merged with latest changes and made some additional corrections
This commit is contained in:
Carl Fredrik Samson
@@ -177,7 +177,7 @@ that it's time to lay down the work and start over tomorrow with a fresh mind.</
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</blockquote>
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<p>On a more serious note, I feel obliged to mention that there are valid reasons
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for the names that were chosen. Naming is not easy, and I considered renaming
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<code>Unpin</code> and <code>!Unpin</code> in this book to make them easier to reason about. </p>
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<code>Unpin</code> and <code>!Unpin</code> in this book to make them easier to reason about.</p>
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<p>However, an experienced member of the Rust community convinced me that that there
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is just too many nuances and edge-cases to consider which is easily overlooked when
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naively giving these markers different names, and I'm convinced that we'll
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@@ -211,11 +211,11 @@ impl Test {
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let self_ref: *const String = &self.a;
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self.b = self_ref;
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}
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fn a(&self) -> &str {
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&self.a
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}
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}
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fn b(&self) -> &String {
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unsafe {&*(self.b)}
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}
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@@ -246,7 +246,7 @@ you see, this works as expected:</p>
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# a: String,
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# b: *const String,
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# }
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#
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#
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# impl Test {
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# fn new(txt: &str) -> Self {
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# let a = String::from(txt);
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@@ -255,17 +255,17 @@ you see, this works as expected:</p>
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# b: std::ptr::null(),
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# }
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# }
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#
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#
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# // We need an `init` method to actually set our self-reference
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# fn init(&mut self) {
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# let self_ref: *const String = &self.a;
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# self.b = self_ref;
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# }
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#
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#
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# fn a(&self) -> &str {
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# &self.a
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# }
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#
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# }
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#
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# fn b(&self) -> &String {
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# unsafe {&*(self.b)}
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# }
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@@ -296,7 +296,7 @@ which <code>test1</code> is pointing to with the data stored at the memory locat
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# a: String,
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# b: *const String,
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# }
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#
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#
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# impl Test {
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# fn new(txt: &str) -> Self {
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# let a = String::from(txt);
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@@ -305,16 +305,16 @@ which <code>test1</code> is pointing to with the data stored at the memory locat
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# b: std::ptr::null(),
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# }
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# }
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#
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#
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# fn init(&mut self) {
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# let self_ref: *const String = &self.a;
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# self.b = self_ref;
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# }
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#
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#
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# fn a(&self) -> &str {
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# &self.a
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# }
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#
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# }
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#
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# fn b(&self) -> &String {
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# unsafe {&*(self.b)}
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# }
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@@ -352,7 +352,7 @@ be tied to the lifetime of <code>test2</code> anymore.</p>
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# a: String,
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# b: *const String,
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# }
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#
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#
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# impl Test {
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# fn new(txt: &str) -> Self {
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# let a = String::from(txt);
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@@ -361,16 +361,16 @@ be tied to the lifetime of <code>test2</code> anymore.</p>
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# b: std::ptr::null(),
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# }
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# }
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#
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#
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# fn init(&mut self) {
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# let self_ref: *const String = &self.a;
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# self.b = self_ref;
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# }
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#
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#
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# fn a(&self) -> &str {
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# &self.a
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# }
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#
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# }
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#
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# fn b(&self) -> &String {
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# unsafe {&*(self.b)}
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# }
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@@ -381,7 +381,7 @@ it's easy to create serious bugs using this code.</p>
