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Amend RFC 1758 (repr(transparent)) w.r.t. repr(align) 

The RFC made contradictory statements on whether repr(align(N)) can be combined with repr(transparent). [Discussion on the tracking issue][discuss] did not find a good reason to support it, so this commit makes the RFC internally consistent by prohibiting align(N) on transparent structs.

[discuss]: https://github.com/rust-lang/rust/issues/43036#issuecomment-352172909
This commit is contained in:
Robin Kruppe 2017-12-29 17:29:32 +01:00
parent b38be1133a
commit f07f9499ca
1 changed files with 3 additions and 5 deletions
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@ -117,16 +117,14 @@ have the same representation as their underlying types.
[RFC 1649]: https://github.com/rust-lang/rfcs/pull/1649
This new representation cannot be used with any other representation attribute
but alignment, to be able to specify a transparent wrapper with additional
alignment constraints:
This new representation cannot be used with any other representation attribute:
```rust
#[repr(transparent, align = "128")]
struct OverAligned(f64); // Behaves as a bare f64 with 128 bits alignment.
struct BogusAlign(f64); // Error, must be aligned like the underlying type.
#[repr(C, transparent)]
struct BogusRepr(f64); // Nonsensical, repr cannot be C and transparent.
struct BogusRepr(f64); // Error, repr cannot be C and transparent.
```
As a matter of optimisation, eligible `#[repr(Rust)]` structs behave as if