const_eval: Consider array length constant even if array is not

compiler/rustc_const_eval/src/interpret/step.rs

@@ -4,6 +4,7 @@
 
 use rustc_middle::mir;
 use rustc_middle::mir::interpret::{InterpResult, Scalar};
+use rustc_middle::ty;
 use rustc_middle::ty::layout::LayoutOf;
 
 use super::{InterpCx, Machine};
@@ -251,9 +252,14 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 
             Len(place) => {
                 let src = self.eval_place(place)?;
-                let mplace = self.force_allocation(&src)?;
-                let len = mplace.len(self)?;
-                self.write_scalar(Scalar::from_machine_usize(len, self), &dest)?;
+                if let &ty::Array(_, len) = src.layout.ty.kind() {
+                    let len = self.const_to_op(len, Some(dest.layout))?;
+                    self.copy_op(&len, &dest, /*allow_transmute*/ false)?;
+                } else {
+                    let mplace = self.force_allocation(&src)?;
+                    let len = mplace.len(self)?;
+                    self.write_scalar(Scalar::from_machine_usize(len, self), &dest)?;
+                }
             }
 
             AddressOf(_, place) | Ref(_, _, place) => {

src/test/ui/const_prop/issue-98444-const-index-out-of-bounds.rs

@@ -0,0 +1,8 @@
+// build-fail
+// Need to use build-fail because check doesn't run constant propagation.
+
+fn main() {
+    let xs: [i32; 5] = [1, 2, 3, 4, 5];
+    let _ = &xs;
+    let _ = xs[7]; //~ ERROR this operation will panic at runtime
+}

src/test/ui/const_prop/issue-98444-const-index-out-of-bounds.stderr

@@ -0,0 +1,10 @@
+error: this operation will panic at runtime
+  --> $DIR/issue-98444-const-index-out-of-bounds.rs:7:13
+   |
+LL |     let _ = xs[7];
+   |             ^^^^^ index out of bounds: the length is 5 but the index is 7
+   |
+   = note: `#[deny(unconditional_panic)]` on by default
+
+error: aborting due to previous error
+

src/test/ui/consts/issue-65348.rs

@@ -1,4 +1,5 @@
 // check-pass
+#![allow(unconditional_panic)]
 
 struct Generic<T>(T);