Match guard creates reference to scrutinee, observable via `repr(packed)`

[theemathas]

I'm not sure if this is a bug or working as intended, but I'm filing this issue because I find the behavior to be rather strange.

I tried this code:

#[repr(packed)]
#[derive(Copy, Clone)]
struct Thing {
    a: i16,
    b: i32,
}

fn foo() {
    let x = Thing { a: 1, b: 1 };
    match x.b {
        _temp if true => {}
        _ => {}
    }
}

This produces the following error, even though I'm not explicitly creating a reference anywhere:

error[E0793]: reference to packed field is unaligned
  --> src/lib.rs:11:9
   |
11 |         _temp if true => {}
   |         ^^^^^
   |
   = note: packed structs are only aligned by one byte, and many modern architectures penalize unaligned field accesses
   = note: creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
   = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)

For more information about this error, try `rustc --explain E0793`.

See also #144939, which probably has the same root cause.

@rustbot labels +A-patterns +A-repr-packed

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Reproducible on the playground with version 1.91.0-nightly (2025-08-05 ec7c02612527d185c379)