Add rustc_peek test for liveness with borrows

Author: ecstatic-morse

src/test/ui/mir-dataflow/liveness-ptr.rs

@@ -0,0 +1,28 @@
+#![feature(core_intrinsics, rustc_attrs)]
+
+use std::intrinsics::rustc_peek;
+
+#[rustc_mir(rustc_peek_liveness, stop_after_dataflow)]
+fn foo() -> i32 {
+    let mut x: i32;
+    let mut p: *const i32;
+
+    x = 0;
+
+    // `x` is live here since it is used in the next statement...
+    unsafe { rustc_peek(x); }
+
+    p = &x;
+
+    // ... but not here, even while it can be accessed through `p`.
+    unsafe { rustc_peek(x); } //~ ERROR rustc_peek: bit not set
+    let tmp = unsafe { *p };
+
+    x = tmp + 1;
+
+    unsafe { rustc_peek(x); }
+
+    x
+}
+
+fn main() {}

src/test/ui/mir-dataflow/liveness-ptr.stderr

@@ -0,0 +1,10 @@
+error: rustc_peek: bit not set
+  --> $DIR/liveness-ptr.rs:18:14
+   |
+LL |     unsafe { rustc_peek(x); }
+   |              ^^^^^^^^^^^^^
+
+error: stop_after_dataflow ended compilation
+
+error: aborting due to 2 previous errors
+