v2.0.108

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• Parse unrecognized or invalid literals as Lit::Verbatim (#​1925)

v2.0.107

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• Improve panic message when constructing a LitInt, LitFloat, or Lit from invalid syntax (#​1917)
• Improve panic message on Punctuated index out of bounds (#​1922)

v2.0.106

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• Replace ~const syntax with [const] conditionally const syntax in trait bounds (#​1896, rust-lang/rust#139858)
• Support conditionally const impl Trait types (#​1897)
• Reject polarity modifier and lifetime binder used in the same trait bound (#​1899, rust-lang/rust#127054)
• Parse const trait bounds with bound lifetimes (#​1902)
• Parse bound lifetimes with lifetime bounds (#​1903)
• Allow type parameters and const parameters in trait bounds and generic closures (#​1904, #​1907, #​1908, #​1909)

v2.0.105

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• Disallow "negative" inherent impls like impl !T {} (#​1881, rust-lang/rust#144386)

v2.0.104

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• Disallow attributes on range expression (#​1872)

v2.0.103

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• Insert parentheses around binary operation with attribute (#​1871)

v2.0.102

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• Fix printing of nested Expr::Index and Expr::Tuple in non-full mode (#​1869)

v2.0.101

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• Improve TypeGenerics::as_turbofish to return longer-lived return value (#​1861)

v2.0.100

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• Add Visit::visit_token_stream, VisitMut::visit_token_stream_mut, Fold::fold_token_stream for processing TokenStream during syntax tree traversals (#​1852)

v2.0.99

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• Documentation improvements

v2.0.98

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• Allow lifetimes in function pointer return values in ParseStream::call and Punctuated parsers (#​1847)

v2.0.97

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• Documentation improvements

v2.0.96

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• Expression precedence fixes (#​1834, #​1836, #​1837, #​1838, #​1839, #​1840)

v2.0.95

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• Fix parenthesization of struct literals in let-chains (#​1832)

v2.0.94

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• Expression precedence fixes (#​1811, #​1812, #​1813, #​1814, #​1815, #​1816, #​1818, #​1819, #​1820, #​1825, #​1826, #​1827, #​1828, #​1829, #​1830)

v2.0.93

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• Fix expression parsing edge cases: x as T <<= y (#​1804), break as T (#​1805), match x { _ if .. => {} } (#​1807)
• Fix expression printing edge cases: || -> T 'a: {} (#​1806), if break x {} (#​1808, #​1810)

v2.0.92

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• Fix parenthesization of jumps inside ranges (#​1798)

v2.0.91

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• Support parsing Vec<Arm> using parse_quote! (#​1796, #​1797)

v2.0.90

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• Fix automatic parenthesization of subexpressions containing outer attributes, such as (#[attr] thing).field (#​1785)
• Fix automatic parenthesization of function calls via a struct field, such as (thing.field)() and thing.0() (#​1786)

v2.0.89

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• Fix "compiler/fallback mismatch 949" panic (dtolnay/proc-macro2#483)

v2.0.88

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• Improve error recovery in parse_str (#​1783)

v2.0.87

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• Add Cursor::any_group (#​1777)
• Add Expr::peek (#​1778)
• Improve syntax support for enum discriminant expressions in non-"full" mode (#​1779)

v2.0.86

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• Support peeking the end of a parse stream (#​1689)
• Allow parse_quote! to produce Vec<Attribute> (#​1775)

v2.0.85

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• Preserve extern static unsafety in ForeignItem::Verbatim (#​1773)

v2.0.84

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• Parse safe and explicitly unsafe extern items (#​1768, rust-lang/rust#123743, rust-lang/rfcs#3484)
• Parse self captures: impl Sized + use<Self> (#​1772)

v2.0.83

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• Documentation improvements

v2.0.82

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• Provide Parse impls for PreciseCapture and CapturedParam (#​1757, #​1758)
• Support parsing unsafe attributes (#​1759)
• Add Fold and VisitMut methods for Vec<Attribute> (#​1762)

