Rust 45 🦀 Static Lifetime (‘static) Mechanics
The 'static lifetime is the longest lifetime in Rust. It means “valid for the entire duration of the program,” and it is the lifetime of every string literal, every global constant, and every value that is stored in the binary itself. The annotation appears in three distinct contexts, and the three contexts have different meanings. As a reference lifetime, &'static T means a reference that is valid for the entire program. As a trait bound, T: 'static means that the type T contains no references with a shorter lifetime — it does not mean that the value lives forever, only that it could. As a lifetime parameter, 'static is a concrete lifetime that outlives every other lifetime in the program.
The most common misunderstanding is the trait bound. A value of a type that satisfies T: 'static does not necessarily live for the entire program. A String owns its data and contains no references, so String: 'static is true, and a String can be moved into a thread that requires 'static even though the String is dropped at the end of the function. The bound means that the value can be kept for as long as needed, because it does not borrow from anything shorter-lived. The distinction between “lives forever” and “could live forever” is the source of most confusion with 'static.
The 'static lifetime is also the only lifetime that can be written in a const or static declaration. A static item is a value that lives for the entire program, and its type must satisfy 'static. A const item is inlined at each use site, and it can be a 'static reference to a string literal. The 'static lifetime is what makes these declarations possible, and it is what guarantees that the value is valid when the program starts.
This chapter covers three areas. First, why 'static exists — the global-data problem and the thread-safety problem that the lifetime solves. Second, the three meanings of 'static — the reference lifetime, the trait bound, and the lifetime parameter, with the distinction between “lives forever” and “could live forever.” Third, the common patterns and the errors — returning &'static str, the T: 'static bound, the Box<dyn Trait + 'static> bound, and the lifetime errors that the 'static bound causes. The chapter ends with a complete example session, a quick reference, best practices, common pitfalls, real-world examples, and diagrams showing the three meanings.
Key point: The 'static lifetime means “valid for the entire program” when it is a reference lifetime (&'static T). As a trait bound (T: 'static), it means the type contains no references with a shorter lifetime — it does not mean the value lives forever. The 'static lifetime is the lifetime of string literals and global constants, and it is the only lifetime that can be written in a static or const declaration.
Why ‘static exists
The global-data problem. A global variable is accessible from every part of the program, and it must be valid for the entire execution. A global that is initialized before main runs and is never dropped has the 'static lifetime. The 'static lifetime is the type-level guarantee that the value is valid whenever it is accessed. String literals are the most common example: they are stored in the read-only data segment of the binary, and they are valid for the entire program.
The thread-safety problem. A value that is moved into a thread must not borrow from the thread’s parent, because the parent may return before the thread finishes. The T: 'static bound is the way to express this: the value must own its data, so it can outlive the scope where it was created. The bound does not mean the value lives forever; it means the value does not borrow from anything shorter-lived. A String satisfies 'static, and it can be moved into a thread, even though the String is dropped when the thread finishes.
The trait-object problem. A trait object Box<dyn Trait> has an implicit 'static bound. The box owns the value, and the value must not borrow from anything shorter-lived, because the box can be moved anywhere. If the trait object needs to borrow, the bound must be written explicitly: Box<dyn Trait + 'a>. The 'static bound is the default because it is the safest, and the explicit bound is required when the trait object needs to be shorter-lived.
The return-type problem. A function that returns a reference must specify the lifetime. A function that returns a 'static reference returns one that is valid for the entire program. A string literal is the canonical example: fn greeting() -> &'static str { "hello" }. The 'static lifetime is the only lifetime that can be returned without borrowing from an input, because it is the only lifetime that is guaranteed to outlive the function call.
The const-and-static problem. A static item is a value that lives for the entire program, and its type must satisfy 'static. A const item is inlined at each use site, and it can be a 'static reference. The 'static lifetime is what makes these declarations valid: the value is guaranteed to be available whenever it is used, because it is part of the binary.
The trade-off. The 'static lifetime is the most restrictive lifetime, because it requires the value to outlive everything. The T: 'static bound is often used when a weaker bound would suffice, which can make an API less flexible than it needs to be. The Box<dyn Trait> default is 'static, which means a trait object that borrows must use an explicit lifetime. The trade-off is between the safety of the 'static guarantee and the flexibility of a shorter lifetime. The recommendation is to use 'static only when the value genuinely needs to outlive everything, and to use an explicit shorter lifetime when the value borrows.
a. The three meanings of ‘static
The 'static lifetime appears in three contexts, and the three contexts have different meanings.