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<p>I created a diagram to help visualize what's going on:</p>
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<p><strong>Fig 1: Before and after swap</strong>
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<img src="./assets/swap_problem.jpg" alt="swap_problem" /></p>
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<p>As you can see this results in unwanted behavior. It's easy to get this to
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<p>As you can see this results in unwanted behavior. It's easy to get this to
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segfault, show UB and fail in other spectacular ways as well.</p>
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<h2><a class="header" href="#pinning-to-the-stack" id="pinning-to-the-stack">Pinning to the stack</a></h2>
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<p>Now, we can solve this problem by using <code>Pin</code> instead. Let's take a look at what
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@@ -434,7 +434,7 @@ we'll show in a second.</p>
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// Notice how we shadow `test1` to prevent it from beeing accessed again
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let mut test1 = unsafe { Pin::new_unchecked(&mut test1) };
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Test::init(test1.as_mut());
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let mut test2 = Test::new("test2");
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let mut test2 = unsafe { Pin::new_unchecked(&mut test2) };
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Test::init(test2.as_mut());
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@@ -444,15 +444,15 @@ we'll show in a second.</p>
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}
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# use std::pin::Pin;
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# use std::marker::PhantomPinned;
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#
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#
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# #[derive(Debug)]
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# struct Test {
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# a: String,
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# b: *const String,
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# _marker: PhantomPinned,
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# }
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#
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#
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#
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#
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# impl Test {
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# fn new(txt: &str) -> Self {
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# let a = String::from(txt);
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@@ -468,11 +468,11 @@ we'll show in a second.</p>
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# let this = unsafe { self.get_unchecked_mut() };
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# this.b = self_ptr;
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# }
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#
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#
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# fn a<'a>(self: Pin<&'a Self>) -> &'a str {
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# &self.get_ref().a
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# }
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#
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#
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# fn b<'a>(self: Pin<&'a Self>) -> &'a String {
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# unsafe { &*(self.b) }
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# }
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@@ -484,7 +484,7 @@ you'll get a compilation error.</p>
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let mut test1 = Test::new("test1");
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let mut test1 = unsafe { Pin::new_unchecked(&mut test1) };
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Test::init(test1.as_mut());
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let mut test2 = Test::new("test2");
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let mut test2 = unsafe { Pin::new_unchecked(&mut test2) };
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Test::init(test2.as_mut());
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@@ -495,15 +495,15 @@ you'll get a compilation error.</p>
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}
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# use std::pin::Pin;
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# use std::marker::PhantomPinned;
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#
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#
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# #[derive(Debug)]
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# struct Test {
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# a: String,
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# b: *const String,
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# _marker: PhantomPinned,
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# }
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#
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#
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#
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#
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# impl Test {
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# fn new(txt: &str) -> Self {
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# let a = String::from(txt);
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@@ -519,11 +519,11 @@ you'll get a compilation error.</p>
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# let this = unsafe { self.get_unchecked_mut() };
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# this.b = self_ptr;
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# }
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#
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#
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# fn a<'a>(self: Pin<&'a Self>) -> &'a str {
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# &self.get_ref().a
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# }
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#
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#
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# fn b<'a>(self: Pin<&'a Self>) -> &'a String {
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# unsafe { &*(self.b) }
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# }
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@@ -536,7 +536,7 @@ us from swapping the pinned pointers.</p>
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stack frame we're in, so we can't create a self referential object in one
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stack frame and return it since any pointers we take to "self" is invalidated.</p>
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<p>It also puts a lot of responsibility in your hands if you pin a value to the
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stack. A mistake that is easy to make is, forgetting to shadow the original variable
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stack. A mistake that is easy to make is, forgetting to shadow the original variable
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since you could drop the pinned pointer and access the old value
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after it's initialized like this:</p>
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<pre><pre class="playpen"><code class="language-rust">fn main() {
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@@ -544,7 +544,7 @@ after it's initialized like this:</p>
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let mut test1_pin = unsafe { Pin::new_unchecked(&mut test1) };
