Enum syn::Expr

pub enum Expr {
    Array(ExprArray),
    Assign(ExprAssign),
    AssignOp(ExprAssignOp),
    Async(ExprAsync),
    Await(ExprAwait),
    Binary(ExprBinary),
    Block(ExprBlock),
    Box(ExprBox),
    Break(ExprBreak),
    Call(ExprCall),
    Cast(ExprCast),
    Closure(ExprClosure),
    Continue(ExprContinue),
    Field(ExprField),
    ForLoop(ExprForLoop),
    Group(ExprGroup),
    If(ExprIf),
    Index(ExprIndex),
    Let(ExprLet),
    Lit(ExprLit),
    Loop(ExprLoop),
    Macro(ExprMacro),
    Match(ExprMatch),
    MethodCall(ExprMethodCall),
    Paren(ExprParen),
    Path(ExprPath),
    Range(ExprRange),
    Reference(ExprReference),
    Repeat(ExprRepeat),
    Return(ExprReturn),
    Struct(ExprStruct),
    Try(ExprTry),
    TryBlock(ExprTryBlock),
    Tuple(ExprTuple),
    Type(ExprType),
    Unary(ExprUnary),
    Unsafe(ExprUnsafe),
    Verbatim(TokenStream),
    While(ExprWhile),
    Yield(ExprYield),
    // some variants omitted
}

A Rust expression.

This type is available only if Syn is built with the "derive" or "full" feature, but most of the variants are not available unless “full” is enabled.

Syntax tree enums

This type is a syntax tree enum. In Syn this and other syntax tree enums are designed to be traversed using the following rebinding idiom.

let expr: Expr = /* ... */;
match expr {
    Expr::MethodCall(expr) => {
        /* ... */
    }
    Expr::Cast(expr) => {
        /* ... */
    }
    Expr::If(expr) => {
        /* ... */
    }

    /* ... */

We begin with a variable expr of type Expr that has no fields (because it is an enum), and by matching on it and rebinding a variable with the same name expr we effectively imbue our variable with all of the data fields provided by the variant that it turned out to be. So for example above if we ended up in the MethodCall case then we get to use expr.receiver, expr.args etc; if we ended up in the If case we get to use expr.cond, expr.then_branch, expr.else_branch.

This approach avoids repeating the variant names twice on every line.

// Repetitive; recommend not doing this.
match expr {
    Expr::MethodCall(ExprMethodCall { method, args, .. }) => {

In general, the name to which a syntax tree enum variant is bound should be a suitable name for the complete syntax tree enum type.

// Binding is called `base` which is the name I would use if I were
// assigning `*discriminant.base` without an `if let`.
if let Expr::Tuple(base) = *discriminant.base {

A sign that you may not be choosing the right variable names is if you see names getting repeated in your code, like accessing receiver.receiver or pat.pat or cond.cond.

Variants

Array

A slice literal expression: [a, b, c, d].

Tuple Fields of Array

0: ExprArray

Assign

An assignment expression: a = compute().

Tuple Fields of Assign

0: ExprAssign

AssignOp

A compound assignment expression: counter += 1.

Tuple Fields of AssignOp

0: ExprAssignOp

Async

An async block: async { ... }.

Tuple Fields of Async

0: ExprAsync

Await

An await expression: fut.await.

Tuple Fields of Await

0: ExprAwait

Binary

A binary operation: a + b, a * b.

Tuple Fields of Binary

0: ExprBinary

Block

A blocked scope: { ... }.

Tuple Fields of Block

0: ExprBlock

Box

A box expression: box f.

Tuple Fields of Box

0: ExprBox

Break

A break, with an optional label to break and an optional expression.

Tuple Fields of Break

0: ExprBreak

Call

A function call expression: invoke(a, b).

Tuple Fields of Call

0: ExprCall

Cast

A cast expression: foo as f64.

Tuple Fields of Cast

0: ExprCast

Closure

A closure expression: |a, b| a + b.

Tuple Fields of Closure

0: ExprClosure

Continue

A continue, with an optional label.

Tuple Fields of Continue

0: ExprContinue

Field

Access of a named struct field (obj.k) or unnamed tuple struct field (obj.0).

Tuple Fields of Field

0: ExprField

ForLoop

A for loop: for pat in expr { ... }.

Tuple Fields of ForLoop

0: ExprForLoop

Group

An expression contained within invisible delimiters.

This variant is important for faithfully representing the precedence of expressions and is related to None-delimited spans in a TokenStream.

Tuple Fields of Group

0: ExprGroup

If

An if expression with an optional else block: if expr { ... } else { ... }.

The else branch expression may only be an If or Block expression, not any of the other types of expression.

