Add test for closure migration where body is a block fragment.

Author: m-ou-se

src/test/ui/closures/2229_closure_analysis/migrations/closure-body-macro-fragment.fixed

@@ -0,0 +1,25 @@
+// run-rustfix
+// edition:2018
+// check-pass
+#![warn(rust_2021_compatibility)]
+
+macro_rules! m {
+    (@ $body:expr) => {{
+        let f = || $body;
+        //~^ WARNING: drop order
+        f();
+    }};
+    ($body:block) => {{
+        m!(@ $body);
+    }};
+}
+
+fn main() {
+    let a = (1.to_string(), 2.to_string());
+    m!({
+        let _ = &a;
+        //~^ HELP: add a dummy
+        let x = a.0;
+        println!("{}", x);
+    });
+}

src/test/ui/closures/2229_closure_analysis/migrations/closure-body-macro-fragment.rs

@@ -0,0 +1,24 @@
+// run-rustfix
+// edition:2018
+// check-pass
+#![warn(rust_2021_compatibility)]
+
+macro_rules! m {
+    (@ $body:expr) => {{
+        let f = || $body;
+        //~^ WARNING: drop order
+        f();
+    }};
+    ($body:block) => {{
+        m!(@ $body);
+    }};
+}
+
+fn main() {
+    let a = (1.to_string(), 2.to_string());
+    m!({
+        //~^ HELP: add a dummy
+        let x = a.0;
+        println!("{}", x);
+    });
+}

src/test/ui/closures/2229_closure_analysis/migrations/closure-body-macro-fragment.stderr

@@ -0,0 +1,37 @@
+warning: changes to closure capture in Rust 2021 will affect drop order
+  --> $DIR/closure-body-macro-fragment.rs:8:17
+   |
+LL |           let f = || $body;
+   |  _________________^
+LL | |
+LL | |         f();
+LL | |     }};
+   | |     - in Rust 2018, `a` is dropped here, but in Rust 2021, only `a.0` will be dropped here as part of the closure
+LL | |     ($body:block) => {{
+LL | |         m!(@ $body);
+   | |__________________^
+...
+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: 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: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/disjoint-capture-in-closures.html>
+   = 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
+