v2.0.81

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• Add TypeParamBound::PreciseCapture to represent precise capture syntax impl Trait + use<'a, T> (#​1752, #​1753, #​1754)

v2.0.80

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• Add Expr::RawAddr (#​1743)
• Reject precise captures and ~const in inappropriate syntax positions (#​1747)
• Reject trait bound containing only precise capture (#​1748)

v2.0.79

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• Fix infinite loop on parsing chained ranges (#​1741)
• Fix panic in parsing use items containing absolute paths (#​1742)

v2.0.78

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• Fix infinite loop on chained comparison (#​1739)

v2.0.77

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• Support parsing Expr::Tuple in non-"full" mode (#​1727)

v2.0.76

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• Enforce that tail call become keyword is followed by an expression (#​1725)

v2.0.75

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• Automatically fill in missing turbofish when printing ExprPath and other paths in expression position (#​1722)

v2.0.74

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• Fix "temporary is dropped and runs the destructor for type `impl Iterator`" regression affecting certain use of Generics iterator methods (#​1719)

v2.0.73

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• Support parsing unnamed C varargs within function pointer types (#​1711)
• Improve synthesized error message on unexpected tokens at the end of the expected contents of a delimited group (#​1713)
• Support parsing unstable tail call syntax (#​1714, rust-lang/rust#112788)
• Add Fields::members iterator (#​1716, thanks @​Fancyflame)

v2.0.72

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• Parse use<'a, T> precise capturing bounds (#​1707, thanks @​compiler-errors)

v2.0.71

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• Do not require mutable borrow in Punctuated::get() (#​1706, thanks @​lemunozm)

v2.0.70

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• Improve parenthesization of closures, jumps, ranges, chained comparisons, and let (#​1694, #​1695, #​1698, #​1699, #​1700)

v2.0.69

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• Correctly parenthesize labeled loops inside a break value (#​1692)
• Add Punctuated::get and get_mut (#​1693)

v2.0.68

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• Improve panic location when parse_quote! parses invalid syntax (#​1690, thanks @​stepancheg)
• More efficient peek implementation for Group and Lifetime (#​1687)

v2.0.67

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• Produce more accurate error message locations for errors located at the end of a nested group (#​1679, #​1680)
• Support peeking LitCStr in ParseStream::peek (#​1682)

v2.0.66

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• Allow braced structs when parsing ExprLet (#​1671)

v2.0.65

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• Optimize the implementation of Fold to compile faster (#​1666, #​1667, #​1668)

v2.0.64

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• Support using ParseBuffer across catch_unwind (#​1646)
• Validate that the expression in a let-else ends in brace as required by rustc (#​1648, #​1649)
• Legalize invalid const generic arguments by wrapping in braces (#​1654, #​1655)
• Fix some expression precedence edge cases involving break and return in loop headers (#​1656)
• Always print closure bodies with a brace when the closure has an explicit return type (#​1658)
• Automatically insert necessary parentheses in ToTokens for Expr when required by expression precedence (#​1659)
• Support struct literal syntax in match guard expressions (#​1662)

v2.0.63

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• Parse and print long if-else-if chains without reliance on deep recursion to avoid overflowing stack (#​1644, #​1645)

v2.0.62

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• Reject invalid unparenthesized range and comparison operator expressions (#​1642, #​1643)

v2.0.61

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• Check for legal binding name in the ident of Pat::Ident (#​1627)
• Resolve unexpected_cfgs warning (#​1635)

v2.0.60

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• Improve how None-delimited groups are counted by peek (#​1625)

v2.0.59

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• Parse c"…" and cr"…" C-string literal syntax as Lit::CStr (#​1502)

v2.0.58

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• Support $ in custom_punctuation! macro (#​1611)

v2.0.57

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• Eliminate dependency on quote when built with default-features disabled and the "proc-macro" feature enabled (#​1608, thanks @​BD103)

v2.0.56

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• Apply RUSTC_WORKSPACE_WRAPPER when deciding whether to run nightly-only tests (#​1605)

