ctutils: enforce Bytes* trait usage at compile-time
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The `Bytes*` traits exist because we don't have specialization[1] and
the generic implementation of these traits for `[T]` and `[T; N]` will
operate a byte-at-a-time on byte slices and byte arrays, when efficient
implementations from the `cmov` crate can instead perform the same
operations a word-at-a-time.
Adds != 1 const size assertions for `T` in the impls of `CtAssign`,
`CtEq` for `[T]` and `[T; N]` as well as the `CtSelect` impl for
`[T; N]` which cause a compile-time error if they're requested to be
used with 1-byte values:
error[E0080]: evaluation panicked: use `BytesCtEq::bytes_ct_eq` when working with byte-sized values
--> ctutils/src/traits/ct_eq.rs:81:13
|
81 | / assert!(
82 | | size_of::<T>() != 1,
83 | | "use `BytesCtEq::bytes_ct_eq` when working with byte-sized values"
84 | | );
| |_____________^ evaluation of `<[u8] as traits::ct_eq::CtEq>::ct_eq::{constant#0}` failed here
Each of them has been configured with a message that steers the user
instead towards the appropriate `BytesCt*` trait to use instead.
[1]: https://rust-lang.github.io/rfcs/1210-impl-specialization.html
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The
Bytes*traits exist because we don't have specialization1 and the generic implementation of these traits for[T]and[T; N]will operate a byte-at-a-time on byte slices and byte arrays, when efficient implementations from thecmovcrate can instead perform the same operations a word-at-a-time.Adds != 1 const size assertions for
Tin the impls ofCtAssign,CtEqfor[T]and[T; N]as well as theCtSelectimpl for[T; N]which cause a compile-time error if they're requested to be used with 1-byte values:Each of them has been configured with a message that steers the user instead towards the appropriate
BytesCt*trait to use instead.