Enum syn::Expr[][src]

pub enum Expr {
Show 40 variants 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
}
This is supported on crate features full or derive only.
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

Array(ExprArray)

Tuple Fields

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

Assign(ExprAssign)

Tuple Fields

An assignment expression: a = compute().

AssignOp(ExprAssignOp)

Tuple Fields

A compound assignment expression: counter += 1.

Async(ExprAsync)

Tuple Fields

An async block: async { ... }.

Await(ExprAwait)

Tuple Fields

An await expression: fut.await.

Binary(ExprBinary)

Tuple Fields

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

Block(ExprBlock)

Tuple Fields

A blocked scope: { ... }.

Box(ExprBox)

Tuple Fields

A box expression: box f.

Break(ExprBreak)

Tuple Fields

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

Call(ExprCall)

Tuple Fields

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

Cast(ExprCast)

Tuple Fields

A cast expression: foo as f64.

Closure(ExprClosure)

Tuple Fields

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

Continue(ExprContinue)

Tuple Fields

A continue, with an optional label.

Field(ExprField)

Tuple Fields

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

ForLoop(ExprForLoop)

Tuple Fields

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

Group(ExprGroup)

Tuple Fields

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)

Tuple Fields

0: 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)

Tuple Fields

A square bracketed indexing expression: vector[2].

Let(ExprLet)

Tuple Fields

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

Lit(ExprLit)

Tuple Fields

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

Loop(ExprLoop)

Tuple Fields

Conditionless loop: loop { ... }.

Macro(ExprMacro)

Tuple Fields

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

Match(ExprMatch)

Tuple Fields

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

MethodCall(ExprMethodCall)

Tuple Fields

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

Paren(ExprParen)

Tuple Fields

A parenthesized expression: (a + b).

Path(ExprPath)

Tuple Fields

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)

Tuple Fields

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

Reference(ExprReference)

Tuple Fields

A referencing operation: &a or &mut a.

Repeat(ExprRepeat)

Tuple Fields

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

Return(ExprReturn)

Tuple Fields

A return, with an optional value to be returned.

Struct(ExprStruct)

Tuple Fields

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)

Tuple Fields

A try-expression: expr?.

TryBlock(ExprTryBlock)

Tuple Fields

A try block: try { ... }.

Tuple(ExprTuple)

Tuple Fields

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

Type(ExprType)

Tuple Fields

A type ascription expression: foo: f64.

Unary(ExprUnary)

Tuple Fields

A unary operation: !x, *x.

Unsafe(ExprUnsafe)

Tuple Fields

An unsafe block: unsafe { ... }.

Verbatim(TokenStream)

Tuple Fields

Tokens in expression position not interpreted by Syn.

While(ExprWhile)

Tuple Fields

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

Yield(ExprYield)

Tuple Fields

A yield expression: yield expr.

Implementations

This is supported on crate features full and parsing only.

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

Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

Formats the value using the given formatter. Read more

Performs the conversion.

Performs the conversion.

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Feeds this value into the given Hasher. Read more

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

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

This method tests for !=.

Write self to the given TokenStream. Read more

Convert self directly into a TokenStream object. Read more

Convert self directly into a TokenStream object. Read more

Auto Trait Implementations

Blanket Implementations

Gets the TypeId of self. Read more

Immutably borrows from an owned value. Read more

Mutably borrows from an owned value. Read more

Performs the conversion.

Performs the conversion.

This is supported on crate features parsing and printing only.

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

The resulting type after obtaining ownership.

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

🔬 This is a nightly-only experimental API. (toowned_clone_into)

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

The type returned in the event of a conversion error.

Performs the conversion.

The type returned in the event of a conversion error.

Performs the conversion.