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<main>
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<h1 id="static-and-t-static"><a class="header" href="#static-and-t-static">&'static and T: 'static</a></h1>
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<p><code>'static</code> is a reserved lifetime name, you might have encountered it several times:</p>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>// A reference with 'static lifetime:
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let s: &'static str = "hello world";
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// 'static as part of a trait bound:
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fn generic<T>(x: T) where T: 'static {}
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<span class="boring">}</span></code></pre></pre>
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<p>Though they are all <code>'static</code>, but subtly different.</p>
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<h2 id="static"><a class="header" href="#static">&'static</a></h2>
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<p>As a reference lifetime, <code>&'static</code> indicates the data pointed to by the reference lives as long as the running program. But it can still be coerced to a shorter lifetime.</p>
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<ol>
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<li>🌟🌟 There are several ways to make a variable with <code>'static</code> lifetime, two of them are stored in the read-only memory of the binary.</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">
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/* Fill in the blank in two ways */
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fn main() {
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__;
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need_static(v);
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println!("Success!")
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}
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fn need_static(r : &'static str) {
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assert_eq!(r, "hello");
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}</code></pre></pre>
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<ol start="2">
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<li>🌟🌟🌟🌟 Another way to make <code>'static</code> lifetime is using <code>Box::leak</code></li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">#[derive(Debug)]
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struct Config {
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a: String,
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b: String,
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}
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static mut config: Option<&mut Config> = None;
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/* Make it work without changing the function signatures of `init`*/
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fn init() -> Option<&'static mut Config> {
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Some(&mut Config {
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a: "A".to_string(),
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b: "B".to_string(),
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})
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}
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fn main() {
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unsafe {
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config = init();
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println!("{:?}",config)
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}
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}</code></pre></pre>
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<ol start="3">
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<li>🌟 <code>&'static</code> only indicates that the data can live forever, not the reference. The latter one will be constrained by its scope.</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">fn main() {
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{
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// Make a `string` literal and print it:
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let static_string = "I'm in read-only memory";
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println!("static_string: {}", static_string);
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// When `static_string` goes out of scope, the reference
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// can no longer be used, but the data remains in the binary.
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}
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println!("static_string reference remains alive: {}", static_string);
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}</code></pre></pre>
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<ol start="4">
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<li><code>&'static</code> can be coerced to a shorter lifetime.</li>
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</ol>
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<p><strong>Example</strong></p>
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<pre><pre class="playground"><code class="language-rust editable edition2021">// Make a constant with `'static` lifetime.
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static NUM: i32 = 18;
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// Returns a reference to `NUM` where its `'static`
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// lifetime is coerced to that of the input argument.
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fn coerce_static<'a>(_: &'a i32) -> &'a i32 {
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&NUM
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}
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fn main() {
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{
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// Make an integer to use for `coerce_static`:
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let lifetime_num = 9;
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// Coerce `NUM` to lifetime of `lifetime_num`:
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let coerced_static = coerce_static(&lifetime_num);
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println!("coerced_static: {}", coerced_static);
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}
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println!("NUM: {} stays accessible!", NUM);
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}</code></pre></pre>
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<h2 id="t-static"><a class="header" href="#t-static">T: 'static</a></h2>
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<p>As a trait bound, it means the type does not contain any non-static references. Eg. the receiver can hold on to the type for as long as they want and it will never become invalid until they drop it.</p>
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<p>It's important to understand this means that any owned data always passes a <code>'static </code>lifetime bound, but a reference to that owned data generally does not.</p>
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<ol start="5">
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<li>🌟🌟</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">/* Make it work */
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use std::fmt::Debug;
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fn print_it<T: Debug + 'static>( input: T) {
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println!( "'static value passed in is: {:?}", input );
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}
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fn print_it1( input: impl Debug + 'static ) {
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println!( "'static value passed in is: {:?}", input );
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}
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fn print_it2<T: Debug + 'static>( input: &T) {
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println!( "'static value passed in is: {:?}", input );
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}
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fn main() {
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// i is owned and contains no references, thus it's 'static:
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let i = 5;
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print_it(i);
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// oops, &i only has the lifetime defined by the scope of
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// main(), so it's not 'static:
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print_it(&i);
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print_it1(&i);
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// but this one WORKS !
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print_it2(&i);
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}</code></pre></pre>
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<ol start="6">
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<li>🌟🌟🌟</li>
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</ol>
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<pre><pre class="playground"><code class="language-rust editable edition2021">use std::fmt::Display;
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fn main() {
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let mut string = "First".to_owned();
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string.push_str(string.to_uppercase().as_str());
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print_a(&string);
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print_b(&string);
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print_c(&string); // Compilation error
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print_d(&string); // Compilation error
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print_e(&string);
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print_f(&string);
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print_g(&string); // Compilation error
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}
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fn print_a<T: Display + 'static>(t: &T) {
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println!("{}", t);
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}
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fn print_b<T>(t: &T)
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where
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T: Display + 'static,
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{
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println!("{}", t);
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}
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fn print_c(t: &'static dyn Display) {
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println!("{}", t)
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}
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fn print_d(t: &'static impl Display) {
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println!("{}", t)
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}
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fn print_e(t: &(dyn Display + 'static)) {
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println!("{}", t)
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}
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fn print_f(t: &(impl Display + 'static)) {
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println!("{}", t)
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}
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fn print_g(t: &'static String) {
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println!("{}", t);
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}</code></pre></pre>
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<blockquote>
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<p>You can find the solutions <a href="https://github.com/sunface/rust-by-practice/blob/master/solutions/lifetime/static.md">here</a>(under the solutions path), but only use it when you need it :)</p>
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</blockquote>
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