Rollup merge of rust-lang#64856 - jonhoo:format-temporaries, r=sfackler

Scope format! temporaries

This places the temporaries that `format!` generates to refer to its arguments (through `&dyn Trait`) in a short-lived scope surrounding just the invocation of `format!`. This enables `format!` to be used in generators without the temporaries preventing the generator from being `Send` (due to `dyn Trait` not being `Sync`).

See rust-lang#64477 for details.

Author: Centril

src/liballoc/macros.rs

@@ -98,5 +98,8 @@ macro_rules! vec {
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
 macro_rules! format {
-    ($($arg:tt)*) => ($crate::fmt::format($crate::__export::format_args!($($arg)*)))
+    ($($arg:tt)*) => {{
+        let res = $crate::fmt::format($crate::__export::format_args!($($arg)*));
+        res
+    }}
 }

src/test/pretty/issue-4264.pp

@@ -29,33 +29,37 @@
 
 
 
-                  ((::alloc::fmt::format as
-                       for<'r> fn(std::fmt::Arguments<'r>) -> std::string::String {std::fmt::format})(((::core::fmt::Arguments::new_v1
-                                                                                                           as
-                                                                                                           fn(&[&str], &[std::fmt::ArgumentV1<'_>]) -> std::fmt::Arguments<'_> {std::fmt::Arguments::<'_>::new_v1})((&([("test"
-                                                                                                                                                                                                                            as
-                                                                                                                                                                                                                            &'static str)]
-                                                                                                                                                                                                                          as
-                                                                                                                                                                                                                          [&str; 1])
-                                                                                                                                                                                                                        as
-                                                                                                                                                                                                                        &[&str; 1]),
-                                                                                                                                                                                                                    (&(match (()
+                  ({
+                       let res =
+                           ((::alloc::fmt::format as
+                                for<'r> fn(std::fmt::Arguments<'r>) -> std::string::String {std::fmt::format})(((::core::fmt::Arguments::new_v1
+                                                                                                                    as
+                                                                                                                    fn(&[&str], &[std::fmt::ArgumentV1<'_>]) -> std::fmt::Arguments<'_> {std::fmt::Arguments::<'_>::new_v1})((&([("test"
+                                                                                                                                                                                                                                     as
+                                                                                                                                                                                                                                     &'static str)]
+                                                                                                                                                                                                                                   as
+                                                                                                                                                                                                                                   [&str; 1])
                                                                                                                                                                                                                                  as
-                                                                                                                                                                                                                                 ())
-                                                                                                                                                                                                                           {
-                                                                                                                                                                                                                           ()
-                                                                                                                                                                                                                           =>
-                                                                                                                                                                                                                           ([]
-                                                                                                                                                                                                                               as
-                                                                                                                                                                                                                               [std::fmt::ArgumentV1<'_>; 0]),
-                                                                                                                                                                                                                       }
-                                                                                                                                                                                                                          as
-                                                                                                                                                                                                                          [std::fmt::ArgumentV1<'_>; 0])
-                                                                                                                                                                                                                        as
-                                                                                                                                                                                                                        &[std::fmt::ArgumentV1<'_>; 0]))
-                                                                                                          as
-                                                                                                          std::fmt::Arguments<'_>))
-                      as std::string::String);
+                                                                                                                                                                                                                                 &[&str; 1]),
+                                                                                                                                                                                                                             (&(match (()
+                                                                                                                                                                                                                                          as
+                                                                                                                                                                                                                                          ())
+                                                                                                                                                                                                                                    {
+                                                                                                                                                                                                                                    ()
+                                                                                                                                                                                                                                    =>
+                                                                                                                                                                                                                                    ([]
+                                                                                                                                                                                                                                        as
+                                                                                                                                                                                                                                        [std::fmt::ArgumentV1<'_>; 0]),
+                                                                                                                                                                                                                                }
+                                                                                                                                                                                                                                   as
+                                                                                                                                                                                                                                   [std::fmt::ArgumentV1<'_>; 0])
+                                                                                                                                                                                                                                 as
+                                                                                                                                                                                                                                 &[std::fmt::ArgumentV1<'_>; 0]))
+                                                                                                                   as
+                                                                                                                   std::fmt::Arguments<'_>))
+                               as std::string::String);
+                       (res as std::string::String)
+                   } as std::string::String);
               } as ())
 pub type Foo = [i32; (3 as usize)];
 pub struct Bar {

src/test/ui/async-await/issues/issue-64477-2.rs

@@ -0,0 +1,22 @@
+// Another regression test for #64477.
+//
+// In the past, the code generated by `format!` produced temporaries in the surrounding scope that
+// borrowed the arguments through `&dyn Trait`. These temporaries do not implement `Send`, which
+// meant that when `format!` was used in an async block, the resulting generator was not `Send`.
+// See https://github.com/rust-lang/rust/issues/64477#issuecomment-534669068 for details
+// and https://github.com/rust-lang/rust/issues/64477#issuecomment-531882958 for an example.
+//
+// check-pass
+// edition:2018
+
+async fn foo(_: String) {}
+
+fn bar() -> impl Send {
+    async move {
+        foo(format!("{}:{}", 1, 2)).await;
+    }
+}
+
+fn main() {
+    let _ = bar();
+}