rust - 未命名值的范围是什么？

Question

未命名的值何时超出范围，何时删除该值？

我正在寻找基于 official docs 的答案，并非基于实验。

示例 1:

f(foo().bar());

示例 2:

match foo().bar() {
    // ...
}

如果 bar 是 fn bar(self) -> ... 它获得传递值的所有权，并且它像往常一样被删除，但是如果 bar 借用，即 fn bar(&self) -> ...? bar 的结果是否取决于 &self 的生命周期是否重要？

也就是说，foo 可能会返回一个 MutexGuard；重要的是要知道守卫何时被丢弃(并且互斥体被解锁)。

实验方法表明，未命名值在创建它的语句“完成”后被删除；强制“提前”删除 let 语句是必需的。

Playground

#[derive(Debug)]
pub struct Foo;
pub fn foo() -> Foo {
    println!("foo()");
    Foo
}
impl Foo {
    pub fn bar(&self) {
    }
}
impl Drop for Foo {
    fn drop(&mut self) {
        println!("Foo::drop()");
    }
}

fn main() {
    println!("--- scope test start");
    println!("value: {:?}", foo().bar());
    println!("--- end");

    println!("--- scope test start");
    match foo().bar() {
        v => println!("value: {:?}", v),
    }
    println!("--- end");

    println!("--- scope test start");
    let v = foo().bar();
    println!("value: {:?}", v);
    println!("--- end");
}

打印:

--- scope test start
foo()
value: ()
Foo::drop()
--- end
--- scope test start
foo()
value: ()
Foo::drop()
--- end
--- scope test start
foo()
Foo::drop()
value: ()
--- end

Answer 1

来自 the reference :

When using an rvalue in most lvalue contexts, a temporary unnamed lvalue is created and used instead, if not promoted to 'static. Promotion of an rvalue expression to a 'static slot occurs when the expression could be written in a constant, borrowed, and dereferencing that borrow where the expression was the originally written, without changing the runtime behavior. That is, the promoted expression can be evaluated at compile-time and the resulting value does not contain interior mutability or destructors (these properties are determined based on the value where possible, e.g. &None always has the type &'static Option<_>, as it contains nothing disallowed). Otherwise, the lifetime of temporary values is typically

• the innermost enclosing statement; the tail expression of a block is considered part of the statement that encloses the block, or

• the condition expression or the loop conditional expression if the temporary is created in the condition expression of an if or an if/else or in the loop conditional expression of a while expression.

When a temporary rvalue is being created that is assigned into a let declaration, however, the temporary is created with the lifetime of the enclosing block instead, as using the enclosing statement (the let declaration) would be a guaranteed error (since a pointer to the temporary would be stored into a variable, but the temporary would be freed before the variable could be used). The compiler uses simple syntactic rules to decide which values are being assigned into a let binding, and therefore deserve a longer temporary lifetime.

引用文献中有这些规则的示例。