# Expr in syn - Rust
A Rust expression.

syn

# Enum Expr

Source

    #[non_exhaustive]pub enum Expr {
    Show 40 variants    Array(ExprArray),
        Assign(ExprAssign),
        Async(ExprAsync),
        Await(ExprAwait),
        Binary(ExprBinary),
        Block(ExprBlock),
        Break(ExprBreak),
        Call(ExprCall),
        Cast(ExprCast),
        Closure(ExprClosure),
        Const(ExprConst),
        Continue(ExprContinue),
        Field(ExprField),
        ForLoop(ExprForLoop),
        Group(ExprGroup),
        If(ExprIf),
        Index(ExprIndex),
        Infer(ExprInfer),
        Let(ExprLet),
        Lit(ExprLit),
        Loop(ExprLoop),
        Macro(ExprMacro),
        Match(ExprMatch),
        MethodCall(ExprMethodCall),
        Paren(ExprParen),
        Path(ExprPath),
        Range(ExprRange),
        RawAddr(ExprRawAddr),
        Reference(ExprReference),
        Repeat(ExprRepeat),
        Return(ExprReturn),
        Struct(ExprStruct),
        Try(ExprTry),
        TryBlock(ExprTryBlock),
        Tuple(ExprTuple),
        Unary(ExprUnary),
        Unsafe(ExprUnsafe),
        Verbatim(TokenStream),
        While(ExprWhile),
        Yield(ExprYield),
    }

Expand description

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 (Non-exhaustive)§

This enum is marked as 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.
§

### Array(ExprArray)

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

§

### Assign(ExprAssign)

An assignment expression: `a = compute()`.

§

### Async(ExprAsync)

An async block: `async { ... }`.

§

### Await(ExprAwait)

An await expression: `fut.await`.

§

### Binary(ExprBinary)

A binary operation: `a + b`, `a += b`.

§

### Block(ExprBlock)

A blocked scope: `{ ... }`.

§

### Break(ExprBreak)

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

§

### Call(ExprCall)

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

§

### Cast(ExprCast)

A cast expression: `foo as f64`.

§

### Closure(ExprClosure)

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

§

### Const(ExprConst)

A const block: `const { ... }`.

§

### Continue(ExprContinue)

A `continue`, with an optional label.

§

### Field(ExprField)

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

§

### ForLoop(ExprForLoop)

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

§

### Group(ExprGroup)

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`.

§

### If(ExprIf)

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.

§

### Index(ExprIndex)

A square bracketed indexing expression: `vector[2]`.

§

### Infer(ExprInfer)

The inferred value of a const generic argument, denoted `_`.

§

### Let(ExprLet)

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

§

### Lit(ExprLit)

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

§

### Loop(ExprLoop)

Conditionless loop: `loop { ... }`.

§

### Macro(ExprMacro)

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

§

### Match(ExprMatch)

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

§

### MethodCall(ExprMethodCall)

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

§

### Paren(ExprParen)

A parenthesized expression: `(a + b)`.

§

### Path(ExprPath)

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.

§

### Range(ExprRange)

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

§

### RawAddr(ExprRawAddr)

Address-of operation: `&raw const place` or `&raw mut place`.

§

### Reference(ExprReference)

A referencing operation: `&a` or `&mut a`.

§

### Repeat(ExprRepeat)

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

§

### Return(ExprReturn)

A `return`, with an optional value to be returned.

§

### Struct(ExprStruct)

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 }`.

§

### Try(ExprTry)

A try-expression: `expr?`.

§

### TryBlock(ExprTryBlock)

A try block: `try { ... }`.

§

### Tuple(ExprTuple)

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

§

### Unary(ExprUnary)

A unary operation: `!x`, `*x`.

§

### Unsafe(ExprUnsafe)

An unsafe block: `unsafe { ... }`.

§

### Verbatim(TokenStream)

Tokens in expression position not interpreted by Syn.

§

### While(ExprWhile)

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

§

### Yield(ExprYield)

A yield expression: `yield expr`.

