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. 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());
}