Indroduce feature flag for RFC-2229

Signed-off-by: Aman Arora <me@aman-arora.com>

src/test/ui/closures/2229_closure_analysis/feature-gate-capture_disjoint_fields.rs

@@ -0,0 +1,12 @@
+#![feature(capture_disjoint_fields)]
+//~^ WARNING the feature `capture_disjoint_fields` is incomplete
+#![feature(rustc_attrs)]
+
+fn main() {
+    let s = format!("s");
+
+    let c = #[rustc_capture_analysis] || {
+    //~^ ERROR: attributes on expressions are experimental
+        println!("This uses new capture analyysis to capture s={}", s);
+    };
+}

src/test/ui/closures/2229_closure_analysis/feature-gate-capture_disjoint_fields.stderr

@@ -0,0 +1,21 @@
+error[E0658]: attributes on expressions are experimental
+  --> $DIR/feature-gate-capture_disjoint_fields.rs:8:13
+   |
+LL |     let c = #[rustc_capture_analysis] || {
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #15701 <https://github.com/rust-lang/rust/issues/15701> for more information
+   = help: add `#![feature(stmt_expr_attributes)]` to the crate attributes to enable
+
+warning: the feature `capture_disjoint_fields` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/feature-gate-capture_disjoint_fields.rs:1:12
+   |
+LL | #![feature(capture_disjoint_fields)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #53488 <https://github.com/rust-lang/rust/issues/53488> for more information
+
+error: aborting due to previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/closures/2229_closure_analysis/feature-gate-capture_disjoint_fields.stdout

@@ -0,0 +1 @@
+For closure=DefId(0:4 ~ feature_gate_capture_disjoint_fields[317d]::main::{closure#0}): Using new-style capture analysis