## Implementations§

Source §

### impl Expr

Source

#### pub const PLACEHOLDER: Self

An unspecified invalid expression.

    use quote::ToTokens;
    use std::mem;
    use syn::{parse_quote, Expr};

    fn unparenthesize(e: &mut Expr) {
        while let Expr::Paren(paren) = e {
            *e = mem::replace(&mut *paren.expr, Expr::PLACEHOLDER);
        }
    }

    fn main() {
        let mut e: Expr = parse_quote! { ((1 + 1)) };
        unparenthesize(&mut e);
        assert_eq!("1 + 1", e.to_token_stream().to_string());
    }

Source

#### 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.

Source

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

An alternative to the primary `Expr::parse` parser (from the `Parse` trait) for syntactic positions in which expression boundaries are placed more eagerly than done by the typical expression grammar. This includes expressions at the head of a statement or in the right-hand side of a `match` arm.

Compare the following cases:

    let _ = match result {
        () if guard => if cond { f } else { g }
        () => false,
    };

    let _ = || {
        if cond { f } else { g }
        ()
    };

    let _ = [if cond { f } else { g } ()];

The same sequence of tokens `if cond { f } else { g } ()` appears in expression position 3 times. The first two syntactic positions use eager placement of expression boundaries, and parse as `Expr::If`, with the adjacent `()` becoming `Pat::Tuple` or `Expr::Tuple`. In contrast, the third case uses standard expression boundaries and parses as `Expr::Call`.

As with `parse_without_eager_brace`, this ambiguity in the Rust grammar is independent of precedence.

Source

#### pub fn peek(input: ParseStream<'_>) -> bool

Returns whether the next token in the parse stream is one that might possibly form the beginning of an expr.

This classification is a load-bearing part of the grammar of some Rust expressions, notably `return` and `break`. For example `return < …` will never parse `<` as a binary operator regardless of what comes after, because `<` is a legal starting token for an expression and so it’s required to be continued as a return value, such as `return <Struct as Trait>::CONST`. Meanwhile `return > …` treats the `>` as a binary operator because it cannot be a starting token for any Rust expression.

