Returning a reference to a function scoped value works for some reason in rust, need explanation

Question

I am using rustc 1.70.1

I have a function which I thought would crash, but it doesn't:

fn testset<'a> () -> std::collections::HashSet<&'a i64> {
    let mut set = HashSet::new();
    set.insert(&123456);
    set
}

fn main() {
    let test = testset();
    println!("{:?}", test);
}

Application output: {123456}

From my understanding, the 123456 only lives within the function, right? Shouldn't it "disappear" with the call stack of the function?

However, if I put the number as a variable, then pass the variable as a reference, I get the crash I expect:

fn testset<'a> () -> std::collections::HashSet<&'a i64> {
    let mut set = HashSet::new();
    let x = 123456;
    set.insert(&x);
    set
}

Compiler output:

error[E0515]: cannot return value referencing local variable `x`
   --> src/main.rs:245:5
    |
244 |     set.insert(&x);
    |                -- `x` is borrowed here
245 |     set
    |     ^^^ returns a value referencing data owned by the current function

For more information about this error, try `rustc --explain E0515`.
error: could not compile `radix-rust` (bin "radix-rust") due to previous error

I need help to understand what is happening here.