How do I fix E0310 (the parameter type may not live long enough)?

Add the bound the compiler suggests: Or allow a shorter lifetime:

Rust errors / Ownership and borrowing

Rust compiler error · checked with rustc 1.99.0

Rust error E0310: the parameter type may not live long enough

Q: What does Rust error E0310 mean?

A generic type T is stored somewhere that must live for a lifetime (often 'static, as in Box<dyn Trait> or a spawned thread), but T could contain short-lived references.

What it meansA generic type T is stored somewhere that must live for a lifetime (often 'static, as in Box<dyn Trait> or a spawned thread), but T could contain short-lived references.

What rustc prints

error[E0310]: the parameter type `T` may not live long enough
 --> main.rs:4:10
  |
4 |     foo: &'static T
  |          ^^^^^^^^^^
  |          |
  |          the parameter type `T` must be valid for the static lifetime...
  |          ...so that the reference type `&'static T` does not outlive the data it points at
  |
help: consider adding an explicit lifetime bound
  |
3 | struct Foo<T: 'static> {
  |             +++++++++

Real output of rustc 1.99.0 compiling the official example below (2021 edition). Your names and line numbers will differ; the first line is the part to match.

Why it happens

Boxing a generic into Box<dyn Any> or Box<dyn Trait> (which means + 'static).
Sending a generic value into thread::spawn or tokio::spawn.

How to fix it

1. Add the bound the compiler suggests

fn store<T: Send + 'static>(v: T) -> Box<dyn std::any::Any + Send> {
    Box::new(v)
}

2. Or allow a shorter lifetime

fn wrap<'a, T: Trait + 'a>(v: T) -> Box<dyn Trait + 'a> { Box::new(v) }

Other wordings of this error

the parameter type may not live long enough, so that the type will meet its required lifetime bounds, consider adding an explicit lifetime bound `T: 'static`

E0310Ownership and borrowing

Activein rustc 1.99.0

8related errors linked below

The compiler team's explanation

A parameter type is missing a lifetime constraint or has a lifetime that does not live long enough.

Erroneous code example:

// This won't compile because T is not constrained to the static lifetime
// the reference needs
struct Foo<T> {
    foo: &'static T
}

Type parameters in type definitions have lifetimes associated with them that represent how long the data stored within them is guaranteed to live. This lifetime must be as long as the data needs to be alive, and missing the constraint that denotes this will cause this error.

This will compile, because it has the constraint on the type parameter:

struct Foo<T: 'static> {
    foo: &'static T
}

From rustc --explain E0310 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.

Up next

Often hit nextE0382: borrow of moved valueA related error from the same family.Read it next →

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