Tuple Fields of If

0: ExprIf

Index

A square bracketed indexing expression: vector[2].

Tuple Fields of Index

0: ExprIndex

Let

A let guard: let Some(x) = opt.

Tuple Fields of Let

0: ExprLet

Lit

A literal in place of an expression: 1, "foo".

Tuple Fields of Lit

0: ExprLit

Loop

Conditionless loop: loop { ... }.

Tuple Fields of Loop

0: ExprLoop

Macro

A macro invocation expression: format!("{}", q).

Tuple Fields of Macro

0: ExprMacro

Match

A match expression: match n { Some(n) => {}, None => {} }.

Tuple Fields of Match

0: ExprMatch

MethodCall

A method call expression: x.foo::<T>(a, b).

Tuple Fields of MethodCall

0: ExprMethodCall

Paren

A parenthesized expression: (a + b).

Tuple Fields of Paren

0: ExprParen

Path

A path like std::mem::replace possibly containing generic parameters and a qualified self-type.

A plain identifier like x is a path of length 1.

Tuple Fields of Path

0: ExprPath

Range

A range expression: 1..2, 1.., ..2, 1..=2, ..=2.

Tuple Fields of Range

0: ExprRange

Reference

A referencing operation: &a or &mut a.

Tuple Fields of Reference

0: ExprReference

Repeat

An array literal constructed from one repeated element: [0u8; N].

Tuple Fields of Repeat

0: ExprRepeat

Return

A return, with an optional value to be returned.

Tuple Fields of Return

0: ExprReturn

Struct

A struct literal expression: Point { x: 1, y: 1 }.

The rest provides the value of the remaining fields as in S { a: 1, b: 1, ..rest }.

Tuple Fields of Struct

0: ExprStruct

Try

A try-expression: expr?.

Tuple Fields of Try

0: ExprTry

TryBlock

A try block: try { ... }.

Tuple Fields of TryBlock

0: ExprTryBlock

Tuple

A tuple expression: (a, b, c, d).

Tuple Fields of Tuple

0: ExprTuple

Type

A type ascription expression: foo: f64.

Tuple Fields of Type

0: ExprType

Unary

A unary operation: !x, *x.

Tuple Fields of Unary

0: ExprUnary

Unsafe

An unsafe block: unsafe { ... }.

Tuple Fields of Unsafe

0: ExprUnsafe

Verbatim

Tokens in expression position not interpreted by Syn.

Tuple Fields of Verbatim

0: TokenStream

While

A while loop: while expr { ... }.

Tuple Fields of While

0: ExprWhile

Yield

A yield expression: yield expr.

Tuple Fields of Yield

0: ExprYield

Implementations

impl Expr

pub fn parse_without_eager_brace(input: ParseStream<'_>) -> Result<Expr>

An alternative to the primary Expr::parse parser (from the Parse trait) for ambiguous syntactic positions in which a trailing brace should not be taken as part of the expression.

Rust grammar has an ambiguity where braces sometimes turn a path expression into a struct initialization and sometimes do not. In the following code, the expression S {} is one expression. Presumably there is an empty struct struct S {} defined somewhere which it is instantiating.

let _ = *S {};

// parsed by rustc as: `*(S {})`

We would want to parse the above using Expr::parse after the = token.

But in the following, S {} is not a struct init expression.

if *S {} {}

// parsed by rustc as:
//
//    if (*S) {
//        /* empty block */
//    }
//    {
//        /* another empty block */
//    }

For that reason we would want to parse if-conditions using Expr::parse_without_eager_brace after the if token. Same for similar syntactic positions such as the condition expr after a while token or the expr at the top of a match.

The Rust grammar’s choices around which way this ambiguity is resolved at various syntactic positions is fairly arbitrary. Really either parse behavior could work in most positions, and language designers just decide each case based on which is more likely to be what the programmer had in mind most of the time.

if return S {} {}

// parsed by rustc as:
//
//    if (return (S {})) {
//    }
//
// but could equally well have been this other arbitrary choice:
//
//    if (return S) {
//    }
//    {}

Note the grammar ambiguity on trailing braces is distinct from precedence and is not captured by assigning a precedence level to the braced struct init expr in relation to other operators. This can be illustrated by return 0..S {} vs match 0..S {}. The former parses as return (0..(S {})) implying tighter precedence for struct init than .., while the latter parses as match (0..S) {} implying tighter precedence for .. than struct init, a contradiction.

Trait Implementations

impl Clone for Expr

fn clone(&self) -> Self

Returns a copy of the value.