v2.0.55

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• Restore compatibility with rustc 1.56 through 1.59 (#​1603)

v2.0.54

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• Fix dead code warning in generated code when using custom_keyword! with syn's "printing" feature disabled (#​1602)

v2.0.53

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• Implement Copy, Clone, and ToTokens for syn::parse::Nothing (#​1597)

v2.0.52

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• Add an expression parser that uses match-arm's boundary rules (#​1593)

v2.0.51

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• Resolve non_local_definitions warnings in generated code under rustc 1.78-nightly

v2.0.50

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• Fix unused_imports warnings when compiled by rustc 1.78

v2.0.49

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• Improve error location when parsing from an empty string literal using LitStr::parse (#​1590)

v2.0.48

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• Improve error message on unexpected token after else (#​1578)

v2.0.47

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• Improve error messages related to proc_macro::LexError (#​1575)

v2.0.46

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• Update proc-macro2 to fix caching issue when using a rustc-wrapper such as sccache

v2.0.45

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• Parse unsupported expressions in enum discriminants of DeriveInput as Expr::Verbatim in non-"full" mode, instead of error (#​1513)
• Support parsing PatType with parse_quote! (#​1573)

v2.0.44

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• Documentation improvements

v2.0.43

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• Insert trailing comma if not already present when printing a 1-tuple in pattern position (#​1553)

v2.0.42

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• Documentation improvements

v2.0.41

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• Support parsing syn::Field in parse_quote! (#​1548)

v2.0.40

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• Fix some edge cases of handling None-delimited groups in expression parser (#​1539, #​1541, #​1542, #​1543, #​1544, #​1545)

v2.0.39

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• Fix parsing of return expression in match guards (#​1528)
• Improve error message on labeled loop as value expression for break (#​1531)

v2.0.38

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• Fix "method 'peek' has an incompatible type for trait" error when defining bool as a custom keyword (#​1518, thanks @​Vanille-N)

v2.0.37

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• Work around incorrect future compatibility warning in rustc 1.74.0-nightly

v2.0.36

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• Restore compatibility with --generate-link-to-definition documentation builds (#​1514)

v2.0.35

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• Make rust-analyzer produce preferred brackets for invocations of Token! macro (#​1510, #​1512)

v2.0.34

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• Documentation improvements

v2.0.33

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• Special handling for the (/*ERROR*/) placeholder that rustc uses for macros that fail to expand

v2.0.32

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• Add Path::require_ident accessor (#​1496, thanks @​Fancyflame)

v2.0.31

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• Parse generics and where-clause on const items (rust-lang/rust#113521)

v2.0.30

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• Parse unnamed struct/union type syntax (rust-lang/rust#49804)

v2.0.29

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• Partially work around rust-analyzer bug (rust-lang/rust-analyzer#9911)

v2.0.28

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• Fix inconsistency between full and non-full expression parse errors (#​1491)

v2.0.27

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• Documentation improvements (thanks @​GuillaumeGomez)

v2.0.26

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• Implement Spanned for QSelf (#​1465)

v2.0.25

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• Support single identifier as unbraced const generic argument (#​1483)
• Produce error message when LitStr::parse is used on a suffixed string literal (#​1484)

v2.0.24

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• Fix duplication of braces around const generic argument in non-full mode (#​1482)

v2.0.23

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• Preserve attributes on verbatim Item in statement position (#​1476)
• Support generic_const_exprs where-clauses such as where [(); { T::COUNT }]: in non-"full" mode (#​1478)

v2.0.22

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• Parse c"…" c-string literals (tracking issue: rust-lang/rust#105723)

v2.0.21

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• Fix value computed by LitByteStr::value in the case of a cooked byte string literal containing form feed or vertical tab characters following an escaped newline (#​1474)

v2.0.20

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• Documentation improvements

v2.0.19

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• Improve rendering of compile errors within 2015-edition code calling a 2018+ edition proc macro (#​1467, thanks @​danielhenrymantilla)