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Test::init(test1_pin.as_mut());
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drop(test1_pin);
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let mut test2 = Test::new("test2");
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mem::swap(&mut test1, &mut test2);
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println!("Not self referential anymore: {:?}", test1.b);
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@@ -552,15 +552,15 @@ after it's initialized like this:</p>
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# use std::pin::Pin;
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# use std::marker::PhantomPinned;
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# use std::mem;
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#
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#
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# #[derive(Debug)]
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# struct Test {
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# a: String,
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# b: *const String,
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# _marker: PhantomPinned,
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# }
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#
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#
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#
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#
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# impl Test {
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# fn new(txt: &str) -> Self {
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# let a = String::from(txt);
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@@ -576,11 +576,11 @@ after it's initialized like this:</p>
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# let this = unsafe { self.get_unchecked_mut() };
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# this.b = self_ptr;
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# }
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#
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#
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# fn a<'a>(self: Pin<&'a Self>) -> &'a str {
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# &self.get_ref().a
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# }
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#
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#
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# fn b<'a>(self: Pin<&'a Self>) -> &'a String {
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# unsafe { &*(self.b) }
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# }
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@@ -659,7 +659,7 @@ certain operations on this value.</p>
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</li>
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<li>
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<p>Most standard library types implement <code>Unpin</code>. The same goes for most
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"normal" types you encounter in Rust. <code>Futures</code> and <code>Generators</code> are two
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"normal" types you encounter in Rust. <code>Future</code>s and <code>Generator</code>s are two
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exceptions.</p>
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</li>
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<li>
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@@ -688,8 +688,8 @@ by adding <code>std::marker::PhantomPinned</code> to your type on stable.</p>
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</li>
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</ol>
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<blockquote>
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<p>Unsafe code does not mean it's literally "unsafe", it only relieves the
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guarantees you normally get from the compiler. An <code>unsafe</code> implementation can
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<p>Unsafe code does not mean it's literally "unsafe", it only relieves the
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guarantees you normally get from the compiler. An <code>unsafe</code> implementation can
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be perfectly safe to do, but you have no safety net.</p>
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</blockquote>
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<h3><a class="header" href="#projectionstructural-pinning" id="projectionstructural-pinning">Projection/structural pinning</a></h3>
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@@ -702,7 +702,7 @@ for that.</p>
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In the <code>Drop</code> implementation you take a mutable reference to <code>self</code>, which means
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extra care must be taken when implementing <code>Drop</code> for pinned types.</p>
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<h2><a class="header" href="#putting-it-all-together" id="putting-it-all-together">Putting it all together</a></h2>
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<p>This is exactly what we'll do when we implement our own <code>Futures</code> stay tuned,
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<p>This is exactly what we'll do when we implement our own <code>Future</code>, so stay tuned,
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we're soon finished.</p>
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<h2><a class="header" href="#bonus-section-fixing-our-self-referential-generator-and-learning-more-about-pin" id="bonus-section-fixing-our-self-referential-generator-and-learning-more-about-pin">Bonus section: Fixing our self-referential generator and learning more about Pin</a></h2>
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<p>But now, let's prevent this problem using <code>Pin</code>. I've commented along the way to
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@@ -726,7 +726,7 @@ pub fn main() {
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let mut pinned1 = Box::pin(gen1);
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let mut pinned2 = Box::pin(gen2);
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// Uncomment these if you think it's safe to pin the values to the stack instead
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// Uncomment these if you think it's safe to pin the values to the stack instead
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// (it is in this case). Remember to comment out the two previous lines first.
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//let mut pinned1 = unsafe { Pin::new_unchecked(&mut gen1) };
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//let mut pinned2 = unsafe { Pin::new_unchecked(&mut gen2) };
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@@ -734,7 +734,7 @@ pub fn main() {
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if let GeneratorState::Yielded(n) = pinned1.as_mut().resume() {
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println!("Gen1 got value {}", n);
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}
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if let GeneratorState::Yielded(n) = pinned2.as_mut().resume() {
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println!("Gen2 got value {}", n);
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};
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@@ -749,8 +749,8 @@ pub fn main() {
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}
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enum GeneratorState<Y, R> {
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Yielded(Y),
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Complete(R),
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Yielded(Y),
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Complete(R),
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}
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trait Generator {
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Reference in New Issue
Block a user