Auto merge of #111590 - dtolnay:panictemporaries, r=bjorn3

Shorten even more panic temporary lifetimes

Followup to #104134. As pointed out by `@bjorn3` in https://github.com/rust-lang/rust/pull/104134#pullrequestreview-1425585948, there are other cases in the panic macros which would also benefit from dropping their non-Send temporaries as soon as possible, avoiding pointlessly holding them across an await point.

For the tests added in this PR, here are the failures you get today on master without the macro changes in this PR:

<details>
<summary>tests/ui/macros/panic-temporaries-2018.rs</summary>

```console
error: future cannot be sent between threads safely
  --> tests/ui/macros/panic-temporaries-2018.rs:52:18
   |
LL |     require_send(panic_display());
   |                  ^^^^^^^^^^^^^^^ future returned by `panic_display` is not `Send`
   |
   = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `*const u8`
note: future is not `Send` as this value is used across an await
  --> tests/ui/macros/panic-temporaries-2018.rs:35:31
   |
LL |     f(panic!("{}", NOT_SEND)).await;
   |                    --------   ^^^^^- `NOT_SEND` is later dropped here
   |                    |          |
   |                    |          await occurs here, with `NOT_SEND` maybe used later
   |                    has type `NotSend` which is not `Send`
note: required by a bound in `require_send`
  --> tests/ui/macros/panic-temporaries-2018.rs:48:25
   |
LL | fn require_send(_: impl Send) {}
   |                         ^^^^ required by this bound in `require_send`

error: future cannot be sent between threads safely
  --> tests/ui/macros/panic-temporaries-2018.rs:52:18
   |
LL |     require_send(panic_display());
   |                  ^^^^^^^^^^^^^^^ future returned by `panic_display` is not `Send`
   |
   = help: within `NotSend`, the trait `Sync` is not implemented for `*const u8`
note: future is not `Send` as this value is used across an await
  --> tests/ui/macros/panic-temporaries-2018.rs:35:31
   |
LL |     f(panic!("{}", NOT_SEND)).await;
   |       ----------------------  ^^^^^- the value is later dropped here
   |       |                       |
   |       |                       await occurs here, with the value maybe used later
   |       has type `&NotSend` which is not `Send`
note: required by a bound in `require_send`
  --> tests/ui/macros/panic-temporaries-2018.rs:48:25
   |
LL | fn require_send(_: impl Send) {}
   |                         ^^^^ required by this bound in `require_send`

error: future cannot be sent between threads safely
  --> tests/ui/macros/panic-temporaries-2018.rs:53:18
   |
LL |     require_send(panic_str());
   |                  ^^^^^^^^^^^ future returned by `panic_str` is not `Send`
   |
   = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `*const u8`
note: future is not `Send` as this value is used across an await
  --> tests/ui/macros/panic-temporaries-2018.rs:40:36
   |
LL |     f(panic!((NOT_SEND, "...").1)).await;
   |               --------             ^^^^^- `NOT_SEND` is later dropped here
   |               |                    |
   |               |                    await occurs here, with `NOT_SEND` maybe used later
   |               has type `NotSend` which is not `Send`
note: required by a bound in `require_send`
  --> tests/ui/macros/panic-temporaries-2018.rs:48:25
   |
LL | fn require_send(_: impl Send) {}
   |                         ^^^^ required by this bound in `require_send`

error: future cannot be sent between threads safely
  --> tests/ui/macros/panic-temporaries-2018.rs:54:18
   |
LL |     require_send(unreachable_display());
   |                  ^^^^^^^^^^^^^^^^^^^^^ future returned by `unreachable_display` is not `Send`
   |
   = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `*const u8`
note: future is not `Send` as this value is used across an await
  --> tests/ui/macros/panic-temporaries-2018.rs:45:31
   |
LL |     f(unreachable!(NOT_SEND)).await;
   |                    --------   ^^^^^- `NOT_SEND` is later dropped here
   |                    |          |
   |                    |          await occurs here, with `NOT_SEND` maybe used later
   |                    has type `NotSend` which is not `Send`
note: required by a bound in `require_send`
  --> tests/ui/macros/panic-temporaries-2018.rs:48:25
   |
LL | fn require_send(_: impl Send) {}
   |                         ^^^^ required by this bound in `require_send`

error: future cannot be sent between threads safely
  --> tests/ui/macros/panic-temporaries-2018.rs:54:18
   |
LL |     require_send(unreachable_display());
   |                  ^^^^^^^^^^^^^^^^^^^^^ future returned by `unreachable_display` is not `Send`
   |
   = help: within `NotSend`, the trait `Sync` is not implemented for `*const u8`
note: future is not `Send` as this value is used across an await
  --> tests/ui/macros/panic-temporaries-2018.rs:45:31
   |
LL |     f(unreachable!(NOT_SEND)).await;
   |       ----------------------  ^^^^^- the value is later dropped here
   |       |                       |
   |       |                       await occurs here, with the value maybe used later
   |       has type `&NotSend` which is not `Send`
note: required by a bound in `require_send`
  --> tests/ui/macros/panic-temporaries-2018.rs:48:25
   |
LL | fn require_send(_: impl Send) {}
   |                         ^^^^ required by this bound in `require_send`

error: aborting due to 5 previous errors
```
</details>

