From 3df4f2a63f176d0f947cee4860d65c9153b8debd Mon Sep 17 00:00:00 2001
From: "Champlin, Saji" <saji.champlin@tranetechnologies.com>
Date: Tue, 6 May 2025 10:34:49 -0500
Subject: [PATCH] add rust lifetimes presentation

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+<section>
+	<h2> Lifetimes in Rust </h2>
+	<p> It was always there, but now it's explicit </p>
+</section>
+
+<section>
+	<h3>Recap: References </h3>
+	<ul>
+		<li><code>&MyStruct</code> is a reference/borrow of a struct</li>
+		<li class="fragment">If the underlying struct is <em>removed</em>, the reference is invalid </li>
+		<li class="fragment">In C++, it is entirely on you to manage this</li>
+		<li class="fragment"pre>Rust just makes this management explicit </li>
+	</ul>
+</section>
+
+<section>
+	<h3>Motivation</h3>
+	<pre ><code class="rust" data-trim data-line-numbers>
+	let x;
+	{
+		let y = 5;
+		x = &y;
+	} // uh oh
+	println!("x: {}", x); // hmmm
+	</code></pre>
+	<p>Rust will not compile this code</p>
+</section>
+
+<section>
+	<h3>Lifetimes</h3>
+	<ul>
+		<li> These always exist. </li>
+		<li class="fragment"> Sometimes the compiler needs some hints </li>
+		<li class="fragment"> We can <em>annotate</em> lifetimes to add hints </li>
+	</ul>
+</section>
+
+<section>
+	<h3>Annotation Example</h3>
+	<pre><code class="rust" data-trim data-line-numbers="1-10|12-23"><script type="text/template">
+		// 'a is a generic lifetime parameter. the borrow checker
+		// will infer which lifetime it represents and ensure
+		// that the relationships are valid.
+		fn longer_string<'a>(x: &'a str, y: &'a str) -> &'a str {
+			if x.len() > y.len() {
+				x
+			} else {
+				y
+			}
+		}
+
+		fn main() {
+			let string1 = String::from("longer");
+			let result;
+
+			{
+				let string2 = String::from("short");
+				result = longest(&string1, &string2);
+				println("Longest: {}", result);
+			}
+			// can't do this. why?
+			// println("Longest: {}", result);
+		}
+	</script></code></pre>
+	<p class="fragment">The borrow checker will use the shortest lifetime</p>
+</section>
+
+<section>
+	<h3>Struct Example</h3>
+	<pre><code class="rust" data-trim data-line-numbers="1-5|7-12"><script type="text/template">
+		// again, 'a is a generic lifetime parameter.
+		struct Keyword<'a> {
+			word: &'a str,
+			parent: &'a str,
+		}
+
+		fn main() {
+			let example = String::from("This is the demo text");
+			let demo = example.get(12..16);
+			let kw = Keyword { word: demo, parent: example };
+			// kw cannot outlive demo or example
+		}
+	</script></code></pre>
+</section>
+
+
+<section>
+	<section>
+	<h3>Challenge: Circular references</h3>
+	<pre><code class="rust" data-trim data-line-numbers><script type="text/template">
+		struct Node<'a> {
+			data: u32,
+			next: Option<&'a Node<'a>>, // what does this mean?
+		}
+		let mut x = Node {value: 42, next: None };
+		let y = Node { value: 7, next: Some(&x) };
+		// and this is where we perish.
+		x.next = Some(&y);
+	</script></code></pre>
+	</section>
+	<section>
+		<h3>Rc + Refcell</h3>
+		<pre><code class="rust" data-trim data-line-numbers><script type="text/template">
+			use std::cell::RefCell;
+			use std::rc::Rc;
+			struct Node {
+				data: u32,
+				next: Option<Rc<RefCell<Node>>>,
+			}
+		</script></code></pre>
+		<ul>
+			<li> + safety of ownership/mutability at runtime</li>
+			<li> - there is a performance penalty</li>
+			<li> - a bit awkward to use.</li>
+		</ul>
+	</section>
+	<section>
+		<h3>Vec + Index</h3>
+		<pre><code class="rust" data-trim data-line-numbers><script type="text/template">
+			struct Graph {
+				nodes: Vec<Option<Node>>
+			}
+			struct Node {
+				data: u32,
+				next: usize, // index into graph.
+			}
+		</script></code></pre>
+		<ul>
+			<li> + fast, no weird Rc stuff</li>
+			<li> - bug prone, self managed, can dangle </li>
+		</ul>
+	</section>
+	<section>
+		<h3>3rd party libraries</h3>
+		<p><a href="https://github.com/orlp/slotmap/blob/master/examples/doubly_linked_list.rs">slotmap</a>: Like a hashmap, but with unique keys</p>
+		<p><a href="https://docs.rs/typed-arena/latest/typed_arena/#safe-cycles">typed-arena</a>: Arena allocator. Can only drop the whole arena</p>
+	</section>
+
+</section>
+
+
+