Rollup merge of #63387 - Centril:async-block-control-flow-tests, r=cramertj

Test interaction between `async { ... }` and `?`, `return`, and `break`

Per the second checkbox in https://github.com/rust-lang/rust/issues/62121#issuecomment-506884048, test that `async { .. }` blocks:
1. do not allow `break` expressions.
2. get targeted by `return` and not the parent function.
3. get targeted by `?` and not the parent function.

Works towards resolving blockers in #63209.

r? @cramertj

src/test/ui/async-await/async-block-control-flow-static-semantics.rs

+// Test that `async { .. }` blocks:
+// 1. do not allow `break` expressions.
+// 2. get targeted by `return` and not the parent function.
+// 3. get targeted by `?` and not the parent function.
+//
+// edition:2018
+// ignore-tidy-linelength
+
+#![feature(async_await)]
+
+fn main() {}
+
+use core::future::Future;
+
+fn return_targets_async_block_not_fn() -> u8 {
+    //~^ ERROR mismatched types
+    let block = async {
+        return 0u8;
+    };
+    let _: &dyn Future<Output = ()> = &block;
+    //~^ ERROR type mismatch resolving `<impl std::future::Future as std::future::Future>::Output == ()`
+}
+
+async fn return_targets_async_block_not_async_fn() -> u8 {
+    //~^ ERROR type mismatch resolving
+    let block = async {
+        return 0u8;
+    };
+    let _: &dyn Future<Output = ()> = &block;
+    //~^ ERROR type mismatch resolving `<impl std::future::Future as std::future::Future>::Output == ()`
+}
+
+fn no_break_in_async_block() {
+    async {
+        break 0u8; //~ ERROR `break` inside of a closure
+        // FIXME: This diagnostic is pretty bad.
+    };
+}
+
+fn no_break_in_async_block_even_with_outer_loop() {
+    loop {
+        async {
+            break 0u8; //~ ERROR `break` inside of a closure
+        };
+    }
+}
+
+struct MyErr;
+fn err() -> Result<u8, MyErr> { Err(MyErr) }
+
+fn rethrow_targets_async_block_not_fn() -> Result<u8, MyErr> {
+    //~^ ERROR mismatched types
+    let block = async {
+        err()?;
+        Ok(())
+    };
+    let _: &dyn Future<Output = Result<(), MyErr>> = &block;
+}
+
+fn rethrow_targets_async_block_not_async_fn() -> Result<u8, MyErr> {
+    //~^ ERROR mismatched types
+    let block = async {
+        err()?;
+        Ok(())
+    };
+    let _: &dyn Future<Output = Result<(), MyErr>> = &block;
+}

src/test/ui/async-await/async-block-control-flow-static-semantics.stderr

+error[E0267]: `break` inside of a closure
+  --> $DIR/async-block-control-flow-static-semantics.rs:35:9
+   |
+LL |         break 0u8;
+   |         ^^^^^^^^^ cannot break inside of a closure
+
+error[E0267]: `break` inside of a closure
+  --> $DIR/async-block-control-flow-static-semantics.rs:43:13
+   |
+LL |             break 0u8;
+   |             ^^^^^^^^^ cannot break inside of a closure
+
+error[E0308]: mismatched types
+  --> $DIR/async-block-control-flow-static-semantics.rs:15:43
+   |
+LL | fn return_targets_async_block_not_fn() -> u8 {
+   |    ---------------------------------      ^^ expected u8, found ()
+   |    |
+   |    this function's body doesn't return
+   |
+   = note: expected type `u8`
+              found type `()`
+
+error[E0271]: type mismatch resolving `<impl std::future::Future as std::future::Future>::Output == ()`
+  --> $DIR/async-block-control-flow-static-semantics.rs:20:39
+   |
+LL |     let _: &dyn Future<Output = ()> = &block;
+   |                                       ^^^^^^ expected u8, found ()
+   |
+   = note: expected type `u8`
+              found type `()`
+   = note: required for the cast to the object type `dyn std::future::Future<Output = ()>`
+
+error[E0271]: type mismatch resolving `<impl std::future::Future as std::future::Future>::Output == ()`
+  --> $DIR/async-block-control-flow-static-semantics.rs:29:39
+   |
+LL |     let _: &dyn Future<Output = ()> = &block;
+   |                                       ^^^^^^ expected u8, found ()
+   |
+   = note: expected type `u8`
+              found type `()`
+   = note: required for the cast to the object type `dyn std::future::Future<Output = ()>`
+
+error[E0271]: type mismatch resolving `<impl std::future::Future as std::future::Future>::Output == u8`
+  --> $DIR/async-block-control-flow-static-semantics.rs:24:55
+   |
+LL | async fn return_targets_async_block_not_async_fn() -> u8 {
+   |                                                       ^^ expected (), found u8
+   |
+   = note: expected type `()`
+              found type `u8`
+   = note: the return type of a function must have a statically known size
+
+error[E0308]: mismatched types
+  --> $DIR/async-block-control-flow-static-semantics.rs:51:44
+   |
+LL | fn rethrow_targets_async_block_not_fn() -> Result<u8, MyErr> {
+   |    ----------------------------------      ^^^^^^^^^^^^^^^^^ expected enum `std::result::Result`, found ()
+   |    |
+   |    this function's body doesn't return
+   |
+   = note: expected type `std::result::Result<u8, MyErr>`
+              found type `()`
+
+error[E0308]: mismatched types
+  --> $DIR/async-block-control-flow-static-semantics.rs:60:50
+   |
+LL | fn rethrow_targets_async_block_not_async_fn() -> Result<u8, MyErr> {
+   |    ----------------------------------------      ^^^^^^^^^^^^^^^^^ expected enum `std::result::Result`, found ()
+   |    |
+   |    this function's body doesn't return
+   |
+   = note: expected type `std::result::Result<u8, MyErr>`
+              found type `()`
+
+error: aborting due to 8 previous errors
+
+Some errors have detailed explanations: E0267, E0271, E0308.
+For more information about an error, try `rustc --explain E0267`.