`#[repr()]` attribute has inconsistent handling of duplicates.

[theemathas]

The #[repr()] attribute can be given various options. It has the following inconsistent behavior when given duplicate attributes.

---

Multiple Rust reprs or multiple C reprs on structs are seemingly ignored.

Example

#[repr(Rust, Rust)]
struct Thing1(u8);

#[repr(C, C)]
struct Thing2(u8);

This code compiles without errors.

Multiple transparent reprs are a compile error.

Example

#[repr(transparent, transparent)]
struct Thing(u8);

Error:

error[E0692]: transparent struct cannot have other repr hints
 --> src/lib.rs:1:8
  |
1 | #[repr(transparent, transparent)]
  |        ^^^^^^^^^^^  ^^^^^^^^^^^

For more information about this error, try `rustc --explain E0692`.

---

Multiple align reprs are accepted. The greatest value is used as the actual alignment.

Example

#[repr(align(2), align(8), align(4))]
struct Thing(u8);

fn main() {
    assert_eq!(align_of::<Thing>(), 8);
}

This code compiles and runs without errors.

Multiple packed reprs with the same alignment value are accepted.

Example

#[repr(packed(2), packed(2))]
struct Thing(u32);

fn main() {
    assert_eq!(align_of::<Thing>(), 2);
}

This code compiles and runs without errors.

Multiple packed reprs with different alignment values are a compile error.

Example

#[repr(packed(2), packed(1))]
struct Thing(u32);

Error:

error[E0634]: type has conflicting packed representation hints
 --> src/lib.rs:2:1
  |
2 | struct Thing(u32);
  | ^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0634`.

---

Multiple integer reprs on enums are a FCW deny-by-default lint. See #68585.

Example

#[repr(u8, u8)]
enum Thing {
    Variant,
}

Error:

error[E0566]: conflicting representation hints
 --> src/lib.rs:1:8
  |
1 | #[repr(u8, u8)]
  |        ^^  ^^
  |
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #68585 <https://github.com/rust-lang/rust/issues/68585>
  = note: `#[deny(conflicting_repr_hints)]` (part of `#[deny(future_incompatible)]`) on by default

Multiple Rust reprs or multiple C reprs on enums on their own are seemingly ignored.

Example

#[repr(Rust, Rust)]
enum Thing1 {
    Variant(u8),
}

#[repr(C, C)]
enum Thing2 {
    Variant(u8),
}

This code compiles without errors.

Multiple Rust reprs, when combined with an integer repr, are a compile error.

Example

#[repr(Rust, Rust, u8)]
enum Thing {
    Variant(u8),
}

Error:

error[E0566]: conflicting representation hints
 --> src/lib.rs:1:8
  |
1 | #[repr(Rust, Rust, u8)]
  |        ^^^^  ^^^^  ^^

For more information about this error, try `rustc --explain E0566`.

Multiple C reprs, when combined with an integer repr, are seemingly ignored.

Example

#[repr(C, C, u8)]
enum Thing {
    Variant(u8),
}

This code compiles without errors.

Multiple integer reprs, when combined with a Rust repr, emits the FCW in #68585, but also emits a hard error. (This hard error is not emitted when Rust is replaced with C.)

Example

#[repr(Rust, u8, u8)]
enum Thing {
    Variant(u8),
}

Error:

error[E0566]: conflicting representation hints
 --> src/lib.rs:1:8
  |
1 | #[repr(Rust, u8, u8)]
  |        ^^^^  ^^  ^^

error[E0566]: conflicting representation hints
 --> src/lib.rs:1:8
  |
1 | #[repr(Rust, u8, u8)]
  |        ^^^^  ^^  ^^
  |
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #68585 <https://github.com/rust-lang/rust/issues/68585>
  = note: `#[deny(conflicting_repr_hints)]` (part of `#[deny(future_incompatible)]`) on by default

For more information about this error, try `rustc --explain E0566`.

---

cc @JonathanBrouwer @jdonszelmann

Meta

Reproducible on the playground with version 1.97.0-nightly (2026-04-29 c935696dd07ca51e6fba)

[el-ev]

I'd be happy to look into this issue. What is the preferred handling of duplicated attributes?