returning pointer to local variable from its block

What it does

the borrow checker checks when a reference got out of its scope, but not for raw pointers, where thing might get weird.

for example:

fn local_ptr() -> *const i32 {
    let temp: i32 = 123;
    &temp as *const i32
}
fn main() {
    let p = local_ptr();
    unsafe {
        println!("{}", *p);
    }
}

The above code prints 0 (Because temp was allocated in the stack???), run it in playground.

I don't think there was anyway to help people avoid such code, if there is, please let me know.

Advantage

• Help reducing undefined behaviors

Drawbacks

Might be hard to cover all the cases while not causing false positive results.

Example

fn local_ptr() -> *const i32 {
    let temp: i32 = 123;
    &temp as *const i32
}

Could be written as:

fn local_ptr() -> *const i32 {
    static temp: i32 = 123;
    &temp as *const i32
}

Start with the Rust examples in the issue, especially local_ptr, and investigate how a Clippy lint could identify raw pointers derived from block-local variables. Done means the unsafe pattern is diagnosed while avoiding the false positives noted in the issue.