As a reference lifetime: &'static T. This means a reference that is valid for the entire program. The referent is stored in the binary or is leaked, and it is never dropped. A string literal has this type:
let s: &'static str = "hello";The reference is valid for the entire execution, and it can be used anywhere. A &'static T can be stored in a static item, returned from a function, and moved into a thread without a lifetime error.
As a trait bound: T: 'static. This means that the type T contains no references with a shorter lifetime. It does not mean that the value lives forever; it means that the value could live forever if it were kept. A String satisfies String: 'static, because it owns its data and contains no references:
fn spawn<F: Fn() + 'static>(f: F) {
std::thread::spawn(f);
}
fn main() {
let s = String::from("hello");
spawn(move || println!("{}", s));
}The closure captures s by move, so the closure owns the String. The String satisfies 'static, so the closure satisfies 'static, and the closure can be moved into the thread. The String is dropped when the thread finishes, not when the program ends.
As a lifetime parameter. The 'static lifetime can be used as a parameter in a generic function or a struct. It is a concrete lifetime that outlives every other lifetime in the program. A function that takes a &'static str requires the caller to pass a string literal or a leaked string, not a String reference with a shorter lifetime.
fn store(s: &'static str) {
// s is valid for the entire program
}The three meanings are related: the reference lifetime &'static T is a reference whose lifetime parameter is 'static, and the trait bound T: 'static means that the type T is valid for any lifetime, including 'static. The distinction is that the reference lifetime is about the reference, and the trait bound is about the type.
b. Returning ‘static references and the T: ‘static bound
A function that returns a 'static reference returns a reference that is valid for the entire program. The canonical example is a function that returns a string literal:
fn greeting() -> &'static str {
"hello"
}The string literal is stored in the binary, so it is valid for the entire execution. The 'static lifetime is the only lifetime that can be returned without borrowing from an input, because it is the only lifetime that is guaranteed to outlive the function call.
The T: 'static bound is used when a value must not borrow from anything shorter-lived. The most common use is spawning a thread:
use std::thread;
fn spawn<F: FnOnce() + Send + 'static>(f: F) {
thread::spawn(f);
}
fn main() {
let message = String::from("hello from the thread");
spawn(move || {
println!("{}", message);
});
}The spawn function requires F: 'static, which means the closure must not borrow from the surrounding scope. The move keyword moves the message string into the closure, so the closure owns it, and the closure satisfies 'static. The thread can outlive the main function’s scope, but the message is owned by the closure, so there is no dangling reference.
The T: 'static bound is also used in trait objects. A Box<dyn Trait> has an implicit 'static bound, which means the boxed value must not borrow from anything shorter-lived:
fn make_box() -> Box<dyn std::fmt::Debug> {
Box::new(42)
}The 42 is an i32, which satisfies 'static, so the Box<dyn Debug> is valid. If the boxed value borrowed from a local, the bound would fail:
// ❌ Does not compile: borrowed data escapes
// fn make_box(s: &str) -> Box<dyn std::fmt::Debug> {
// Box::new(s)
// }The fix is to write the lifetime explicitly:
// ✅ The boxed value borrows from s
fn make_box<'a>(s: &'a str) -> Box<dyn std::fmt::Debug + 'a> {
Box::new(s)
}The + 'a bound says that the trait object is valid for the lifetime 'a, which is the lifetime of the input. The box borrows from s, and the caller must ensure that s outlives the box.
c. The ‘static lifetime in statics and consts
A static item is a value that lives for the entire program. Its type must satisfy 'static, and it is initialized before main runs:
static GREETING: &str = "hello";
static MAX_SIZE: usize = 1024;The GREETING static is a &'static str, because string literals are 'static. The MAX_SIZE static is a usize, which satisfies 'static because it contains no references.
A const item is inlined at each use site. It can be a 'static reference to a string literal, or a computed constant:
const PI: f64 = 3.14159;
const GREETING: &str = "hello";
const MAX_SIZE: usize = 1024;The const items are not stored in a fixed location; they are substituted into the code at each use. The 'static lifetime of a const reference is the lifetime of the string literal it points to.
