Auto merge of #51052 - nikomatsakis:obsolete-arrow, r=petrochenkov

restore emplacement syntax (obsolete)

Fix https://github.com/rust-lang/rust/issues/50832

r? @petrochenkov

src/test/ui/obsolete-in-place/bad.bad.stderr

@@ -0,0 +1,18 @@
+error: emplacement syntax is obsolete (for now, anyway)
+  --> $DIR/bad.rs:19:5
+   |
+LL |     x <- y; //[bad]~ ERROR emplacement syntax is obsolete
+   |     ^^^^^^
+   |
+   = note: for more information, see <https://github.com/rust-lang/rust/issues/27779#issuecomment-378416911>
+
+error: emplacement syntax is obsolete (for now, anyway)
+  --> $DIR/bad.rs:20:5
+   |
+LL |     in(foo) { bar }; //[bad]~ ERROR emplacement syntax is obsolete
+   |     ^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see <https://github.com/rust-lang/rust/issues/27779#issuecomment-378416911>
+
+error: aborting due to 2 previous errors
+

src/test/ui/obsolete-in-place/bad.rs

@@ -0,0 +1,25 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Check that `<-` and `in` syntax gets a hard error.
+
+// revisions: good bad
+//[good] run-pass
+
+#[cfg(bad)]
+fn main() {
+    let (x, y, foo, bar);
+    x <- y; //[bad]~ ERROR emplacement syntax is obsolete
+    in(foo) { bar }; //[bad]~ ERROR emplacement syntax is obsolete
+}
+
+#[cfg(good)]
+fn main() {
+}