v2.0.18

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• Permit empty attr in syn::meta::parser (#​1460)

v2.0.17

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• Enable proc_macro support on wasm targets (#​1459)

v2.0.16

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• Parse builtin # syntax as Expr::Verbatim (rust-lang/rust#110680, #​1454)

v2.0.15

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• Ensure Type::Tuple of length 1 prints as a tuple even if trailing comma is not provided in the Punctuated (#​1444, thanks @​Fancyflame)

v2.0.14

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• Add Punctuated::pop_punct() (#​1442, thanks @​programmerjake)

v2.0.13

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• Improve spans of Expr::Field parsed from a float Literal (#​1433, #​1436)

v2.0.12

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• Refer to compile_error! by absolute path in token stream produced by syn::Error::to_compile_error (#​1431, thanks @​smoelius)

v2.0.11

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• Improve error message on empty parens inside parse_nested_meta (#​1428)

v2.0.10

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• Fix visibility being parsed incorrectly on macro invocations inside of a trait

v2.0.9

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• Disallow type items in an extern block, trait, or module from being marked default
• Disallow ImplItemFn from having an omitted function body, as in impl T { fn f(&self); } — omitted function bodies are allowed by TraitItemFn, but in impl blocks this syntax is now parsed as ImplItem::Verbatim rather than ImplItem::Fn

v2.0.8

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• Treat try keyword as 2015-edition identifier in definition of try macro (#​1422)

v2.0.7

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• Fix parsing of mut self inside of Type::BareFn

v2.0.6

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• Improve error message on missing ';' between statements (#​1419)
• Keep non-brace macro invocations in trailing expr position as Expr::Macro (#​1420)

v2.0.5

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• Expose ExprMacro data structure even when features="full" is not used (#​1417)

v2.0.4

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• Improve error reporting when parsing identifiers and paths (#​1415, #​1416)

v2.0.3

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• Expose ExprGroup data structure even when features="full" is not used (#​1412)

v2.0.2

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• Documentation improvements

v2.0.1

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• Add methods on syn::Meta for reporting error on an incorrect kind of attribute (#​1409)

v2.0.0

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This release contains a batch of syntax tree improvements to incorporate ongoing Rust language development from the past 3.5 years since syn 1.

It never seems like an ideal time to finalize a syntax tree design, considering the frankly alarming number of syntax-disrupting language features currently in flight: keyword generics, restrictions, capabilities and contexts, conditional constness, new varieties of literals, dyn revamp such as explicitly dyn-safe traits and dyn-star, expression syntax in various phases of being added or being torn out (const blocks, try blocks, raw references), auto traits and negative impls, generalizations to higher rank trait bounds, async closures and static async trait methods, postfix keywords, pattern types, return type notation, unsafe attributes, …

The plan continues to be the same as laid out originally in the 1.0.0 release announcement:

Be aware that the underlying Rust language will continue to evolve. Syn is able to accommodate most kinds of Rust grammar changes via the nonexhaustive enums and Verbatim variants in the syntax tree, but we will plan to put out new major versions on a 12 to 24 month cadence to incorporate ongoing language changes as needed.

If anything, the takeaway from the 3.5 year longevity of syn 1 is that this period was tamer from a language development perspective than anticipated, but that is unlikely to last and I think around 24 months is still the correct cadence to expect between releases going forward.

[API documentation for 2.0]

Breaking changes

• Minimum required Rust version is raised from rustc 1.31 to 1.56.

Expressions

• Support for box expr syntax has been deleted, as it has been deleted recently from rustc.

• Support for type ascription syntax expr: Type in expression position has been deleted.

• Support for unstable &raw const expr raw-pointer reference syntax has been deleted.

• The representation of generic arguments has been unified between method calls and non-method paths into a single GenericArgument type, which supersedes the previous GenericMethodArgument and MethodTurbofish.

• Generic arguments now distinguish between associated types (AssocType) and associated constant values (AssocConst). Previously these would be parsed ambiguously as Binding.

