Rollup merge of #55071 - oli-obk:const_cast_🍨, r=RalfJung

Fix ICE and report a human readable error

fixes #55063

r? @RalfJung

src/test/ui/consts/min_const_fn/bad_const_fn_body_ice.rs

@@ -0,0 +1,5 @@
+const fn foo(a: i32) -> Vec<i32> {
+    vec![1, 2, 3] //~ ERROR heap allocations are not allowed in const fn
+}
+
+fn main() {}

src/test/ui/consts/min_const_fn/bad_const_fn_body_ice.stderr

@@ -0,0 +1,10 @@
+error: heap allocations are not allowed in const fn
+  --> $DIR/bad_const_fn_body_ice.rs:2:5
+   |
+LL |     vec![1, 2, 3] //~ ERROR heap allocations are not allowed in const fn
+   |     ^^^^^^^^^^^^^
+   |
+   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
+
+error: aborting due to previous error
+

src/test/ui/consts/min_const_fn/cast_errors.rs

@@ -0,0 +1,14 @@
+fn main() {}
+
+const fn unsize(x: &[u8; 3]) -> &[u8] { x }
+//~^ ERROR unsizing casts are not allowed in const fn
+const fn closure() -> fn() { || {} }
+//~^ ERROR function pointers in const fn are unstable
+const fn closure2() {
+    (|| {}) as fn();
+//~^ ERROR function pointers in const fn are unstable
+}
+const fn reify(f: fn()) -> unsafe fn() { f }
+//~^ ERROR function pointers in const fn are unstable
+const fn reify2() { main as unsafe fn(); }
+//~^ ERROR function pointers in const fn are unstable

src/test/ui/consts/min_const_fn/cast_errors.stderr

@@ -0,0 +1,32 @@
+error: unsizing casts are not allowed in const fn
+  --> $DIR/cast_errors.rs:3:41
+   |
+LL | const fn unsize(x: &[u8; 3]) -> &[u8] { x }
+   |                                         ^
+
+error: function pointers in const fn are unstable
+  --> $DIR/cast_errors.rs:5:23
+   |
+LL | const fn closure() -> fn() { || {} }
+   |                       ^^^^
+
+error: function pointers in const fn are unstable
+  --> $DIR/cast_errors.rs:8:5
+   |
+LL |     (|| {}) as fn();
+   |     ^^^^^^^^^^^^^^^
+
+error: function pointers in const fn are unstable
+  --> $DIR/cast_errors.rs:11:28
+   |
+LL | const fn reify(f: fn()) -> unsafe fn() { f }
+   |                            ^^^^^^^^^^^
+
+error: function pointers in const fn are unstable
+  --> $DIR/cast_errors.rs:13:21
+   |
+LL | const fn reify2() { main as unsafe fn(); }
+   |                     ^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 5 previous errors
+