The static and const declarations are the only places where a value can be declared without an explicit lifetime annotation, because the 'static lifetime is implied. A static GREETING: &str is a static GREETING: &'static str, and the annotation is elided.
A static mut item is a mutable global, and it requires unsafe to access. It is rarely used, and the recommendation is to use a synchronization primitive like Mutex or AtomicUsize instead:
use std::sync::atomic::{AtomicUsize, Ordering};
static COUNTER: AtomicUsize = AtomicUsize::new(0);
fn increment() {
COUNTER.fetch_add(1, Ordering::SeqCst);
}The AtomicUsize is 'static, and it provides safe mutation through atomic operations.
Complete Example Session
// ============================================
// PART 1: A 'static REFERENCE
// ============================================
fn greeting() -> &'static str {
"hello"
}
fn main() {
let s: &'static str = greeting();
println!("{}", s); // hello
}
// String literals are stored in the binary.
// They are valid for the entire program.
// ============================================
// PART 2: THE T: 'static BOUND
// ============================================
fn print_static<T: 'static + std::fmt::Display>(value: T) {
println!("{}", value);
}
fn main() {
print_static(42);
print_static(String::from("hello"));
print_static("literal");
}
// The bound says the type contains no references
// with a shorter lifetime. It does not mean the
// value lives forever.
// ============================================
// PART 3: SPAWNING A THREAD
// ============================================
use std::thread;
fn main() {
let message = String::from("hello from the thread");
let handle = thread::spawn(move || {
println!("{}", message);
});
handle.join().unwrap();
}
// The closure captures message by move.
// The closure owns the String, so it satisfies 'static.
// The thread can outlive the main scope.
// ============================================
// PART 4: THE 'static BOUND FAILS WITH A BORROW
// ============================================
// ❌ Does not compile:
// fn main() {
// let message = String::from("hello");
// let handle = thread::spawn(|| {
// println!("{}", message);
// });
// handle.join().unwrap();
// }
// error[E0373]: closure may outlive the current function,
// but it borrows `message`
// ✅ The move keyword fixes it:
fn main() {
let message = String::from("hello");
let handle = thread::spawn(move || {
println!("{}", message);
});
handle.join().unwrap();
}
// ============================================
// PART 5: A TRAIT OBJECT WITH 'static
// ============================================
fn make_box() -> Box<dyn std::fmt::Debug> {
Box::new(42)
}
// Box<dyn Debug> has an implicit 'static bound.
// The value 42 is an i32, which satisfies 'static.
// ============================================
// PART 6: A TRAIT OBJECT WITH AN EXPLICIT LIFETIME
// ============================================
fn make_box_borrowed<'a>(s: &'a str) -> Box<dyn std::fmt::Debug + 'a> {
Box::new(s)
}
fn main() {
let s = String::from("hello");
let boxed = make_box_borrowed(&s);
println!("{:?}", boxed);
}
// The + 'a bound says the trait object is valid
// for the lifetime of the input.
// The box borrows from s.
// ============================================
// PART 7: STATIC ITEMS
// ============================================
static GREETING: &str = "hello";
static MAX_SIZE: usize = 1024;
fn main() {
println!("{}", GREETING); // hello
println!("{}", MAX_SIZE); // 1024
}
// static items live for the entire program.
// Their type must satisfy 'static.
// ============================================
// PART 8: CONST ITEMS
// ============================================
const PI: f64 = 3.14159;
const GREETING: &str = "hello";
fn main() {
println!("{}", PI); // 3.14159
println!("{}", GREETING); // hello
}
// const items are inlined at each use site.
// A const reference has the 'static lifetime
// of the string literal it points to.
// ============================================
// PART 9: STATIC MUT WITH ATOMIC
// ============================================
use std::sync::atomic::{AtomicUsize, Ordering};
static COUNTER: AtomicUsize = AtomicUsize::new(0);
fn increment() {
COUNTER.fetch_add(1, Ordering::SeqCst);
}
fn main() {
increment();
increment();
println!("{}", COUNTER.load(Ordering::SeqCst)); // 2
}
// Static mut requires unsafe.
// AtomicUsize provides safe mutation.
