结构与枚举的生命周期差异

Question

Why does this work

#[derive(Debug)]
pub struct Foo<'a,'b> {
    s : &'a str,
    n : &'b i32
}
#[test]
fn test_struct() {
    let f = Foo { s : &"bar" , n : &17 };
    println!("{:?}",f);
}

but this doesn't

#[derive(Debug)]
pub enum Bar<'a,'b> {
    Baz ( &'a str),
    Fub ( &'b i32)
}
#[test]
fn test_struct() {
    let b = Bar::Baz(&"Foo");
    let c = Bar::Fub(&17);
    println!("{:?} {:?}",b,c);
}

The error is (part of a bigger file so ignore line numbers)

src\lib.rs:176:27: 176:29 error: borrowed value does not live long enough
src\lib.rs:176         let c = Bar::Fub(&17);
                       ^~~~~~~~~~~~~~~~~~~~~~

To me it seems like let c = Bar::Fub(&17) , the 17 lasts the same life time as the previous line where "Foo" is created on the stack. If I modify it slightly and do

let h = &17;
let c = Bar::Fub(&h);

In which case it's completely clear that h lasts longer than Bar::Fub(). SoI'm not sure how I can can get this to work.

This is a follow up to Lifetime parameters for an enum within a struct

Answer 1

To me it seems like let c = Bar::Fub(&17), the 17 lasts the same life time as the previous line where "Foo" is created on the stack

A string literal always has 'static lifetime and will therefor always live long enough.

I think the issue is that you are hitting is the fact that an enum expression is actually somewhat of aa function call. Somewhat meaning that the lifetime of the argument is ignored in the computation of the Enum's lifetime. The Enum's lifetime apparently is marginally larger, as if you wrote:

let c: Bar;
let x = &17;
c = Bar::Fub(x);

which is already addressed in Scope of addresses: Does not live long enough

In which case it's completely clear that h lasts longer than Bar::Fub().

Yes, the lifetimes are clear here, and it works in the Playpen :

let x = &17;
let c = Bar::Fub(x);

so I'm not sure what you are asking.