循环可以是表达式吗？

Question

我正在阅读 Rust 的内部结构，https://github.com/rust-lang/rust/blob/master/compiler/rustc_hir/src/hir.rs 它有

pub enum ExprKind<'hir> {
    /// A `box x` expression.
    Box(&'hir Expr<'hir>),
    /// Allow anonymous constants from an inline `const` block
    ConstBlock(AnonConst),
    /// An array (e.g., `[a, b, c, d]`).
    Array(&'hir [Expr<'hir>]),
    /// A function call.
    ///
    /// The first field resolves to the function itself (usually an `ExprKind::Path`),
    /// and the second field is the list of arguments.
    /// This also represents calling the constructor of
    /// tuple-like ADTs such as tuple structs and enum variants.
    Call(&'hir Expr<'hir>, &'hir [Expr<'hir>]),
    /// A method call (e.g., `x.foo::<'static, Bar, Baz>(a, b, c, d)`).
    ///
    /// The `PathSegment`/`Span` represent the method name and its generic arguments
    /// (within the angle brackets).
    /// The first element of the vector of `Expr`s is the expression that evaluates
    /// to the object on which the method is being called on (the receiver),
    /// and the remaining elements are the rest of the arguments.
    /// Thus, `x.foo::<Bar, Baz>(a, b, c, d)` is represented as
    /// `ExprKind::MethodCall(PathSegment { foo, [Bar, Baz] }, [x, a, b, c, d])`.
    /// The final `Span` represents the span of the function and arguments
    /// (e.g. `foo::<Bar, Baz>(a, b, c, d)` in `x.foo::<Bar, Baz>(a, b, c, d)`
    ///
    /// To resolve the called method to a `DefId`, call [`type_dependent_def_id`] with
    /// the `hir_id` of the `MethodCall` node itself.
    ///
    /// [`type_dependent_def_id`]: ../ty/struct.TypeckResults.html#method.type_dependent_def_id
    MethodCall(&'hir PathSegment<'hir>, Span, &'hir [Expr<'hir>], Span),
    /// A tuple (e.g., `(a, b, c, d)`).
    Tup(&'hir [Expr<'hir>]),
    /// A binary operation (e.g., `a + b`, `a * b`).
    Binary(BinOp, &'hir Expr<'hir>, &'hir Expr<'hir>),
    /// A unary operation (e.g., `!x`, `*x`).
    Unary(UnOp, &'hir Expr<'hir>),
    /// A literal (e.g., `1`, `"foo"`).
    Lit(Lit),
    /// A cast (e.g., `foo as f64`).
    Cast(&'hir Expr<'hir>, &'hir Ty<'hir>),

它看起来像是一个枚举，其中包含被视为表达式的内容。表达式是可以被评估为一个值的东西。所以函数调用、闭包、MethodCall 等都可以看作是表达式。但是这个枚举有 Loop 之类的东西。可以对循环进行评估吗？数组呢？结构？

Answer 1

Rust 是一种面向表达式的语言，所以很多在其他语言中传统上被认为是语句的东西实际上是表达式，比如大多数控制流。例如，循环是表达式 See the rust book for more details。但是，在各种形式的循环中，只有loop表达式可以有!或()以外的值，因为所有其他循环表达式都有隐式中断条件；

对于array和struct，这些代表对应的字面值。所以数组表达式 kind 指的是数组文字： [1, 2, 3] 数组文字值是指定的那些值的数组。结构体也是如此。

循环可以是表达式吗？

Can a loop be an expression?

syntax

rust