Unsoundness due to calling `#[track_caller]` function in Generic Const Expression

[rodrimati1992]

I tried running this code in miri:

#![feature(generic_const_exprs)]
#![allow(incomplete_features)]

#[track_caller]
const fn sneaky() -> usize {
    std::panic::Location::caller().column() as usize % 10
}

fn foo<const N: usize>(array: [u8; N + sneaky()]) -> [u8; N + sneaky()] {
    array
}

fn main() {
    dbg!(foo::<0>([]));
}

I expected to see this happen: the code fails to compile because sneaky returns different values for the length of the two array types.

Instead, this happened: the code compiles and produces UB:

error: Undefined Behavior: transmuting from 0-byte type to 3-byte type: `[u8; 0]` -> `[u8; 3]`
  --> src/main.rs:10:5
   |
10 |     array
   |     ^^^^^ 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
   = note: BACKTRACE:
   = note: inside `foo::<0>` at src/main.rs:10:5: 10:10
note: inside `main`
  --> src/main.rs:14:10
   |
14 |     dbg!(foo::<0>([]));
   |          ^^^^^^^^^^^^
   = note: inside `<fn() as std::ops::FnOnce<()>>::call_once - shim(fn())` at /home/me/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5: 250:71
   = note: inside `std::sys::backtrace::__rust_begin_short_backtrace::<fn(), ()>` at /home/me/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys/backtrace.rs:158:18: 158:21
   = note: inside closure at /home/me/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:206:18: 206:75
   = note: inside `std::ops::function::impls::<impl std::ops::FnOnce<()> for &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>::call_once` at /home/me/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:287:13: 287:31
   = note: inside `std::panicking::catch_unwind::do_call::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>` at /home/me/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:590:40: 590:43
   = note: inside `std::panicking::catch_unwind::<i32, &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>` at /home/me/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:553:19: 553:88
   = note: inside `std::panic::catch_unwind::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>` at /home/me/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panic.rs:359:14: 359:40
   = note: inside closure at /home/me/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:175:24: 175:49
   = note: inside `std::panicking::catch_unwind::do_call::<{closure@std::rt::lang_start_internal::{closure#0}}, isize>` at /home/me/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:590:40: 590:43
   = note: inside `std::panicking::catch_unwind::<isize, {closure@std::rt::lang_start_internal::{closure#0}}>` at /home/me/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:553:19: 553:88
   = note: inside `std::panic::catch_unwind::<{closure@std::rt::lang_start_internal::{closure#0}}, isize>` at /home/me/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panic.rs:359:14: 359:40
   = note: inside `std::rt::lang_start_internal` at /home/me/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:171:5: 193:7
   = note: inside `std::rt::lang_start::<()>` at /home/me/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:205:5: 210:6

Meta

rustc --version --verbose:

rustc 1.92.0-nightly (595b9a498 2025-10-03)
binary: rustc
commit-hash: 595b9a498bc55fcd30111e430d8e4290ed833b4c
commit-date: 2025-10-03
host: x86_64-unknown-linux-gnu
release: 1.92.0-nightly
LLVM version: 21.1.2

[workingjubilee]

@BoxyUwU do we want to blame this one on GCE or on the standard library?

[BoxyUwU]

i expect gce