<details>
<summary>tests/ui/macros/panic-temporaries.rs</summary>

```console
error: future cannot be sent between threads safely
  --> tests/ui/macros/panic-temporaries.rs:42:18
   |
LL |     require_send(panic_display());
   |                  ^^^^^^^^^^^^^^^ future returned by `panic_display` is not `Send`
   |
   = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `*const u8`
note: future is not `Send` as this value is used across an await
  --> tests/ui/macros/panic-temporaries.rs:35:31
   |
LL |     f(panic!("{}", NOT_SEND)).await;
   |                    --------   ^^^^^- `NOT_SEND` is later dropped here
   |                    |          |
   |                    |          await occurs here, with `NOT_SEND` maybe used later
   |                    has type `NotSend` which is not `Send`
note: required by a bound in `require_send`
  --> tests/ui/macros/panic-temporaries.rs:38:25
   |
LL | fn require_send(_: impl Send) {}
   |                         ^^^^ required by this bound in `require_send`

error: future cannot be sent between threads safely
  --> tests/ui/macros/panic-temporaries.rs:42:18
   |
LL |     require_send(panic_display());
   |                  ^^^^^^^^^^^^^^^ future returned by `panic_display` is not `Send`
   |
   = help: within `NotSend`, the trait `Sync` is not implemented for `*const u8`
note: future is not `Send` as this value is used across an await
  --> tests/ui/macros/panic-temporaries.rs:35:31
   |
LL |     f(panic!("{}", NOT_SEND)).await;
   |       ----------------------  ^^^^^- the value is later dropped here
   |       |                       |
   |       |                       await occurs here, with the value maybe used later
   |       has type `&NotSend` which is not `Send`
note: required by a bound in `require_send`
  --> tests/ui/macros/panic-temporaries.rs:38:25
   |
LL | fn require_send(_: impl Send) {}
   |                         ^^^^ required by this bound in `require_send`

error: aborting due to 2 previous errors
```
</details>

r? bjorn3

2 files changed, 83 insertions, 4 deletions

diff --git a/tests/ui/macros/panic-temporaries-2018.rs b/tests/ui/macros/panic-temporaries-2018.rs
new file mode 100644
index 00000000000..d914df38062
--- /dev/null
+++ b/tests/ui/macros/panic-temporaries-2018.rs
@@ -0,0 +1,55 @@
+// check-pass
+// edition:2018
+
+#![allow(non_fmt_panics, unreachable_code)]
+
+use std::fmt::{self, Display};
+use std::marker::PhantomData;
+
+struct NotSend {
+    marker: PhantomData<*const u8>,
+}
+
+const NOT_SEND: NotSend = NotSend { marker: PhantomData };
+
+impl Display for NotSend {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("this value does not implement Send")
+    }
+}
+
+async fn f(_: u8) {}
+
+// Exercises this matcher in panic_2015:
+// ($fmt:expr, $($arg:tt)+) => $crate::panicking::panic_fmt(...)
+async fn panic_fmt() {
+    // Panic returns `!`, so the await is never reached, and in particular the
+    // temporaries inside the formatting machinery are not still alive at the
+    // await point.
+    let todo = "...";
+    f(panic!("not yet implemented: {}", todo)).await;
+}
+
+// Exercises ("{}", $arg:expr) => $crate::panicking::panic_display(&$arg)
+async fn panic_display() {
+    f(panic!("{}", NOT_SEND)).await;
+}
+
+// Exercises ($msg:expr) => $crate::panicking::panic_str($msg)
+async fn panic_str() {
+    f(panic!((NOT_SEND, "...").1)).await;
+}
+
+// Exercises ($msg:expr) => $crate::panicking::unreachable_display(&$msg)
+async fn unreachable_display() {
+    f(unreachable!(NOT_SEND)).await;
+}
+
+fn require_send(_: impl Send) {}
+
+fn main() {
+    require_send(panic_fmt());
+    require_send(panic_display());
+    require_send(panic_str());
+    require_send(unreachable_display());
+}
diff --git a/tests/ui/macros/panic-temporaries.rs b/tests/ui/macros/panic-temporaries.rs
index 5b5b8b7c2d9..db65601fb73 100644
--- a/tests/ui/macros/panic-temporaries.rs
+++ b/tests/ui/macros/panic-temporaries.rs
@@ -3,17 +3,41 @@
 
 #![allow(unreachable_code)]
 
+use std::fmt::{self, Display};
+use std::marker::PhantomData;
+
+struct NotSend {
+    marker: PhantomData<*const u8>,
+}
+
+const NOT_SEND: NotSend = NotSend { marker: PhantomData };
+
+impl Display for NotSend {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("this value does not implement Send")
+    }
+}
+
 async fn f(_: u8) {}
 
-async fn g() {
-    // Todo returns `!`, so the await is never reached, and in particular the
+// Exercises this matcher in panic_2021:
+// ($($t:tt)+) => $crate::panicking::panic_fmt(...)
+async fn panic_fmt() {
+    // Panic returns `!`, so the await is never reached, and in particular the
     // temporaries inside the formatting machinery are not still alive at the
     // await point.
-    f(todo!("...")).await;
+    let todo = "...";
+    f(panic!("not yet implemented: {}", todo)).await;
+}
+
+// Exercises ("{}", $arg:expr) => $crate::panicking::panic_display(&$arg)
+async fn panic_display() {
+    f(panic!("{}", NOT_SEND)).await;
 }
 
 fn require_send(_: impl Send) {}
 
 fn main() {
-    require_send(g());
+    require_send(panic_fmt());
+    require_send(panic_display());
 }