why fn return &str, consider using the `'static` lifetime: `&'static `, but struct fn is correct?

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why fn return &str should static lifetime, but struct's fn not?

fn say() -> &str {
    "this is str"
}
// consider using the `'static` lifetime: `&'static `

but cat.hi ,is correct

struct Cat {}
impl Cat {
    fn hi(&self) -> &str {
        "hi"
    }
}

why fn return &str tips consider using the 'static lifetime: &'static , but struct fn not?

rust
1 Answers

In general every reference needs a lifetime. The compiler will autocomplete them in some cases. These cases are described here https://doc.rust-lang.org/nomicon/lifetime-elision.html.

There is no rule for the first example, so you have to provide a lifetime.

In your second example the rule If there are multiple input lifetime positions, but one of them is &self or &mut self, the lifetime of self is assigned to all elided output lifetimes. is applied which makes it to

struct Cat {}
impl Cat {
    fn hi<'a>(&'a self) -> &'a str {
        "hi"
    }
}

Which is actually different from

struct Cat {}
impl Cat {
    fn hi<'a>(&'a self) -> &'static str {
        "hi"
    }
}
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