Detect dereference of null pointer

[estebank]

What it does

Tracks the creation of null pointers with std::ptr::{null, null_mut}, and any binding that holds them that are never touched until they are dereferenced. When such a case is encountered, a deny-by-default lint is triggered.

Advantage

• compile time detection of assured UB
• slightly lowered need for miri

Drawbacks

• detection logic to avoid false positives might be tricky
• miri already catches this, this could be considered redundant

Example

fn foo(x: *mut i32) -> i32 {
    unsafe {
        if x.is_null() {
            *x
        } else {
            0
        }
    }
}
fn main() {
    foo(std::ptr::null_mut());
}

Should complain about the access to a null pointer at compile time on *x.

[y21]

rustc already has the deref-nullptr lint, can that just be extended to cover these additional cases? Or is there a reason for why we'd want another lint in clippy for this specifically?