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<main>
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<h1 id="advance-lifetime"><a class="header" href="#advance-lifetime">Advance lifetime</a></h1>
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<h2 id="trait-bounds"><a class="header" href="#trait-bounds">Trait Bounds</a></h2>
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<p>Just like generic types can be bounded, lifetimes can also be bounded as below:</p>
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<ul>
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<li><code>T: 'a</code>，all references in <code>T</code> must outlive the lifetime <code>'a</code></li>
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<li><code>T: Trait + 'a</code>: <code>T</code> must implement trait <code>Trait</code> and all references in <code>T</code> must outlive <code>'a</code></li>
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</ul>
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<p><strong>Example</strong></p>
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<pre><pre class="playground"><code class="language-rust editable edition2021">use std::fmt::Debug; // Trait to bound with.
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#[derive(Debug)]
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struct Ref<'a, T: 'a>(&'a T);
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// `Ref` contains a reference to a generic type `T` that has
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// an unknown lifetime `'a`. `T` is bounded such that any
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// *references* in `T` must outlive `'a`. Additionally, the lifetime
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// of `Ref` may not exceed `'a`.
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// A generic function which prints using the `Debug` trait.
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fn print<T>(t: T) where
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T: Debug {
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println!("`print`: t is {:?}", t);
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}
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// Here a reference to `T` is taken where `T` implements
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// `Debug` and all *references* in `T` outlive `'a`. In
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// addition, `'a` must outlive the function.
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fn print_ref<'a, T>(t: &'a T) where
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T: Debug + 'a {
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println!("`print_ref`: t is {:?}", t);
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}
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fn main() {
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let x = 7;
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let ref_x = Ref(&x);
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print_ref(&ref_x);
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print(ref_x);
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}</code></pre></pre>
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<ol>
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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">/* Annotate struct with lifetime:
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1. `r` and `s` must have different lifetimes
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2. lifetime of `s` is bigger than that of 'r'
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*/
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struct DoubleRef<T> {
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r: &T,
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s: &T
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}
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fn main() {
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println!("Success!")
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}</code></pre></pre>
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<ol start="2">
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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">/* Adding trait bounds to make it work */
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struct ImportantExcerpt<'a> {
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part: &'a str,
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}
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impl<'a, 'b> ImportantExcerpt<'a> {
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fn announce_and_return_part(&'a self, announcement: &'b str) -> &'b str {
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println!("Attention please: {}", announcement);
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self.part
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}
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}
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fn main() {
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println!("Success!")
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}</code></pre></pre>
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<ol start="3">
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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">/* Adding trait bounds to make it work */
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fn f<'a, 'b>(x: &'a i32, mut y: &'b i32) {
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y = x;
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let r: &'b &'a i32 = &&0;
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}
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fn main() {
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println!("Success!")
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}</code></pre></pre>
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<h2 id="hrtbhigher-ranked-trait-bounds"><a class="header" href="#hrtbhigher-ranked-trait-bounds">HRTB(Higher-ranked trait bounds)</a></h2>
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<p>Type bounds may be higher ranked over lifetimes. These bounds specify a bound is true for all lifetimes. For example, a bound such as <code>for<'a> &'a T: PartialEq<i32></code> would require an implementation like:</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>impl<'a> PartialEq<i32> for &'a T {
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// ...
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}
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<span class="boring">}</span></code></pre></pre>
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<p>and could then be used to compare a <code>&'a T</code> with any lifetime to an <code>i32</code>.</p>
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<p>Only a higher-ranked bound can be used here, because the lifetime of the reference is shorter than any possible lifetime parameter on the function.</p>
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<ol start="4">
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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">/* Adding HRTB to make it work!*/
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fn call_on_ref_zero<'a, F>(f: F) where F: Fn(&'a i32) {
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let zero = 0;
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f(&zero);
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}
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fn main() {
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println!("Success!");
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}</code></pre></pre>
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<h2 id="nll-non-lexical-lifetime"><a class="header" href="#nll-non-lexical-lifetime">NLL (Non-Lexical Lifetime)</a></h2>
