Add a section dedicated to Edition 2024 changes to temporary scopes

Author: dingxiangfei2009

src/destructors.md

@@ -211,6 +211,8 @@ smallest scope that contains the expression and is one of the following:
   guard.
 * The body expression for a match arm.
 * The second operand of a [lazy boolean expression].
+* *Starting from Edition 2024* the pattern-matching condition and consequent body of [`if let`].
+* *Starting from Edition 2024* the entirety of the tail expression of a block.
 
 > **Notes**:
 >
@@ -260,6 +262,96 @@ match PrintOnDrop("Matched value in final expression") {
 }
 ```
 
+#### Changes to temporary scopes by Edition 2024
+
+Edition 2024 introduces two changes to the language regarding temporary scopes.
+First, temporary values generated from evaluating the pattern-matching condition
+of `if let` will be dropped earlier in general.
+
+```rust,edition2024
+struct Droppy(u8);
+impl Droppy {
+    fn get(&self) -> Option<&u8> {
+        Some(&self.0)
+    }
+}
+impl Drop for Droppy {
+    fn drop(&mut self) {
+        println!("drop({})", self.0);
+    }
+}
+
+fn call(x: &u8, y: u8, z: &u8) {
+    println!("call with {x} {y} {z}");
+}
+
+let x1 = 0;
+let x2 = 3;
+
+call(
+    if let Some(x) = Droppy(0).get() {
+        println!("if let consequence");
+        &x1
+    } else {
+        &x2
+    },
+    {
+      let y = Droppy(1);
+      *y.get().unwrap()
+    },
+    Droppy(2).get().unwrap()
+);
+println!("call ended");
+```
+
+This will print the following
+```
+if let consequence
+drop(0)
+drop(1)
+call with 0 1 2
+drop(2)
+call ended
+```
+
+In other words, `Droppy(0)` is dropped before `Droppy(1)` because its temporary scope is
+limited by Edition 2024 to the proper end of the `if let` expression.
+
+Second, temporary values generated from evaluating the tail expression of a block
+or a function body will be dropped earlier than the local variable bindings.
+The following example demonstrates the Edition 2024 order of events that values
+and local variables are dropped.
+
+```rust,edition2024
+struct Droppy(u8);
+impl Droppy {
+    fn get(&self) -> Option<&u8> {
+        Some(&self.0)
+    }
+}
+impl Drop for Droppy {
+    fn drop(&mut self) {
+        println!("drop({})", self.0);
+    }
+}
+
+fn call() -> u8 {
+    let x = Droppy(0);
+    *x.get() + *Droppy(1).get()
+}
+
+call();
+```
+
+The print-out from this program will be the following.
+
+```
+drop(1)
+drop(0)
+```
+
+In other words, `Droppy(1)` is dropped earlier than `x`.
+
 ### Operands
 
 r[destructors.scope.operands]