Improve chapter about Vec<T> (#381)

Co-authored-by: Daniel Henry-Mantilla <[email protected]>

Author: Noratrieb

src/vec/vec-alloc.md

@@ -28,7 +28,6 @@ impl<T> Vec<T> {
             ptr: NonNull::dangling(),
             len: 0,
             cap: 0,
-            _marker: PhantomData,
         }
     }
 }

src/vec/vec-final.md

@@ -10,7 +10,6 @@ use std::ptr::{self, NonNull};
 struct RawVec<T> {
     ptr: NonNull<T>,
     cap: usize,
-    _marker: PhantomData<T>,
 }
 
 unsafe impl<T: Send> Send for RawVec<T> {}
@@ -25,7 +24,6 @@ impl<T> RawVec<T> {
         RawVec {
             ptr: NonNull::dangling(),
             cap: cap,
-            _marker: PhantomData,
         }
     }
 

src/vec/vec-insert-remove.md

@@ -22,9 +22,11 @@ pub fn insert(&mut self, index: usize, elem: T) {
 
     unsafe {
         // ptr::copy(src, dest, len): "copy from src to dest len elems"
-        ptr::copy(self.ptr.as_ptr().add(index),
-                  self.ptr.as_ptr().add(index + 1),
-                  self.len - index);
+        ptr::copy(
+            self.ptr.as_ptr().add(index),
+            self.ptr.as_ptr().add(index + 1),
+            self.len - index,
+        );
         ptr::write(self.ptr.as_ptr().add(index), elem);
         self.len += 1;
     }
@@ -42,9 +44,11 @@ pub fn remove(&mut self, index: usize) -> T {
     unsafe {
         self.len -= 1;
         let result = ptr::read(self.ptr.as_ptr().add(index));
-        ptr::copy(self.ptr.as_ptr().add(index + 1),
-                  self.ptr.as_ptr().add(index),
-                  self.len - index);
+        ptr::copy(
+            self.ptr.as_ptr().add(index + 1),
+            self.ptr.as_ptr().add(index),
+            self.len - index,
+        );
         result
     }
 }

src/vec/vec-into-iter.md

@@ -49,7 +49,6 @@ pub struct IntoIter<T> {
     cap: usize,
     start: *const T,
     end: *const T,
-    _marker: PhantomData<T>,
 }
 ```
 
@@ -61,13 +60,13 @@ impl<T> IntoIterator for Vec<T> {
     type Item = T;
     type IntoIter = IntoIter<T>;
     fn into_iter(self) -> IntoIter<T> {
-        // Can't destructure Vec since it's Drop
-        let ptr = self.ptr;
-        let cap = self.cap;
-        let len = self.len;
-
         // Make sure not to drop Vec since that would free the buffer
-        mem::forget(self);
+        let vec = ManuallyDrop::new(self);
+
+        // Can't destructure Vec since it's Drop
+        let ptr = vec.ptr;
+        let cap = vec.cap;
+        let len = vec.len;
 
         unsafe {
             IntoIter {
@@ -80,7 +79,6 @@ impl<T> IntoIterator for Vec<T> {
                 } else {
                     ptr.as_ptr().add(len)
                 },
-                _marker: PhantomData,
             }
         }
     }

src/vec/vec-layout.md

@@ -15,13 +15,10 @@ pub struct Vec<T> {
 }
 ```
 
-And indeed this would compile. Unfortunately, it would be incorrect. First, the
+And indeed this would compile. Unfortunately, it would be too strict. The
 compiler will give us too strict variance. So a `&Vec<&'static str>`
-couldn't be used where an `&Vec<&'a str>` was expected. More importantly, it
-will give incorrect ownership information to the drop checker, as it will
-conservatively assume we don't own any values of type `T`. See [the chapter
-on ownership and lifetimes][ownership] for all the details on variance and
-drop check.
+couldn't be used where a `&Vec<&'a str>` was expected. See [the chapter
+on ownership and lifetimes][ownership] for all the details on variance.
 
 As we saw in the ownership chapter, the standard library uses `Unique<T>` in place of
 `*mut T` when it has a raw pointer to an allocation that it owns. Unique is unstable,
@@ -30,16 +27,16 @@ so we'd like to not use it if possible, though.
 As a recap, Unique is a wrapper around a raw pointer that declares that:
 
 * We are covariant over `T`
-* We may own a value of type `T` (for drop check)
+* We may own a value of type `T` (this is not relevant for our example here, but see 
+  [the chapter on PhantomData][phantom-data] on why the real `std::vec::Vec<T>` needs this)
 * We are Send/Sync if `T` is Send/Sync
 * Our pointer is never null (so `Option<Vec<T>>` is null-pointer-optimized)
 
 We can implement all of the above requirements in stable Rust. To do this, instead
 of using `Unique<T>` we will use [`NonNull<T>`][NonNull], another wrapper around a
 raw pointer, which gives us two of the above properties, namely it is covariant
-over `T` and is declared to never be null. By adding a `PhantomData<T>` (for drop
-check) and implementing Send/Sync if `T` is, we get the same results as using
-`Unique<T>`:
+over `T` and is declared to never be null. By implementing Send/Sync if `T` is,
+we get the same results as using `Unique<T>`:
 
 ```rust
 use std::ptr::NonNull;
@@ -49,7 +46,6 @@ pub struct Vec<T> {
     ptr: NonNull<T>,
     cap: usize,
     len: usize,
-    _marker: PhantomData<T>,
 }
 
 unsafe impl<T: Send> Send for Vec<T> {}
@@ -58,4 +54,5 @@ unsafe impl<T: Sync> Sync for Vec<T> {}
 ```
 
 [ownership]: ../ownership.html
+[phantom-data]: ../phantom-data.md
 [NonNull]: ../../std/ptr/struct.NonNull.html

src/vec/vec-raw.md

@@ -13,7 +13,6 @@ allocating, growing, and freeing:
 struct RawVec<T> {
     ptr: NonNull<T>,
     cap: usize,
-    _marker: PhantomData<T>,
 }
 
 unsafe impl<T: Send> Send for RawVec<T> {}
@@ -25,7 +24,6 @@ impl<T> RawVec<T> {
         RawVec {
             ptr: NonNull::dangling(),
             cap: 0,
-            _marker: PhantomData,
         }
     }
 

src/vec/vec-zsts.md

@@ -33,14 +33,13 @@ method of `RawVec`.
 ```rust,ignore
 impl<T> RawVec<T> {
     fn new() -> Self {
-        // !0 is usize::MAX. This branch should be stripped at compile time.
-        let cap = if mem::size_of::<T>() == 0 { !0 } else { 0 };
+        // This branch should be stripped at compile time.
+        let cap = if mem::size_of::<T>() == 0 { usize::MAX } else { 0 };
 
         // `NonNull::dangling()` doubles as "unallocated" and "zero-sized allocation"
         RawVec {
             ptr: NonNull::dangling(),
             cap: cap,
-            _marker: PhantomData,
         }
     }