• The binary assignment operators in BinOp have been renamed to align with the naming used by the standard library's core::ops module's traits. For example BinOp::AddEq is now called BinOp::AddAssign.

• Expr::Struct struct construction expressions now support structs which are a variant of an enum associated type of a trait, as in <Type as Trait>::Assoc::Variant { ... }, which has recently been added to Rust.

• Expr::Range now follows the start and end naming used by the standard library's RangeBounds trait, rather than from/to or lo/hi.

• Expr::AssignOp has been merged into Expr::Binary, which now represents both non-assignment and assignment binary operators.

• Stricter parsing of ranges. None of the following are valid expressions, but were previously accepted by syn: ..=, lo..=, ..., ...hi, lo..., lo...hi.

• Expr::Closure now includes a representation for for<...> lifetimes.

Statements

• Variants Stmt::Expr (tail-position expression without trailing semicolon) and Stmt::Semi (non-tail expression with trailing semicolon) have been combined into Stmt::Expr with the optional semicolon represented by Option<Token![;]>.

• The syntax tree for Stmt::Local has been extended to handle let/else syntax.

• Macros in statement position are now uniformly parsed as Stmt::Macro. Previously these would be disambiguated to Stmt::Item, although it was ambiguous whether a macro in statement position would expand to an item (like thread_local! { ... }) vs an expression (like println! { ... }).

Patterns

• Pattern parsing for all the different syntactic positions in which patterns are allowed has been split into Pat::parse_single (for function- and closure-argument position, where top-level | is not allowed), Pat::parse_multi (where | is allowed) and Pat::parse_multi_with_leading_vert (for the pattern of match arms, which allow an optional leading |). Previously only a single parse behavior was supported and behaved like the new parse_single.

• The Pat syntax tree now shares more common data structures with the Expr syntax tree where possible, such as for literals, paths, macros, and ranges in pattern position.

• Parsing of struct field patterns does a better job rejecting bogus syntax such as Struct { 0 asdf } and Struct { ref mut 0: asdf }, which were previously incorrectly accepted.

• Pat::Range now supports one-sided ranges by representing the start and end bound of the range by Option<Expr>.

• Pat::Struct keeps track of attributes on the optional .. "rest" part of the pattern, as in let Struct { x, #[cfg(any())] .. } = _;.

• Parsing unary negation now enforces that only literal patterns can be unarily negated. For example -self::CONST and -const { 0i32 } are not valid syntax in pattern position.

• Pat::TupleStruct no longer wraps a value of type PatTuple but represents that information in its fields directly.

• A single parenthesized pattern without trailing comma inside the parentheses is no longer considered a Pat::Tuple, it will be parsed as Pat::Paren.

• One-sided range patterns are no longer allowed inside of slice patterns. [lo..] and [..=hi] are not considered valid pattern syntax by Rust.

Items

• Typed self in a method signature, such as self: Pin<&mut Self>, will now be parsed as FnArg::Receiver. This means self, whether with or without an explicit type, is always treated as a Receiver. Previously only the &self and &mut self shorthand receivers were parsed as Receiver.

• TraitItem::Method and ImplItem::Method have been renamed to TraitItem::Fn and ImplItem::Fn, as they do not necessarily represent methods if the function signature contains no self.

• Item::Macro2 has been deleted as "macros 2.0" syntax is no longer considered on track for stabilization.

• Various item kinds now hold Generics which didn't used to have them.

• The variadic argument of an extern function signature can now be given an optional parameter name.

• WherePredicate::Eq is no longer supported.

• Visibility::Crate is no longer supported. This syntax has been removed from rustc.

• Public visibility is now represented by a single Token![pub] token rather than the old VisPublic struct.

• LifetimeDef is now called LifetimeParam. This name makes more sense in the context of the GenericParam enum (which also includes TypeParam and ConstParam), and is the name that the Rust Reference uses.

• Modules and extern blocks (Item::Mod and Item::ForeignMod) can now be marked unsafe.

