Rollup merge of #71276 - RalfJung:miri-unleash-box, r=ecstatic-morse

miri-unleashed: test that we detect heap allocations

This removes the second-to-last use of `IS_SUPPORTED_IN_MIRI = false`.

r? @ecstatic-morse @oli-obk

src/test/ui/consts/miri_unleashed/box.rs

@@ -0,0 +1,14 @@
+// compile-flags: -Zunleash-the-miri-inside-of-you
+#![feature(const_mut_refs, box_syntax)]
+#![deny(const_err)]
+
+use std::mem::ManuallyDrop;
+
+fn main() {}
+
+static TEST_BAD: &mut i32 = {
+    &mut *(box 0)
+    //~^ WARN skipping const check
+    //~| ERROR could not evaluate static initializer
+    //~| NOTE heap allocations
+};

src/test/ui/consts/miri_unleashed/box.stderr

@@ -0,0 +1,15 @@
+warning: skipping const checks
+  --> $DIR/box.rs:10:11
+   |
+LL |     &mut *(box 0)
+   |           ^^^^^^^
+
+error[E0080]: could not evaluate static initializer
+  --> $DIR/box.rs:10:11
+   |
+LL |     &mut *(box 0)
+   |           ^^^^^^^ "heap allocations via `box` keyword" needs an rfc before being allowed inside constants
+
+error: aborting due to previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0080`.