Check index value <= 0xFFFF_FF00

compiler/rustc_mir_transform/src/known_panics_lint.rs

@@ -102,8 +102,12 @@ impl<'tcx> Value<'tcx> {
                 }
                 (PlaceElem::Index(idx), Value::Aggregate { fields, .. }) => {
                     let idx = prop.get_const(idx.into())?.immediate()?;
-                    let idx = prop.ecx.read_target_usize(idx).ok()?;
-                    fields.get(FieldIdx::from_u32(idx.try_into().ok()?)).unwrap_or(&Value::Uninit)
+                    let idx = prop.ecx.read_target_usize(idx).ok()?.try_into().ok()?;
+                    if idx <= FieldIdx::MAX_AS_U32 {
+                        fields.get(FieldIdx::from_u32(idx)).unwrap_or(&Value::Uninit)
+                    } else {
+                        return None;
+                    }
                 }
                 (
                     PlaceElem::ConstantIndex { offset, min_length: _, from_end: false },

tests/ui/indexing/index-bounds.rs

@@ -0,0 +1,10 @@
+//@ build-fail
+
+fn main() {
+    let _n = [64][200];
+    //~^ ERROR this operation will panic at runtime [unconditional_panic]
+
+    // issue #121126, test index value between 0xFFFF_FF00 and u32::MAX
+    let _n = [64][u32::MAX as usize - 1];
+    //~^ ERROR this operation will panic at runtime [unconditional_panic]
+}

tests/ui/indexing/index-bounds.stderr

@@ -0,0 +1,16 @@
+error: this operation will panic at runtime
+  --> $DIR/index-bounds.rs:4:14
+   |
+LL |     let _n = [64][200];
+   |              ^^^^^^^^^ index out of bounds: the length is 1 but the index is 200
+   |
+   = note: `#[deny(unconditional_panic)]` on by default
+
+error: this operation will panic at runtime
+  --> $DIR/index-bounds.rs:8:14
+   |
+LL |     let _n = [64][u32::MAX as usize - 1];
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^ index out of bounds: the length is 1 but the index is 4294967294
+
+error: aborting due to 2 previous errors
+