Delay a bug when indexing unsized slices

compiler/rustc_const_eval/src/check_consts/check.rs

@@ -634,7 +634,12 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
             Rvalue::RawPtr(RawPtrKind::FakeForPtrMetadata, place) => {
                 // These are only inserted for slice length, so the place must already be indirect.
                 // This implies we do not have to worry about whether the borrow escapes.
-                assert!(place.is_indirect(), "fake borrows are always indirect");
+                if !place.is_indirect() {
+                    self.tcx.dcx().span_delayed_bug(
+                        self.body.source_info(location).span,
+                        "fake borrows are always indirect",
+                    );
+                }
             }
 
             Rvalue::Cast(

tests/ui/consts/const-slice-array-deref.rs

@@ -0,0 +1,9 @@
+const ONE: [u16] = [1];
+//~^ ERROR the size for values of type `[u16]` cannot be known at compilation time
+//~| ERROR the size for values of type `[u16]` cannot be known at compilation time
+//~| ERROR mismatched types
+
+const TWO: &'static u16 = &ONE[0];
+//~^ ERROR cannot move a value of type `[u16]`
+
+fn main() {}

tests/ui/consts/const-slice-array-deref.stderr

@@ -0,0 +1,33 @@
+error[E0277]: the size for values of type `[u16]` cannot be known at compilation time
+  --> $DIR/const-slice-array-deref.rs:1:12
+   |
+LL | const ONE: [u16] = [1];
+   |            ^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `[u16]`
+
+error[E0308]: mismatched types
+  --> $DIR/const-slice-array-deref.rs:1:20
+   |
+LL | const ONE: [u16] = [1];
+   |                    ^^^ expected `[u16]`, found `[u16; 1]`
+
+error[E0277]: the size for values of type `[u16]` cannot be known at compilation time
+  --> $DIR/const-slice-array-deref.rs:1:20
+   |
+LL | const ONE: [u16] = [1];
+   |                    ^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `[u16]`
+   = note: constant expressions must have a statically known size
+
+error[E0161]: cannot move a value of type `[u16]`
+  --> $DIR/const-slice-array-deref.rs:6:28
+   |
+LL | const TWO: &'static u16 = &ONE[0];
+   |                            ^^^ the size of `[u16]` cannot be statically determined
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0161, E0277, E0308.
+For more information about an error, try `rustc --explain E0161`.