Auto merge of #31739 - steveklabnik:rollup, r=steveklabnik

- Successful merges: #31565, #31679, #31694, #31695, #31703, #31720, #31733
- Failed merges:

RELEASES.md

@@ -94,11 +94,11 @@ Misc
   the `-Z unstable-options` flag.
 * [When running tests with `--test`, rustdoc will pass `--cfg`
   arguments to the compiler][1.7dt].
-* [The compiler is built with RPATH information by default][1.7rp].
+* [The compiler is built with RPATH information by default][1.7rpa].
   This means that it will be possible to run `rustc` when installed in
   unusual configurations without configuring the dynamic linker search
   path explicitly.
-* [`rustc` passes `--enable-new-dtags` to GNU ld][1.7dt]. This makes
+* [`rustc` passes `--enable-new-dtags` to GNU ld][1.7dta]. This makes
   any RPATH entries (emitted with `-C rpath`) *not* take precedence
   over `LD_LIBRARY_PATH`.
 
@@ -132,15 +132,15 @@ Compatibility Notes
 [1.7cp]: https://github.com/rust-lang/cargo/pull/2224
 [1.7d]: https://github.com/rust-lang/rust/pull/30724
 [1.7dt]: https://github.com/rust-lang/rust/pull/30372
-[1.7dt]: https://github.com/rust-lang/rust/pull/30394
+[1.7dta]: https://github.com/rust-lang/rust/pull/30394
 [1.7f]: https://github.com/rust-lang/rust/pull/30672
 [1.7h]: https://github.com/rust-lang/rust/pull/30818
 [1.7j]: https://github.com/rust-lang/rust/pull/30711
 [1.7ll]: https://github.com/rust-lang/rust/pull/30663
 [1.7m]: https://github.com/rust-lang/rust/pull/30381
 [1.7p]: https://github.com/rust-lang/rust/pull/30681
 [1.7rp]: https://github.com/rust-lang/rust/pull/29498
-[1.7rp]: https://github.com/rust-lang/rust/pull/30353
+[1.7rpa]: https://github.com/rust-lang/rust/pull/30353
 [1.7rr]: https://github.com/rust-lang/cargo/pull/2279
 [1.7sf]: https://github.com/rust-lang/rust/pull/30389
 [1.7utf8]: https://github.com/rust-lang/rust/pull/30740

src/doc/book/ownership.md

@@ -124,21 +124,65 @@ special annotation here, it’s the default thing that Rust does.
 ## The details
 
 The reason that we cannot use a binding after we’ve moved it is subtle, but
-important. When we write code like this:
+important. 
+
+When we write code like this:
+
+```rust
+let x = 10;
+```
+
+Rust allocates memory for an integer [i32] on the [stack][sh], copies the bit
+pattern representing the value of 10 to the allocated memory and binds the
+variable name x to this memory region for future reference.
+
+Now consider the following code fragment:
 
 ```rust
 let v = vec![1, 2, 3];
 
-let v2 = v;
+let mut v2 = v;
+```
+
+The first line allocates memory for the vector object `v` on the stack like
+it does for `x` above. But in addition to that it also allocates some memory
+on the [heap][sh] for the actual data (`[1, 2, 3]`). Rust copies the address
+of this heap allocation to an internal pointer, which is part of the vector
+object placed on the stack (let's call it the data pointer). 
+
+It is worth pointing out (even at the risk of stating the obvious) that the
+vector object and its data live in separate memory regions instead of being a
+single contiguous memory allocation (due to reasons we will not go into at
+this point of time). These two parts of the vector (the one on the stack and
+one on the heap) must agree with each other at all times with regards to
+things like the length, capacity etc.
+
+When we move `v` to `v2`, rust actually does a bitwise copy of the vector
+object `v` into the stack allocation represented by `v2`. This shallow copy
+does not create a copy of the heap allocation containing the actual data.
+Which means that there would be two pointers to the contents of the vector
+both pointing to the same memory allocation on the heap. It would violate
+Rust’s safety guarantees by introducing a data race if one could access both
+`v` and `v2` at the same time. 
+
+For example if we truncated the vector to just two elements through `v2`:
+
+```rust
+# let v = vec![1, 2, 3];
+# let mut v2 = v;
+v2.truncate(2);
 ```
 
-The first line allocates memory for the vector object, `v`, and for the data it
-contains. The vector object is stored on the [stack][sh] and contains a pointer
-to the content (`[1, 2, 3]`) stored on the [heap][sh]. When we move `v` to `v2`,
-it creates a copy of that pointer, for `v2`. Which means that there would be two
-pointers to the content of the vector on the heap. It would violate Rust’s
-safety guarantees by introducing a data race. Therefore, Rust forbids using `v`
-after we’ve done the move.
+and `v1` were still accessible we'd end up with an invalid vector since `v1`
+would not know that the heap data has been truncated. Now, the part of the
+vector `v1` on the stack does not agree with the corresponding part on the
+heap. `v1` still thinks there are three elements in the vector and will
+happily let us access the non existent element `v1[2]` but as you might
+already know this is a recipe for disaster. Especially because it might lead
+to a segmentation fault or worse allow an unauthorized user to read from
+memory to which they don't have access.
+
+This is why Rust forbids using `v` after we’ve done the move.
 
 [sh]: the-stack-and-the-heap.html
 

src/libcollections/btree/map.rs

@@ -375,9 +375,10 @@ impl<K: Ord, V> BTreeMap<K, V> {
     ///
     /// If the map did not have this key present, `None` is returned.
     ///
-    /// If the map did have this key present, the key is not updated, the
-    /// value is updated and the old value is returned.
-    /// See the [module-level documentation] for more.
+    /// If the map did have this key present, the value is updated, and the old
+    /// value is returned. The key is not updated, though; this matters for
+    /// types that can be `==` without being identical. See the [module-level
+    /// documentation] for more.
     ///
     /// [module-level documentation]: index.html#insert-and-complex-keys
     ///

src/libcore/mem.rs

@@ -571,9 +571,25 @@ pub const POST_DROP_USIZE: usize = POST_DROP_U64 as usize;
 /// ```
 /// use std::mem;
 ///
-/// let one = unsafe { mem::transmute_copy(&1) };
+/// #[repr(packed)]
+/// struct Foo {
+///     bar: u8,
+/// }
+///
+/// let foo_slice = [10u8];
+///
+/// unsafe {
+///     // Copy the data from 'foo_slice' and treat it as a 'Foo'
+///     let mut foo_struct: Foo = mem::transmute_copy(&foo_slice);
+///     assert_eq!(foo_struct.bar, 10);
+///
+///     // Modify the copied data
+///     foo_struct.bar = 20;
+///     assert_eq!(foo_struct.bar, 20);
+/// }
 ///
-/// assert_eq!(1, one);
+/// // The contents of 'foo_slice' should not have changed
+/// assert_eq!(foo_slice, [10]);
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]