Revise and extend text on extending expressions
In particular, let's note that the final expression of an async block
expression is not extending; we need to say that as we otherwise say
that the final expression of a block expression is extending, and
async blocks are ostensibly blocks in our ontology.
Let's build the pass tests with Rust 2024 to ensure that we're not
relying on the old edition tail expression rule.
When talking about an arm of an extending match expression, let's be
clear that we're talking about the arm expression.
Let's break apart all of the failure tests; otherwise we can't be sure
that they're all failing, and let's mark the expected error code for
each failure.
For each test, let's note the applicable rule for the outermost part
of the expression.
Let's add some tests to show that the operand of `break` is not
extending, as that might be a bit surprising.
Let's demonstrate explicitly that raw borrows count as borrows.
diff --git a/src/destructors.md b/src/destructors.md
index 6d6226d..ff62b00 100644
--- a/src/destructors.md
+++ b/src/destructors.md
@@ -434,9 +434,9 @@
expression], [braced struct][struct expression], or [tuple][tuple expression]
expression.
* The arguments to an extending [tuple struct] or [tuple variant] constructor expression.
-* The final expression of any extending [block expression].
-* The arm(s) of an extending [`match`] expression.
-* The final expressions of an extending [`if`] expression's consequent, `else if`, and `else` blocks.
+* The final expression of an extending [block expression] except for an [async block expression].
+* The final expression of an extending [`if`] expression's consequent, `else if`, or `else` block.
+* An arm expression of an extending [`match`] expression.
So the borrow expressions in `&mut 0`, `(&1, &mut 2)`, and `Some(&mut 3)`
are all extending expressions. The borrows in `&0 + &1` and `f(&mut 0)` are not.
@@ -448,45 +448,95 @@
Here are some examples where expressions have extended temporary scopes:
-```rust
-# fn temp() {}
-// The temporary that stores the result of `temp()` lives in the same scope
-// as x in these cases.
-let x = &temp();
+```rust,edition2024
+# use core::sync::atomic::{AtomicU64, Ordering::Relaxed};
+# static X: AtomicU64 = AtomicU64::new(0);
+# struct S;
+# impl Drop for S { fn drop(&mut self) { X.fetch_add(1, Relaxed); } }
+# const fn temp() -> S { S }
+let x = &temp(); // Operand of borrow.
# x;
-let x = &temp() as &dyn Send;
+let x = &raw const *&temp(); // Operand of raw borrow.
+# assert_eq!(X.load(Relaxed), 0);
+let x = &temp() as &dyn Send; // Operand of cast.
# x;
-let x = (&*&temp(),);
+let x = (&*&temp(),); // Operand of tuple constructor.
# x;
-let x = { [Some(&temp()) ] };
+let x = { [Some(&temp())] }; // Final expr of block.
# x;
-let x = match () { _ => &temp() };
+let x = const { &temp() }; // Final expr of `const` block.
+# x;
+let x = unsafe { &temp() }; // Final expr of `unsafe` block.
# x;
let x = if true { &temp() } else { &temp() };
+// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+// Final exprs of `if`/`else` blocks.
# x;
-let ref x = temp();
+let x = match () { _ => &temp() }; // `match` arm expression.
# x;
-let ref x = *&temp();
+let ref x = temp(); // Initializer expression.
# x;
+let ref x = *&temp(); // Initializer expression.
+# x;
+//
+// All of the temporaries above are still live here.
+# assert_eq!(X.load(Relaxed), 0);
```
Here are some examples where expressions don't have extended temporary scopes:
-```rust,compile_fail
+```rust,compile_fail,E0716
+# fn temp() {}
+// Arguments to function calls are not extending expressions. The
+// temporary is dropped at the semicolon.
+let x = core::convert::identity(&temp()); // ERROR
+# x;
+```
+
+```rust,compile_fail,E0716
# fn temp() {}
# trait Use { fn use_temp(&self) -> &Self { self } }
# impl Use for () {}
-// The temporary that stores the result of `temp()` only lives until the
-// end of the let statement in these cases.
+// Receivers of method calls are not extending expressions.
+let x = (&temp()).use_temp(); // ERROR
+# x;
+```
-let x = std::convert::identity(&temp()); // ERROR
-# x;
-let x = (&temp()).use_temp(); // ERROR
-# x;
+```rust,compile_fail,E0716
+# fn temp() {}
+// Scrutinees of match expressions are not extending expressions.
let x = match &temp() { x => x }; // ERROR
# x;
```
+```rust,compile_fail,E0515
+# fn temp() {}
+// Final expressions of `async` blocks are not extending expressions.
+let x = async { &temp() }; // ERROR
+# x;
+```
+
+```rust,compile_fail,E0515
+# fn temp() {}
+// Final expressions of closures are not extending expressions.
+let x = || &temp(); // ERROR
+# x;
+```
+
+```rust,compile_fail,E0716
+# fn temp() {}
+// Operands of loop breaks are not extending expressions.
+let x = loop { break &temp() }; // ERROR
+# x;
+```
+
+```rust,compile_fail,E0716
+# fn temp() {}
+// Operands of breaks to labels are not extending expressions.
+let x = 'a: { break 'a &temp() }; // ERROR
+# x;
+```
+
r[destructors.forget]
## Not running destructors
@@ -544,6 +594,7 @@
[tuple variant]: type.enum.declaration
[array expression]: expressions/array-expr.md#array-expressions
+[async block expression]: expr.block.async
[block expression]: expressions/block-expr.md
[borrow expression]: expressions/operator-expr.md#borrow-operators
[cast expression]: expressions/operator-expr.md#type-cast-expressions