Attributes

• The syntax tree for Attribute has been redesigned. The new API better accommodates attributes which mix structured and unstructured content at different levels of nesting.

• AttributeArgs has been removed. Use Punctuated<Meta, Token![,]>.

• For parsing attribute contents, parse_meta() is superseded by a new parsing library called syn::meta, and the parse_nested_meta method on Attribute.

Tokens

• In string literals, the handling of non-ASCII whitespace after trailing \ now matches what is implemented by rustc. Space, horizontal tab, line feed, and carriage return are the only 4 whitespace characters which are supposed to be stripped from the beginning of the next line.

• The delimiter tokens syn::token::Paren, Bracket, and Brace now store 2 spans (the open and close punctuation separately) rather than just 1. Use .join() to obtain a single Span spanning the whole group.

• Keyword construction now requires a single span; an array of 1 span is no longer accepted. Use Token![trait](span) instead of Token![trait]([span]).

• Some token types have been renamed to conform with terminology used by the Rust Reference. These are Add->Plus, Bang->Not, Colon2->PathSep, Div->Slash, Dot2->DotDot, Dot3->DotDotDot, Rem->Percent, and Sub->Minus.

More

• Several enums have been made #[non_exhaustive] in anticipation of upcoming language changes. This includes WherePredicate, Lit, and GenericArgument.

• The impl Extend<Pair<T, P>> for Punctuated<T, P> now requires P: Default and will push a default punctuation between the pre-existing elements and the new ones, if there is not already a trailing punctuation. Previously it would panic in this situation.

• ParseStream::parse_terminated now takes a peek-style punctuation argument instead of turbofish. Replace input.parse_terminated::<_, Token![,]>(Thing::parse) with input.parse_terminated(Thing::parse, Token![,]).

v1.0.109

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v1.0.108

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• Fix handling of unusual whitespace after escaped newlines in LitStr::value (#​1381, thanks @​ModProg)

v1.0.107

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• Opt out of -Zrustdoc-scrape-examples on docs.rs for now

v1.0.106

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• Documentation improvements

v1.0.105

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• Improve parse errors related to dyn and impl type syntax (#​1245)

v1.0.104

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• Add PathArguments::is_none()

v1.0.103

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• Implement PartialOrd for Cursor (#​1236, #​1237, thanks @​CAD97)

v1.0.102

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• More efficient internal representation for TokenBuffer (#​1223, thanks @​CAD97)
• Fix parsing of a left shift after macro metavariable in type position (#​1229)

v1.0.101

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• Eliminate a bunch of redundant work done by LitStr::parse (#​1221)

v1.0.100

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• Parse closures with explicit empty lifetime parameter list for<> || -> T {…} (#​1212, rust-lang/rust#97362)
• Parse dyn* provisional syntax (#​1213, rust-lang/rust#91611)
• Parse attributes on the "rest" pattern of a struct in pattern position, Struct { #[attr] .. } (#​1214)
• Parse static async and static async move closures (#​1215, rust-lang/rust#62290)

v1.0.99

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• Add categories and keywords to crates.io metadata

v1.0.98

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• Format example code with rustfmt

v1.0.97

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• Update examples

v1.0.96

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• Add a punct_mut() method on syn::punctuated::Pair to return Option<&mut P> (#​1183)

v1.0.95

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• Replace unicode-xid with unicode-ident crate: https://github.com/dtolnay/unicode-ident

v1.0.94

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• Resolve some unused_macro_rules warnings

v1.0.93

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• Fix compile error when Some means something unusual in the expression namespace of the scope where custom_keyword is invoked (#​1171, thanks @​zakarumych)

v1.0.92

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• Make compatible with Miri's tag-raw-pointers mode (#​1166, thanks @​saethlin)

v1.0.91

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• impl ToTokens for RangeLimits
• impl Parse for ExprAwait
• impl Parse for GenericMethodArgument
• impl Parse for MethodTurbofish

[v1.0.90](https://redirect.github.com/dto