// ============================================
// PART 10: THE COMPLETE EXAMPLE
// ============================================
use std::thread;
static APP_NAME: &str = "my_app";
static VERSION: &str = "1.0.0";
fn app_info() -> String {
format!("{} v{}", APP_NAME, VERSION)
}
fn spawn_worker(id: usize) -> thread::JoinHandle<String> {
thread::spawn(move || {
format!("worker {} finished", id)
})
}
fn main() {
println!("{}", app_info()); // my_app v1.0.0
let handles: Vec<_> = (0..3).map(spawn_worker).collect();
for handle in handles {
println!("{}", handle.join().unwrap());
}
}
// APP_NAME and VERSION are static references.
// spawn_worker returns a JoinHandle with a String,
// which satisfies 'static.The ten parts show a 'static reference, the T: 'static bound, spawning a thread, the 'static bound failing with a borrow, a trait object with 'static, a trait object with an explicit lifetime, static items, const items, static mut with an atomic, and the complete example.
Quick Reference
The Three Meanings
| Context | Meaning |
|---|---|
&'static T | A reference valid for the entire program |
T: 'static | The type contains no shorter-lived references |
'static as parameter | A concrete lifetime that outlives all others |
Common Patterns
| Pattern | Example |
|---|---|
| String literal | let s: &'static str = "hello"; |
| Return a static reference | fn f() -> &'static str { "hi" } |
| Thread closure | `thread::spawn(move |
| Boxed trait object | Box<dyn Debug> (implicit 'static) |
| Borrowed trait object | Box<dyn Debug + 'a> |
| Static item | static GREETING: &str = "hello"; |
| Const item | const PI: f64 = 3.14159; |
| Atomic global | static COUNTER: AtomicUsize = ... |
‘static Bound in Common APIs
| API | Bound |
|---|---|
thread::spawn | F: FnOnce() + Send + 'static |
Box<dyn Trait> | Implicit 'static |
Any trait | T: 'static |
std::error::Error | 'static for the boxed error |
When ‘static Fails
| Situation | Fix |
|---|---|
| Closure borrows a local | Use move to take ownership |
| Trait object borrows | Add + 'a to the bound |
| Reference to a local | Return an owned value |
&'static from a local | Leak the value or use a String |
Best Practices
✅ Do This:
// Use 'static for string literals
fn greeting() -> &'static str { "hello" } // ✅
// Use move to satisfy the 'static bound
thread::spawn(move || println!("{}", s)); // ✅
// Use an explicit lifetime for a borrowed trait object
fn f<'a>(s: &'a str) -> Box<dyn Debug + 'a> { Box::new(s) } // ✅
// Use AtomicUsize instead of static mut
static COUNTER: AtomicUsize = AtomicUsize::new(0); // ✅
// Use static for global constants
static APP_NAME: &str = "my_app"; // ✅
// Use T: 'static only when the value must outlive everything
fn spawn<F: FnOnce() + 'static>(f: F) { ... } // ✅❌ Don’t Do This:
// Don't expect a borrowed value to satisfy 'static
let s = String::from("hello");
thread::spawn(|| println!("{}", s)); // borrow error // ❌
// Don't use 'static when a shorter lifetime suffices
fn f(s: &'static str) { } // too restrictive // ❌
// Don't confuse "lives forever" with "could live forever"
// String: 'static does not mean the String lives forever // ❌
// Don't use static mut
static mut COUNTER: i32 = 0; // requires unsafe // ❌
// Don't return a reference to a local
fn f() -> &'static str { &String::from("hello") } // error // ❌Common Pitfalls
| Pitfall | Why It Happens | Fix |
|---|---|---|
| Closure may outlive the function | Borrows a local | Use move |
| Borrowed data escapes | Trait object has implicit 'static | Add + 'a |
'static bound not satisfied | The type contains a reference | Own the data |
| Cannot return reference to a local | Dangling reference | Return an owned value |
static mut is unsafe | Mutable global | Use an atomic or a mutex |
| Overly restrictive API | 'static bound where a shorter lifetime works | Use a generic lifetime |
Real-World Examples
1. String Literal
let s: &'static str = "hello";2. Return a Static Reference
fn greeting() -> &'static str { "hello" }3. Thread Closure
thread::spawn(move || println!("{}", message));4. T: ‘static Bound
fn print_static<T: 'static + Display>(value: T) { ... }5. Boxed Trait Object
fn make() -> Box<dyn Debug> { Box::new(42) }6. Borrowed Trait Object