## Trait Implementations§

Source §

### impl Clone for Expr

Source §

#### fn clone(&self) -> Self

Returns a copy of the value. Read more
1.0.0 · Source §

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

Performs copy-assignment from `source`. Read more
Source §

### impl Debug for Expr

Source §

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

Formats the value using the given formatter. Read more
Source §

### impl From<ExprArray> for Expr

Source §

#### fn from(e: ExprArray) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprAssign> for Expr

Source §

#### fn from(e: ExprAssign) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprAsync> for Expr

Source §

#### fn from(e: ExprAsync) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprAwait> for Expr

Source §

#### fn from(e: ExprAwait) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprBinary> for Expr

Source §

#### fn from(e: ExprBinary) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprBlock> for Expr

Source §

#### fn from(e: ExprBlock) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprBreak> for Expr

Source §

#### fn from(e: ExprBreak) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprCall> for Expr

Source §

#### fn from(e: ExprCall) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprCast> for Expr

Source §

#### fn from(e: ExprCast) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprClosure> for Expr

Source §

#### fn from(e: ExprClosure) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprConst> for Expr

Source §

#### fn from(e: ExprConst) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprContinue> for Expr

Source §

#### fn from(e: ExprContinue) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprField> for Expr

Source §

#### fn from(e: ExprField) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprForLoop> for Expr

Source §

#### fn from(e: ExprForLoop) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprGroup> for Expr

Source §

#### fn from(e: ExprGroup) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprIf> for Expr

Source §

#### fn from(e: ExprIf) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprIndex> for Expr

Source §

#### fn from(e: ExprIndex) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprInfer> for Expr

Source §

#### fn from(e: ExprInfer) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprLet> for Expr

Source §

#### fn from(e: ExprLet) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprLit> for Expr

Source §

#### fn from(e: ExprLit) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprLoop> for Expr

Source §

#### fn from(e: ExprLoop) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprMacro> for Expr

Source §

#### fn from(e: ExprMacro) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprMatch> for Expr

Source §

#### fn from(e: ExprMatch) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprMethodCall> for Expr

Source §

#### fn from(e: ExprMethodCall) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprParen> for Expr

Source §

#### fn from(e: ExprParen) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprPath> for Expr

Source §

#### fn from(e: ExprPath) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprRange> for Expr

Source §

#### fn from(e: ExprRange) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprRawAddr> for Expr

Source §

#### fn from(e: ExprRawAddr) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprReference> for Expr

Source §

#### fn from(e: ExprReference) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprRepeat> for Expr

Source §

#### fn from(e: ExprRepeat) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprReturn> for Expr

Source §

#### fn from(e: ExprReturn) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprStruct> for Expr

Source §

#### fn from(e: ExprStruct) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprTry> for Expr

Source §

#### fn from(e: ExprTry) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprTryBlock> for Expr

Source §

#### fn from(e: ExprTryBlock) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprTuple> for Expr

Source §

#### fn from(e: ExprTuple) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprUnary> for Expr

Source §

#### fn from(e: ExprUnary) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprUnsafe> for Expr

Source §

#### fn from(e: ExprUnsafe) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprWhile> for Expr

Source §

#### fn from(e: ExprWhile) -> Expr

Converts to this type from the input type.
Source §

### impl From<ExprYield> for Expr

Source §

#### fn from(e: ExprYield) -> Expr

Converts to this type from the input type.
Source §

### impl Hash for Expr

Source §

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

Feeds this value into the given `Hasher`. Read more
1.3.0 · Source §

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

Feeds a slice of this type into the given `Hasher`. Read more
Source §

### impl Parse for Expr

Source §

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

Source §

### impl PartialEq for Expr

Source §

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

Tests for `self` and `other` values to be equal, and is used by `==`.
1.0.0 · Source §

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

Tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason.
Source §

### impl ToTokens for Expr

Source §

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

Write `self` to the given `TokenStream`. Read more
Source §

#### fn to_token_stream(&self) -> TokenStream

Convert `self` directly into a `TokenStream` object. Read more
Source §

#### fn into_token_stream(self) -> TokenStream
where Self: Sized,

Convert `self` directly into a `TokenStream` object. Read more
Source §

### impl Eq for Expr

## Auto Trait Implementations§

§

### impl Freeze for Expr

§

### impl RefUnwindSafe for Expr

§

### impl !Send for Expr

§

### impl !Sync for Expr

§

### impl Unpin for Expr

§

### impl UnwindSafe for Expr

## Blanket Implementations§

Source §

### impl<T> Any for T
where T: 'static + ?Sized,

Source §

#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
Source §

### impl<T> Borrow<T> for T
where T: ?Sized,

Source §

#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
Source §

### impl<T> BorrowMut<T> for T
where T: ?Sized,

Source §

#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
Source §

### impl<T> CloneToUninit for T
where T: Clone,

Source §

#### unsafe fn clone_to_uninit(&self, dst: *mut u8)

🔬This is a nightly-only experimental API. (`clone_to_uninit`)
Performs copy-assignment from `self` to `dst`. Read more
Source §

### impl<T> From<T> for T

Source §

#### fn from(t: T) -> T

Returns the argument unchanged.

Source §

### impl<T, U> Into<U> for T
where U: From<T>,

Source §

#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

Source §

### impl<T> Spanned for T
where T: Spanned + ?Sized,

Source §

#### fn span(&self) -> Span

Returns a `Span` covering the complete contents of this syntax tree node, or `Span::call_site()` if this node is empty.
Source §

### impl<T> ToOwned for T
where T: Clone,

Source §

#### type Owned = T

The resulting type after obtaining ownership.
Source §

#### fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
Source §

#### fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
Source §

### impl<T, U> TryFrom<U> for T
where U: Into<T>,

Source §

#### type Error = Infallible

The type returned in the event of a conversion error.
Source §

#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
Source §

### impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

Source §

#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
Source §

#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.