I tried this code (playpen):
fn main() {
let mut arr = [[Box::new(42)]];
let alias = &mut arr[0][{ let [row] = arr; drop(row); 0 }];
println!("{}", **alias); // use-after-free of arr's dead stack slot
}I expected to see this happen: Compiler rejects code as unsound.
Instead, this happened: Compiler accepts code and prints a garbage number.
When run under Miri, we see this output:
error: Undefined Behavior: constructing invalid value of type std::boxed::Box<i32>: encountered a dangling box (use-after-free)
--> src/main.rs:4:20
|
4 | println!("{}", **alias); // use-after-free of arr's dead stack slot
| ^^^^^^^ Undefined Behavior occurred here
|
= help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
= help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
Hypothesis: after a move _1[ConstantIndex{0 of 1}], a subsequent use of _1[Index(_6)] is not flagged as a use-of-partially-moved. The "any subpath moved" check of borrowck treats ConstantIndex as a sibling of a runtime Index projection, rather than a potential alias.
Meta
Nightly channel
Build using the Nightly version: 1.99.0-nightly
(2026-08-03 5048696)
I tried this code (playpen):
I expected to see this happen: Compiler rejects code as unsound.
Instead, this happened: Compiler accepts code and prints a garbage number.
When run under Miri, we see this output:
Hypothesis: after a
move _1[ConstantIndex{0 of 1}], a subsequent use of_1[Index(_6)]is not flagged as a use-of-partially-moved. The "any subpath moved" check of borrowck treatsConstantIndexas a sibling of a runtimeIndexprojection, rather than a potential alias.Meta
Nightly channel
Build using the Nightly version: 1.99.0-nightly
(2026-08-03 5048696)