Enum syn::Pat

#[non_exhaustive]
pub enum Pat {
    Const(PatConst),
    Ident(PatIdent),
    Lit(PatLit),
    Macro(PatMacro),
    Or(PatOr),
    Paren(PatParen),
    Path(PatPath),
    Range(PatRange),
    Reference(PatReference),
    Rest(PatRest),
    Slice(PatSlice),
    Struct(PatStruct),
    Tuple(PatTuple),
    TupleStruct(PatTupleStruct),
    Type(PatType),
    Verbatim(TokenStream),
    Wild(PatWild),
}

A pattern in a local binding, function signature, match expression, or various other places.

Syntax tree enum

This type is a syntax tree enum.

Variants (Non-exhaustive)

Non-exhaustive enums could have additional variants added in future. Therefore, when matching against variants of non-exhaustive enums, an extra wildcard arm must be added to account for any future variants.

Const(PatConst)

A const block: const { ... }.

Ident(PatIdent)

A pattern that binds a new variable: ref mut binding @ SUBPATTERN.

Lit(PatLit)

A literal pattern: 0.

Macro(PatMacro)

A macro in pattern position.

Or(PatOr)

A pattern that matches any one of a set of cases.

Paren(PatParen)

A parenthesized pattern: (A | B).

Path(PatPath)

A path pattern like Color::Red, optionally qualified with a self-type.

Unqualified path patterns can legally refer to variants, structs, constants or associated constants. Qualified path patterns like <A>::B::C and <A as Trait>::B::C can only legally refer to associated constants.

Range(PatRange)

A range pattern: 1..=2.

Reference(PatReference)

A reference pattern: &mut var.

Rest(PatRest)

The dots in a tuple or slice pattern: [0, 1, ..].

Slice(PatSlice)

A dynamically sized slice pattern: [a, b, ref i @ .., y, z].

Struct(PatStruct)

A struct or struct variant pattern: Variant { x, y, .. }.

Tuple(PatTuple)

A tuple pattern: (a, b).

TupleStruct(PatTupleStruct)

A tuple struct or tuple variant pattern: Variant(x, y, .., z).

Type(PatType)

A type ascription pattern: foo: f64.

Verbatim(TokenStream)

Tokens in pattern position not interpreted by Syn.

Wild(PatWild)

A pattern that matches any value: _.

Implementations

impl Pat

pub fn parse_single(input: ParseStream<'_>) -> Result<Self>

Parse a pattern that does not involve | at the top level.

This parser matches the behavior of the $:pat_param macro_rules matcher, and on editions prior to Rust 2021, the behavior of $:pat.

In Rust syntax, some examples of where this syntax would occur are in the argument pattern of functions and closures. Patterns using | are not allowed to occur in these positions.

fn f(Some(_) | None: Option<T>) {
    let _ = |Some(_) | None: Option<T>| {};
    //       ^^^^^^^^^^^^^^^^^^^^^^^^^??? :(
}

error: top-level or-patterns are not allowed in function parameters
 --> src/main.rs:1:6
  |
1 | fn f(Some(_) | None: Option<T>) {
  |      ^^^^^^^^^^^^^^ help: wrap the pattern in parentheses: `(Some(_) | None)`

pub fn parse_multi(input: ParseStream<'_>) -> Result<Self>

Parse a pattern, possibly involving |, but not a leading |.

pub fn parse_multi_with_leading_vert(input: ParseStream<'_>) -> Result<Self>

Parse a pattern, possibly involving |, possibly including a leading |.

This parser matches the behavior of the Rust 2021 edition’s $:pat macro_rules matcher.

In Rust syntax, an example of where this syntax would occur is in the pattern of a match arm, where the language permits an optional leading |, although it is not idiomatic to write one there in handwritten code.

match wat {
    | None | Some(false) => {}
    | Some(true) => {}
}

The compiler accepts it only to facilitate some situations in macro-generated code where a macro author might need to write:

match $value {
    $(| $conditions1)* $(| $conditions2)* => $then
}

Expressing the same thing correctly in the case that either one (but not both) of $conditions1 and $conditions2 might be empty, without leading |, is complex.

Use Pat::parse_multi instead if you are not intending to support macro-generated macro input.

Trait Implementations

impl Clone for Pat

fn clone(&self) -> Self

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl From<ExprConst> for Pat

fn from(e: PatConst) -> Pat

Converts to this type from the input type.

impl From<ExprLit> for Pat

fn from(e: PatLit) -> Pat

Converts to this type from the input type.

impl From<ExprMacro> for Pat

fn from(e: PatMacro) -> Pat

Converts to this type from the input type.

impl From<ExprPath> for Pat

fn from(e: PatPath) -> Pat

Converts to this type from the input type.

impl From<ExprRange> for Pat

fn from(e: PatRange) -> Pat

Converts to this type from the input type.

impl From<PatIdent> for Pat

fn from(e: PatIdent) -> Pat

Converts to this type from the input type.

impl From<PatOr> for Pat

fn from(e: PatOr) -> Pat

Converts to this type from the input type.

impl From<PatParen> for Pat

fn from(e: PatParen) -> Pat

Converts to this type from the input type.

impl From<PatReference> for Pat

fn from(e: PatReference) -> Pat

Converts to this type from the input type.

impl From<PatRest> for Pat

fn from(e: PatRest) -> Pat

Converts to this type from the input type.

impl From<PatSlice> for Pat

fn from(e: PatSlice) -> Pat

Converts to this type from the input type.

impl From<PatStruct> for Pat

fn from(e: PatStruct) -> Pat

Converts to this type from the input type.

impl From<PatTuple> for Pat

fn from(e: PatTuple) -> Pat

Converts to this type from the input type.

impl From<PatTupleStruct> for Pat

fn from(e: PatTupleStruct) -> Pat

Converts to this type from the input type.

impl From<PatType> for Pat

fn from(e: PatType) -> Pat

Converts to this type from the input type.

impl From<PatWild> for Pat

fn from(e: PatWild) -> Pat

Converts to this type from the input type.

impl ToTokens for Pat

fn to_tokens(&self, tokens: &mut TokenStream)

Write self to the given TokenStream.

fn to_token_stream(&self) -> TokenStream

Convert self directly into a TokenStream object.

fn into_token_stream(self) -> TokenStream

where Self: Sized,

Convert self directly into a TokenStream object.