vec as_ptr/as_mut_ptr should have a deny lint

[TheDan64]

Hi!

Similarly to the temporary_cstring_as_ptr lint, a common mistake I've run into when dealing with FFI a bunch is immediately trying to get a raw pointer from an initialized vec:

let ptr = vec![0; 4].as_mut_ptr();

This is a really easy way to get a dangling pointer and can be easily overlooked. It would be great if we had a clippy lint for it and as_ptr as well.

There are quite a few ways to initialize a vec that should be also taken into consideration, for example:

let ptr = Vec::from_elem(0, 4).as_mut_ptr();

[oli-obk]

we can in fact lint any function call returning a Vec by value that is immediately followed by a call to as_mut_ptr.

[y21]

Closing as this lint exists in the compiler now

warning: a dangling pointer will be produced because the temporary `Vec<i32>` will be dropped
 --> src/main.rs:2:22
  |
2 | let ptr = vec![0; 4].as_mut_ptr();
  |           ---------- ^^^^^^^^^^ this pointer will immediately be invalid
  |           |
  |           this `Vec<i32>` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
  |
  = note: pointers do not have a lifetime; when calling `as_mut_ptr` the `Vec<i32>` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
  = help: for more information, see <https://doc.rust-lang.org/reference/destructors.html>
  = note: `#[warn(dangling_pointers_from_temporaries)]` on by default