E0716 - Error codes index
Description: Book listing all Rust error codes

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# Error code E0716

A temporary value is being dropped while a borrow is still in active use.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    fn foo() -> i32 { 22 }
    fn bar(x: &i32) -> &i32 { x }
    let p = bar(&foo());
             // ------ creates a temporary
    let q = *p;
    }

Here, the expression `&foo()` is borrowing the expression `foo()`. As `foo()` is a call to a function, and not the name of a variable, this creates a temporary -- that temporary stores the return value from `foo()` so that it can be borrowed. You could imagine that `let p = bar(&foo());` is equivalent to the following, which uses an explicit temporary variable.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    fn foo() -> i32 { 22 }
    fn bar(x: &i32) -> &i32 { x }
    let p = {
      let tmp = foo(); // the temporary
      bar(&tmp) // error: `tmp` does not live long enough
    }; // <-- tmp is freed as we exit this block
    let q = p;
    }

Whenever a temporary is created, it is automatically dropped (freed) according to fixed rules. Ordinarily, the temporary is dropped at the end of the enclosing statement -- in this case, after the `let p`. This is illustrated in the example above by showing that `tmp` would be freed as we exit the block.

To fix this problem, you need to create a local variable to store the value in rather than relying on a temporary. For example, you might change the original program to the following:

    #![allow(unused)]
    fn main() {
    fn foo() -> i32 { 22 }
    fn bar(x: &i32) -> &i32 { x }
    let value = foo(); // dropped at the end of the enclosing block
    let p = bar(&value);
    let q = *p;
    }

By introducing the explicit `let value`, we allocate storage that will last until the end of the enclosing block (when `value` goes out of scope). When we borrow `&value`, we are borrowing a local variable that already exists, and hence no temporary is created.

Temporaries are not always dropped at the end of the enclosing statement. In simple cases where the `&` expression is immediately stored into a variable, the compiler will automatically extend the lifetime of the temporary until the end of the enclosing block. Therefore, an alternative way to fix the original program is to write `let tmp = &foo()` and not `let tmp = foo()`:

    #![allow(unused)]
    fn main() {
    fn foo() -> i32 { 22 }
    fn bar(x: &i32) -> &i32 { x }
    let value = &foo();
    let p = bar(value);
    let q = *p;
    }

Here, we are still borrowing `foo()`, but as the borrow is assigned directly into a variable, the temporary will not be dropped until the end of the enclosing block. Similar rules apply when temporaries are stored into aggregate structures like a tuple or struct:

    #![allow(unused)]
    fn main() {
    // Here, two temporaries are created, but
    // as they are stored directly into `value`,
    // they are not dropped until the end of the
    // enclosing block.
    fn foo() -> i32 { 22 }
    let value = (&foo(), &foo());
    }