Add description of auto traits in async blocks

This documents the proposed liveness-based rules for inferring auto
traits for async block. Currently we use a scope-based approach, so this
also serves as a description of we would like to change.

Author: eholk

src/expressions/block-expr.md

@@ -94,6 +94,25 @@ Async blocks capture variables from their environment using the same [capture mo
 Like closures, when written `async { .. }` the capture mode for each variable will be inferred from the content of the block.
 `async move { .. }` blocks however will move all referenced variables into the resulting future.
 
+### 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
+#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.
+
+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 count. 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].
+
 ### Async context
 
 Because async blocks construct a future, they define an **async context** which can in turn contain [`await` expressions].
@@ -189,3 +208,5 @@ 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
+[the scrutinee is live for the entire match block]: ../destructors.md#temporary-scopes