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<p>Before explaining NLL, let's see some code first:</p>
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<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
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let mut s = String::from("hello");
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let r1 = &s;
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let r2 = &s;
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println!("{} and {}", r1, r2);
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let r3 = &mut s;
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println!("{}", r3);
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}</code></pre></pre>
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<p>Based on our current knowledge, this code will cause en error due to violating the borrowing rules in Rust.</p>
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<p>But if you <code>cargo run</code> it, then everything will be ok, so what's going on here?</p>
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<p>The ability of the compiler to tell that a reference is no longer used at a point before the end of the scope, is called <strong>Non-Lexical Lifetimes</strong> (<strong>NLL</strong> for short).</p>
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<p>With this ability the compiler knows when is the last time that a reference is used and optimizing the borrowing rules based on this knowledge.</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>let mut u = 0i32;
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let mut v = 1i32;
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let mut w = 2i32;
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// lifetime of `a` = α ∪ β ∪ γ
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let mut a = &mut u; // --+ α. lifetime of `&mut u` --+ lexical "lifetime" of `&mut u`,`&mut u`, `&mut w` and `a`
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use(a); // | |
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*a = 3; // <-----------------+ |
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... // |
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a = &mut v; // --+ β. lifetime of `&mut v` |
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use(a); // | |
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*a = 4; // <-----------------+ |
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... // |
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a = &mut w; // --+ γ. lifetime of `&mut w` |
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use(a); // | |
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*a = 5; // <-----------------+ <--------------------------+
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<span class="boring">}</span></code></pre></pre>
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<h2 id="reborrow"><a class="header" href="#reborrow">Reborrow</a></h2>
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<p>After learning NLL, we can easily understand reborrow now.</p>
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||
<p><strong>Example</strong></p>
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<pre><pre class="playground"><code class="language-rust edition2021">#[derive(Debug)]
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struct Point {
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x: i32,
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y: i32,
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}
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impl Point {
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fn move_to(&mut self, x: i32, y: i32) {
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self.x = x;
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self.y = y;
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}
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}
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fn main() {
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let mut p = Point { x: 0, y: 0 };
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let r = &mut p;
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// Here comes the reborrow
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let rr: &Point = &*r;
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println!("{:?}", rr); // Reborrow ends here, NLL introduced
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|
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// Reborrow is over, we can continue using `r` now
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r.move_to(10, 10);
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println!("{:?}", r);
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||
}</code></pre></pre>
|
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<ol start="5">
|
||
<li>🌟🌟</li>
|
||
</ol>
|
||
<pre><pre class="playground"><code class="language-rust editable edition2021">/* Make it work by reordering some code */
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||
fn main() {
|
||
let mut data = 10;
|
||
let ref1 = &mut data;
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||
let ref2 = &mut *ref1;
|
||
|
||
*ref1 += 1;
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||
*ref2 += 2;
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||
|
||
println!("{}", data);
|
||
}</code></pre></pre>
|
||
<h2 id="unbound-lifetime"><a class="header" href="#unbound-lifetime">Unbound lifetime</a></h2>
|
||
<p>See more info in <a href="https://doc.rust-lang.org/nomicon/unbounded-lifetimes.html">Nomicon - Unbounded Lifetimes</a>.</p>
|
||
<h2 id="more-elision-rules"><a class="header" href="#more-elision-rules">More elision rules</a></h2>
|
||
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
||
</span><span class="boring">fn main() {
|
||
</span>impl<'a> Reader for BufReader<'a> {
|
||
// 'a is not used in the following methods
|
||
}
|
||
|
||
// can be written as :
|
||
impl Reader for BufReader<'_> {
|
||
|
||
}
|
||
<span class="boring">}</span></code></pre></pre>
|
||
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
||
</span><span class="boring">fn main() {
|
||
</span>// Rust 2015
|
||
struct Ref<'a, T: 'a> {
|
||
field: &'a T
|
||
}
|
||
|
||
// Rust 2018
|
||
struct Ref<'a, T> {
|
||
field: &'a T
|
||
}
|
||
<span class="boring">}</span></code></pre></pre>
|
||
<h2 id="a-difficult-exercise"><a class="header" href="#a-difficult-exercise">A difficult exercise</a></h2>
|
||
<ol start="6">
|
||
<li>🌟🌟🌟🌟</li>
|
||
</ol>
|
||
<pre><pre class="playground"><code class="language-rust editable edition2021">/* Make it work */
|
||
struct Interface<'a> {
|
||
manager: &'a mut Manager<'a>
|
||
}
|
||
|
||
impl<'a> Interface<'a> {
|
||
pub fn noop(self) {
|
||
println!("interface consumed");
|
||
}
|
||
}
|
||
|
||
struct Manager<'a> {
|
||
text: &'a str
|
||
}
|
||
|
||
struct List<'a> {
|
||
manager: Manager<'a>,
|
||
}
|
||
|
||
impl<'a> List<'a> {
|
||
pub fn get_interface(&'a mut self) -> Interface {
|
||
Interface {
|
||
manager: &mut self.manager
|
||
}
|
||
}
|
||
}
|
||
|
||
fn main() {
|
||
let mut list = List {
|
||
manager: Manager {
|
||
text: "hello"
|
||
}
|
||
};
|
||
|
||
list.get_interface().noop();
|
||
|
||
println!("Interface should be dropped here and the borrow released");
|
||
|
||
use_list(&list);
|
||
}
|
||
|
||
fn use_list(list: &List) {
|
||
println!("{}", list.manager.text);
|
||
}</code></pre></pre>
|
||
<blockquote>
|
||
<p>You can find the solutions <a href="https://github.com/sunface/rust-by-practice/blob/master/solutions/lifetime/advance.md">here</a>(under the solutions path), but only use it when you need it :)</p>
|
||
</blockquote>
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||
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</main>
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