fn make<'a>(s: &'a str) -> Box<dyn Debug + 'a> { Box::new(s) }7. Static Item
static APP_NAME: &str = "my_app";8. Const Item
const PI: f64 = 3.14159;9. Atomic Global
static COUNTER: AtomicUsize = AtomicUsize::new(0);10. Complete Example
fn spawn_worker(id: usize) -> thread::JoinHandle<String> {
thread::spawn(move || format!("worker {} finished", id))
}Visual
The Three Meanings of ‘static
┌──────────────────────────────────────────────────────────────┐
│ THE THREE MEANINGS OF 'static │
│ │
│ 1. REFERENCE LIFETIME: &'static T │
│ ┌──────────────────────────────────────────────────────┐ │
│ │ The reference is valid for the entire program. │ │
│ │ │ │
│ │ let s: &'static str = "hello"; │ │
│ │ ┌─────────────────────────────────────────────┐ │ │
│ │ │ binary ──► "hello" │ │ │
│ │ │ s ────────► (points to the binary) │ │ │
│ │ │ │ │ │
│ │ │ valid from program start to program end │ │ │
│ │ └─────────────────────────────────────────────┘ │ │
│ └──────────────────────────────────────────────────────┘ │
│ │
│ 2. TRAIT BOUND: T: 'static │
│ ┌──────────────────────────────────────────────────────┐ │
│ │ The type contains no shorter-lived references. │ │
│ │ It does NOT mean the value lives forever. │ │
│ │ │ │
│ │ String: 'static ← true (owns its data) │ │
│ │ &'a str: 'static ← false (borrows) │ │
│ │ │ │
│ │ let s = String::from("hello"); │ │
│ │ thread::spawn(move || println!("{}", s)); │ │
│ │ // s is moved into the closure, dropped later │ │
│ └──────────────────────────────────────────────────────┘ │
│ │
│ 3. LIFETIME PARAMETER: 'static as a bound │
│ ┌──────────────────────────────────────────────────────┐ │
│ │ A concrete lifetime that outlives all others. │ │
│ │ │ │
│ │ fn store(s: &'static str) { ... } │ │
│ │ // the caller must pass a string literal │ │
│ └──────────────────────────────────────────────────────┘ │
│ │
└──────────────────────────────────────────────────────────────┘The Trait Bound vs the Value
┌──────────────────────────────────────────────────────────────┐
│ THE TRAIT BOUND vs THE VALUE │
│ │
│ String: 'static is TRUE. │
│ ┌──────────────────────────────────────────────────────┐ │
│ │ A String owns its data. │ │
│ │ It contains no references. │ │
│ │ It could live forever if it were kept. │ │
│ │ │ │
│ │ let s = String::from("hello"); │ │
│ │ // s is dropped at the end of the scope │ │
│ │ // The String does NOT live forever │ │
│ │ // But String: 'static is still true │ │
│ └──────────────────────────────────────────────────────┘ │
│ │
│ &'a str: 'static is FALSE. │
│ ┌──────────────────────────────────────────────────────┐ │
│ │ A reference borrows from something. │ │
│ │ The referenced data may not live forever. │ │
│ │ │ │
│ │ let s = String::from("hello"); │ │
│ │ let r = &s; │ │
│ │ // r: &'a str where 'a is the scope of s │ │
│ │ // &'a str does NOT satisfy 'static │ │
│ └──────────────────────────────────────────────────────┘ │
│ │
│ The bound is about the TYPE, not the VALUE. │
│ "Could live forever" ≠ "lives forever". │
│ │
└──────────────────────────────────────────────────────────────┘Thread Spawning with ‘static
┌──────────────────────────────────────────────────────────────┐
│ THREAD SPAWNING WITH 'static │
│ │
│ ❌ WITHOUT move (borrows): │
│ ┌──────────────────────────────────────────────────────┐ │
│ │ let message = String::from("hello"); │ │
│ │ thread::spawn(|| { │ │
│ │ println!("{}", message); ← borrows message │ │
│ │ }); │ │
│ │ │ │
│ │ ERROR: closure may outlive the current function, │ │
│ │ but it borrows `message` │ │
│ │ │ │
│ │ The thread could outlive main, and message could │ │
│ │ be dropped while the thread is using it. │ │
│ └──────────────────────────────────────────────────────┘ │
│ │
│ ✅ WITH move (owns): │
│ ┌──────────────────────────────────────────────────────┐ │
│ │ let message = String::from("hello"); │ │
│ │ thread::spawn(move || { │ │
│ │ println!("{}", message); ← owns message │ │
│ │ }); │ │
│ │ │ │
│ │ The closure owns the String. │ │
│ │ The String satisfies 'static. │ │
│ │ The thread can outlive main. │ │
│ └──────────────────────────────────────────────────────┘ │
│ │
└──────────────────────────────────────────────────────────────┘Trait Objects and ‘static
