warning: changes to closure capture in Rust 2021 will affect drop order
  --> $DIR/closure-body-macro-fragment.rs:16:17
   |
LL |           let f = || $body;
   |                   ^^
...
LL |       }};
   |       - in Rust 2018, `a` is dropped here, but in Rust 2021, only `a.0` will be dropped here as part of the closure
...
LL | /     m!({
LL | |
LL | |         let x = a.0;
   | |                 --- in Rust 2018, this closure captures all of `a`, but in Rust 2021, it will only capture `a.0`
LL | |         println!("{:?}", x);
LL | |     });
   | |______- in this macro invocation
   |
   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/disjoint-capture-in-closures.html>
note: the lint level is defined here
  --> $DIR/closure-body-macro-fragment.rs:4:9
   |
LL | #![warn(rust_2021_compatibility)]
   |         ^^^^^^^^^^^^^^^^^^^^^^^
   = note: `#[warn(rust_2021_incompatible_closure_captures)]` implied by `#[warn(rust_2021_compatibility)]`
   = note: this warning originates in the macro `m` (in Nightly builds, run with -Z macro-backtrace for more info)
help: add a dummy let to cause `a` to be fully captured
   |
LL ~     m!({
LL +         let _ = &a;
   |

warning: 1 warning emitted