A few fixes for the unsafe chapter (#2805)

See individual commits in https://github.com/google/comprehensive-rust/pull/2805. Fixes #2734.

src/unsafe-rust/dereferencing.md

@@ -8,27 +8,32 @@ Creating pointers is safe, but dereferencing them requires `unsafe`:
 
 ```rust,editable
 fn main() {
-    let mut s = String::from("careful!");
+    let mut x = 10;
 
-    let r1 = &raw mut s;
-    let r2 = r1 as *const String;
+    let p1: *mut i32 = &raw mut x;
+    let p2 = p1 as *const i32;
 
-    // SAFETY: r1 and r2 were obtained from references and so are guaranteed to
-    // be non-null and properly aligned, the objects underlying the references
-    // from which they were obtained are live throughout the whole unsafe
-    // block, and they are not accessed either through the references or
-    // concurrently through any other pointers.
+    // SAFETY: p1 and p2 were created by taking raw pointers to a local, so they
+    // are guaranteed to be non-null, aligned, and point into a single (stack-)
+    // allocated object.
+    //
+    // The object underlying the raw pointers lives for the entire function, so
+    // it is not deallocated while the raw pointers still exist. It is not
+    // accessed through references while the raw pointers exist, nor is it
+    // accessed from other threads concurrently.
     unsafe {
-        println!("r1 is: {}", *r1);
-        *r1 = String::from("uhoh");
-        println!("r2 is: {}", *r2);
+        dbg!(*p1);
+        *p1 = 6;
+        // Mutation may soundly be observed through a raw pointer, like in C.
+        dbg!(*p2);
     }
 
-    // NOT SAFE. DO NOT DO THIS.
+    // UNSOUND. DO NOT DO THIS.
     /*
-    let r3: &String = unsafe { &*r1 };
-    drop(s);
-    println!("r3 is: {}", *r3);
+    let r: &i32 = unsafe { &*p1 };
+    dbg!(r);
+    x = 50;
+    dbg!(r); // Object underlying the reference has been mutated. This is UB.
     */
 }
 ```
@@ -52,8 +57,11 @@ In the case of pointer dereferences, this means that the pointers must be
 
 In most cases the pointer must also be properly aligned.
 
-The "NOT SAFE" section gives an example of a common kind of UB bug: `*r1` has
-the `'static` lifetime, so `r3` has type `&'static String`, and thus outlives
-`s`. Creating a reference from a pointer requires _great care_.
+The "UNSOUND" section gives an example of a common kind of UB bug: naïvely
+taking a reference to the dereference of a raw pointer sidesteps the compiler's
+knowledge of what object the reference is actually pointing to. As such, the
+borrow checker does not freeze `x` and so we are able to modify it despite the
+existence of a reference to it. Creating a reference from a pointer requires
+_great care_.
 
 </details>

src/unsafe-rust/mutable-static.md

@@ -14,15 +14,18 @@ fn main() {
 }
 ```
 
-However, since data races can occur, it is unsafe to read and write mutable
-static variables:
+However, mutable static variables are unsafe to read and write because multiple
+threads could do so concurrently without synchronization, constituting a data
+race.
+
+Using mutable statics soundly requires reasoning about concurrency without the
+compiler's help:
 
 ```rust,editable
 static mut COUNTER: u32 = 0;
 
 fn add_to_counter(inc: u32) {
     // SAFETY: There are no other threads which could be accessing `COUNTER`.
-    #[allow(static_mut_refs)]
     unsafe {
         COUNTER += inc;
     }
@@ -32,7 +35,6 @@ fn main() {
     add_to_counter(42);
 
     // SAFETY: There are no other threads which could be accessing `COUNTER`.
-    #[allow(static_mut_refs)]
     unsafe {
         dbg!(COUNTER);
     }
@@ -41,13 +43,12 @@ fn main() {
 
 <details>
 
-- The program here is safe because it is single-threaded. However, the Rust
+- The program here is sound because it is single-threaded. However, the Rust
   compiler reasons about functions individually so can't assume that. Try
   removing the `unsafe` and see how the compiler explains that it is undefined
   behavior to access a mutable static from multiple threads.
-- Rust 2024 edition goes further and makes accessing a mutable static by
-  reference an error by default. We work around this in the example with
-  `#[allow(static_mut_refs)]`. Don't do this.
+- The 2024 Rust edition goes further and makes accessing a mutable static by
+  reference an error by default.
 - Using a mutable static is almost always a bad idea, you should use interior
   mutability instead.
 - There are some cases where it might be necessary in low-level `no_std` code,

src/unsafe-rust/unsafe-functions.md

@@ -7,10 +7,10 @@ minutes: 15
 A function or method can be marked `unsafe` if it has extra preconditions you
 must uphold to avoid undefined behaviour.
 
-There are two main categories:
+Unsafe functions may come from two places:
 
-- Rust functions declared unsafe with `unsafe fn`.
-- Foreign functions in `extern "C"` blocks.
+- Rust functions declared unsafe.
+- Unsafe foreign functions in `extern "C"` blocks.
 
 <details>
 

src/unsafe-rust/unsafe-functions/extern-c.md

@@ -1,6 +1,9 @@
 # Unsafe External Functions
 
-Functions in a foreign language may also be unsafe:
+You can declare foreign functions for access from Rust with `unsafe extern`.
+This is unsafe because the compiler has to way to reason about their behavior.
+Functions declared in an `extern` block must be marked as `safe` or `unsafe`,
+depending on whether they have preconditions for safe use:
 
 ```rust,editable
 use std::ffi::c_char;