Prevent Getting a Raw Pointer to a Temporary

What it does

The borrow checker prevents users from borrowing content from temporaries.

The following code...

fn mistake() {
    let hello = String::from("hello").as_str();
    hello.to_string();
}

gives this compiler error:

error[E0716]: temporary value dropped while borrowed
 --> src/main.rs:4:17
  |
4 |     let hello = String::from("hello").as_str();
  |                 ^^^^^^^^^^^^^^^^^^^^^         - temporary value is freed at the end of this statement
  |                 |
  |                 creates a temporary which is freed while still in use
5 |     hello.to_string();
  |     ----- borrow later used here
  |
  = note: consider using a `let` binding to create a longer lived value

However, the same mistakes with raw pointers is not caught. This is almost always a mistake which can lead to use after free.

Luckily, most standard library APIs are designed to not make this mistake possible, but for some like CString::as_raw there is already a clippy lint.

However, for custom smart pointer types, this issue is not caught by clippy (although it is caught by miri).

Categories (optional)

• Kind: clippy::correctness

What is the advantage of the recommended code over the original code

The use of a pointer to a temporary is almost guaranteed to be incorrect.

Drawbacks

None.

Start by reviewing the existing temporary_cstring_as_ptr Clippy lint and the Rust examples in the issue, then compare the custom smart-pointer case with Miri's behavior. Done means Clippy detects raw pointers obtained from temporaries, including the custom smart-pointer scenario, without flagging valid code.