syn 2 rather than 3, which is already in the tree via clap_derive: nothing here
 # needs the newer API, and matching what is there avoids a second copy.
-syn = { version = "2", features = ["full"] }
+syn = { version = "3", features = ["full"] }
 
 # The generated code refers to usage-argv, so the doc examples need it in scope to
 # compile. Not a real dependency: this crate emits tokens and links nothing.