Document using liveness to infer auto traits for async blocks

src/expressions/block-expr.md

@@ -117,6 +117,30 @@ loop {
 }
 ```
 
+### Auto traits and `async` blocks
+
+Auto trait inference for `async` blocks follow the same [rules as closures] except that values that are live across an `await` expression are also considered.
+Live values are variables or temporaries that are defined before an `await` expression and potentially used afterwards.
+As an example, see below:
+```rust
+# struct Bar;
+# async fn foo() {}
+async {
+    let x = Bar;
+    foo().await;
+    drop(x);
+}
+# ;
+```
+Here the resulting future will be `Send` if `Bar` is send, since `x` is defined before the await and used afterwards.
+If the `drop(x)` line were removed, then `x` would not be considered in auto trait inference because it is not live across the await point.
+
+Note that for values of types that implement `Drop`, there is an implicit use of the value at the end of its lifetime in order to run the destructor.
+
+Besides named variables, temporary values also affect auto trait inference.
+For example in `foo(&bar, baz.await)`, the value `&bar` is considered live across the `await` point.
+This is also true of the scrutinee of the match expression, since [the scrutinee is live for the entire match block][temporary-scopes].
+
 ## `unsafe` blocks
 
 > **<sup>Syntax</sup>**\
@@ -189,3 +213,5 @@ fn is_unix_platform() -> bool {
 [tuple expressions]: tuple-expr.md
 [unsafe operations]: ../unsafety.md
 [value expressions]: ../expressions.md#place-expressions-and-value-expressions
+[rules as closures]: ../special-types-and-traits.md#auto-traits
+[temporary-scopes]: ../destructors.md#temporary-scopes