Fix feature gating for new try bikeshed expressions

tests/ui/feature-gates/feature-gate-try_blocks_heterogeneous.rs

@@ -6,4 +6,11 @@ pub fn main() {
         x
     };
     assert_eq!(try_result, Some(5));
+
+    // The heterogenous form is new, so is gated even under a `cfg(false)`.
+    // See <https://github.com/rust-lang/rust/issues/152501>
+
+    #[cfg(false)]
+    try bikeshed () {}
+    //~^ error `try bikeshed` expression is experimental
 }

tests/ui/feature-gates/feature-gate-try_blocks_heterogeneous.stderr

@@ -12,6 +12,16 @@ LL | |     };
    = help: add `#![feature(try_blocks_heterogeneous)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error: aborting due to 1 previous error
+error[E0658]: `try bikeshed` expression is experimental
+  --> $DIR/feature-gate-try_blocks_heterogeneous.rs:14:5
+   |
+LL |     try bikeshed () {}
+   |     ^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #149488 <https://github.com/rust-lang/rust/issues/149488> for more information
+   = help: add `#![feature(try_blocks_heterogeneous)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0658`.

tests/ui/try-block/try-block-homogeneous-pre-expansion.rs

@@ -0,0 +1,12 @@
+//@ check-pass
+//@ edition: 2018
+
+// For historical reasons this is only a warning, not an error.
+// See <https://github.com/rust-lang/rust/issues/152501>
+
+fn main() {
+    #[cfg(false)]
+    try {}
+    //~^ warn `try` blocks are unstable
+    //~| warn unstable syntax can change at any point
+}

tests/ui/try-block/try-block-homogeneous-pre-expansion.stderr

@@ -0,0 +1,14 @@
+warning: `try` blocks are unstable
+  --> $DIR/try-block-homogeneous-pre-expansion.rs:9:5
+   |
+LL |     try {}
+   |     ^^^^^^
+   |
+   = note: see issue #31436 <https://github.com/rust-lang/rust/issues/31436> for more information
+   = help: add `#![feature(try_blocks)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = warning: unstable syntax can change at any point in the future, causing a hard error!
+   = note: for more information, see issue #65860 <https://github.com/rust-lang/rust/issues/65860>
+
+warning: 1 warning emitted
+