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

Performs copy-assignment from source.

impl Debug for Expr

fn fmt(&self, formatter: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl From<ExprArray> for Expr

fn from(e: ExprArray) -> Expr

Performs the conversion.

impl From<ExprAssign> for Expr

fn from(e: ExprAssign) -> Expr

Performs the conversion.

impl From<ExprAssignOp> for Expr

fn from(e: ExprAssignOp) -> Expr

Performs the conversion.

impl From<ExprAsync> for Expr

fn from(e: ExprAsync) -> Expr

Performs the conversion.

impl From<ExprAwait> for Expr

fn from(e: ExprAwait) -> Expr

Performs the conversion.

impl From<ExprBinary> for Expr

fn from(e: ExprBinary) -> Expr

Performs the conversion.

impl From<ExprBlock> for Expr

fn from(e: ExprBlock) -> Expr

Performs the conversion.

impl From<ExprBox> for Expr

fn from(e: ExprBox) -> Expr

Performs the conversion.

impl From<ExprBreak> for Expr

fn from(e: ExprBreak) -> Expr

Performs the conversion.

impl From<ExprCall> for Expr

fn from(e: ExprCall) -> Expr

Performs the conversion.

impl From<ExprCast> for Expr

fn from(e: ExprCast) -> Expr

Performs the conversion.

impl From<ExprClosure> for Expr

fn from(e: ExprClosure) -> Expr

Performs the conversion.

impl From<ExprContinue> for Expr

fn from(e: ExprContinue) -> Expr

Performs the conversion.

impl From<ExprField> for Expr

fn from(e: ExprField) -> Expr

Performs the conversion.

impl From<ExprForLoop> for Expr

fn from(e: ExprForLoop) -> Expr

Performs the conversion.

impl From<ExprGroup> for Expr

fn from(e: ExprGroup) -> Expr

Performs the conversion.

impl From<ExprIf> for Expr

fn from(e: ExprIf) -> Expr

Performs the conversion.

impl From<ExprIndex> for Expr

fn from(e: ExprIndex) -> Expr

Performs the conversion.

impl From<ExprLet> for Expr

fn from(e: ExprLet) -> Expr

Performs the conversion.

impl From<ExprLit> for Expr

fn from(e: ExprLit) -> Expr

Performs the conversion.

impl From<ExprLoop> for Expr

fn from(e: ExprLoop) -> Expr

Performs the conversion.

impl From<ExprMacro> for Expr

fn from(e: ExprMacro) -> Expr

Performs the conversion.

impl From<ExprMatch> for Expr

fn from(e: ExprMatch) -> Expr

Performs the conversion.

impl From<ExprMethodCall> for Expr

fn from(e: ExprMethodCall) -> Expr

Performs the conversion.

impl From<ExprParen> for Expr

fn from(e: ExprParen) -> Expr

Performs the conversion.

impl From<ExprPath> for Expr

fn from(e: ExprPath) -> Expr

Performs the conversion.

impl From<ExprRange> for Expr

fn from(e: ExprRange) -> Expr

Performs the conversion.

impl From<ExprReference> for Expr

fn from(e: ExprReference) -> Expr

Performs the conversion.

impl From<ExprRepeat> for Expr

fn from(e: ExprRepeat) -> Expr

Performs the conversion.

impl From<ExprReturn> for Expr

fn from(e: ExprReturn) -> Expr

Performs the conversion.

impl From<ExprStruct> for Expr

fn from(e: ExprStruct) -> Expr

Performs the conversion.

impl From<ExprTry> for Expr

fn from(e: ExprTry) -> Expr

Performs the conversion.

impl From<ExprTryBlock> for Expr

fn from(e: ExprTryBlock) -> Expr

Performs the conversion.

impl From<ExprTuple> for Expr

fn from(e: ExprTuple) -> Expr

Performs the conversion.

impl From<ExprType> for Expr

fn from(e: ExprType) -> Expr

Performs the conversion.

impl From<ExprUnary> for Expr

fn from(e: ExprUnary) -> Expr

Performs the conversion.

impl From<ExprUnsafe> for Expr

fn from(e: ExprUnsafe) -> Expr

Performs the conversion.

impl From<ExprWhile> for Expr

fn from(e: ExprWhile) -> Expr

Performs the conversion.

impl From<ExprYield> for Expr

fn from(e: ExprYield) -> Expr

Performs the conversion.

impl Hash for Expr

fn hash<H>(&self, state: &mut H) where
    H: Hasher, 

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H) where
    H: Hasher, 

Feeds a slice of this type into the given Hasher.

impl Parse for Expr

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

impl PartialEq<Expr> for Expr

fn eq(&self, other: &Self) -> bool

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

This method tests for !=.

impl ToTokens for Expr

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

Convert self directly into a TokenStream object.

impl Eq for Expr