┌──────────────────────────────────────────────────────────────┐
│ TRAIT OBJECTS AND 'static │
│ │
│ Box<dyn Debug>: │
│ ┌──────────────────────────────────────────────────────┐ │
│ │ Implicit bound: Box<dyn Debug + 'static> │ │
│ │ │ │
│ │ fn make() -> Box<dyn Debug> { │ │
│ │ Box::new(42) ← i32 satisfies 'static │ │
│ │ } │ │
│ │ │ │
│ │ The boxed value must not borrow. │ │
│ └──────────────────────────────────────────────────────┘ │
│ │
│ Box<dyn Debug + 'a>: │
│ ┌──────────────────────────────────────────────────────┐ │
│ │ Explicit bound: the trait object borrows for 'a. │ │
│ │ │ │
│ │ fn make<'a>(s: &'a str) -> Box<dyn Debug + 'a> { │ │
│ │ Box::new(s) ← borrows s for 'a │ │
│ │ } │ │
│ │ │ │
│ │ The box borrows from the input. │ │
│ └──────────────────────────────────────────────────────┘ │
│ │
│ The default is 'static. The explicit bound is for borrowing.│
│ │
└──────────────────────────────────────────────────────────────┘Summary
| Item | Value |
|---|---|
&'static T | A reference valid for the entire program |
T: 'static | The type contains no shorter-lived references |
'static as parameter | A concrete lifetime that outlives all others |
| String literal | &'static str |
| Thread closure | F: FnOnce() + Send + 'static |
| Boxed trait object | Box<dyn Trait> (implicit 'static) |
| Borrowed trait object | Box<dyn Trait + 'a> |
| Static item | static NAME: Type = value; |
| Const item | const NAME: Type = value; |
| Atomic global | static COUNTER: AtomicUsize = ... |
Key takeaways:
- The
'staticlifetime has three meanings. As a reference lifetime (&'static T), it means the reference is valid for the entire program. As a trait bound (T: 'static), it means the type contains no shorter-lived references. As a lifetime parameter, it is a concrete lifetime that outlives all others. T: 'staticdoes not mean the value lives forever. It means the value could live forever, because it owns its data and contains no references with a shorter lifetime. AStringsatisfiesString: 'static, but aStringis dropped at the end of its scope.- The
'staticlifetime is the lifetime of string literals. A string literal is stored in the binary, so it is valid for the entire execution. A function that returns a'staticreference returns a reference that is valid for the entire program. - The
T: 'staticbound is used for thread spawning. A closure that is moved into a thread must own its data, because the thread can outlive the scope where the closure was created. Themovekeyword moves the data into the closure, and the closure satisfies'static. - A
Box<dyn Trait>has an implicit'staticbound. The box owns the value, and the value must not borrow from anything shorter-lived. A boxed trait object that borrows must use an explicit lifetime:Box<dyn Trait + 'a>. staticitems live for the entire program. Their type must satisfy'static, and they are initialized beforemainruns. Astatic mutitem requiresunsafe, and the recommendation is to use an atomic or a mutex instead.constitems are inlined at each use site. Aconstreference has the'staticlifetime of the string literal it points to. Theconstdeclaration is the only place where the'staticlifetime is implied.- Use
'staticonly when the value must outlive everything. AT: 'staticbound is more restrictive than necessary in many APIs. A generic lifetime parameter allows the caller to pass a value with a shorter lifetime, and it should be used when the value does not need to outlive the entire program.
Remember: The 'static lifetime means “valid for the entire program” when it is a reference lifetime, and “contains no shorter-lived references” when it is a trait bound. The distinction is between “lives forever” and “could live forever.” A string literal has the 'static lifetime because it is stored in the binary. A String satisfies String: 'static because it owns its data, even though it is dropped at the end of its scope. The 'static bound is used for thread spawning, boxed trait objects, and global constants. It is the most restrictive lifetime, and it should be used only when the value genuinely needs to outlive everything.
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