const_prop_lint: Consider array length constant even if array is not

compiler/rustc_mir_transform/src/const_prop_lint.rs

@@ -501,12 +501,23 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
                 return None;
             }
 
+            Rvalue::Len(len_place) => {
+                // To get the length of an array, we don't need to know the value.
+                let ty = len_place.ty(self.local_decls, self.tcx).ty;
+                if let &ty::Array(_, len) = ty.kind() {
+                    return self.use_ecx(source_info, |this| {
+                        let const_len = this.ecx.const_to_op(len, None)?;
+                        let ecx_place = this.ecx.eval_place(place)?;
+                        this.ecx.copy_op(&const_len, &ecx_place, /*allow_transmute*/ false)
+                    });
+                }
+            }
+
             // There's no other checking to do at this time.
             Rvalue::Aggregate(..)
             | Rvalue::Use(..)
             | Rvalue::CopyForDeref(..)
             | Rvalue::Repeat(..)
-            | Rvalue::Len(..)
             | Rvalue::Cast(..)
             | Rvalue::ShallowInitBox(..)
             | Rvalue::Discriminant(..)

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);