Miscompilation of a program projecting field of an extern type
Open
Nobody has claimed this yet.
A-mir-opt
C-bug
F-extern_types
I-miscompile
I-unsound
requires-nightly
T-compiler
T-opsem
- Dominant language
- Rust
- Stars
- 119k
- Forks
- 16.1k
- PR merge metrics
- PR metrics pending
Description
Since #118534 projecting a field of an extern type at non-zero offset panics. MIR transformations do not understand this:
$ cat a.rs
#![feature(extern_types)]
extern "C" {
type Opaque;
}
fn main() {
let a = [0u8; 1];
let b: &(u8, Opaque) = unsafe { &*(&a as *const _ as *const (u8, Opaque)) };
let _ = &b.1;
}
$ rustc a.rs -Zmir-opt-level=1 && ./a
$ rustc a.rs -Zmir-opt-level=0 && ./a
thread 'main' panicked at library/core/src/panicking.rs:221:5:
attempted to compute the size or alignment of extern type `Opaque`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread caused non-unwinding panic. aborting.
Aborted
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Reproduce the issue with the provided a.rs program using rustc at -Zmir-opt-level=1 and -Zmir-opt-level=0, then investigate the MIR transformations implicated in the report. Done means the two optimization levels no longer differ in this case and the program does not panic while projecting the extern type field.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- rust
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100