Trait std::marker::StructuralEq
source · pub trait StructuralEq { }🔬This is a nightly-only experimental API. (
structural_match #31434)Expand description
Required trait for constants used in pattern matches.
Any type that derives Eq automatically implements this trait, regardless
of whether its type parameters implement Eq.
This is a hack to work around a limitation in our type system.
Background
We want to require that types of consts used in pattern matches
have the attribute #[derive(PartialEq, Eq)].
In a more ideal world, we could check that requirement by just checking that
the given type implements both the StructuralPartialEq trait and
the Eq trait. However, you can have ADTs that do derive(PartialEq, Eq),
and be a case that we want the compiler to accept, and yet the constant’s
type fails to implement Eq.
Namely, a case like this:
#[derive(PartialEq, Eq)]
struct Wrap<X>(X);
fn higher_order(_: &()) { }
const CFN: Wrap<fn(&())> = Wrap(higher_order);
fn main() {
match CFN {
CFN => {}
_ => {}
}
}(The problem in the above code is that Wrap<fn(&())> does not implement
PartialEq, nor Eq, because for<'a> fn(&'a _) does not implement those
traits.)
Therefore, we cannot rely on naive check for StructuralPartialEq and
mere Eq.
As a hack to work around this, we use two separate traits injected by each
of the two derives (#[derive(PartialEq)] and #[derive(Eq)]) and check
that both of them are present as part of structural-match checking.
Implementors§
impl StructuralEq for BacktraceStatus
impl StructuralEq for std::cmp::Ordering
impl StructuralEq for TryReserveErrorKind
impl StructuralEq for VarError
impl StructuralEq for std::fmt::Alignment
impl StructuralEq for ErrorKind
impl StructuralEq for SeekFrom
impl StructuralEq for IpAddr
impl StructuralEq for Ipv6MulticastScope
impl StructuralEq for Shutdown
impl StructuralEq for SocketAddr
impl StructuralEq for FpCategory
impl StructuralEq for IntErrorKind
impl StructuralEq for BacktraceStyle
impl StructuralEq for Which
impl StructuralEq for SearchStep
impl StructuralEq for std::sync::atomic::Ordering
impl StructuralEq for RecvTimeoutError
impl StructuralEq for TryRecvError
impl StructuralEq for CpuidResult
impl StructuralEq for FromBytesUntilNulError
impl StructuralEq for AllocError
impl StructuralEq for Layout
impl StructuralEq for LayoutError
impl StructuralEq for TypeId
impl StructuralEq for CharTryFromError
impl StructuralEq for DecodeUtf16Error
impl StructuralEq for ParseCharError
impl StructuralEq for TryFromCharError
impl StructuralEq for TryReserveError
impl StructuralEq for CString
impl StructuralEq for FromBytesWithNulError
impl StructuralEq for FromVecWithNulError
impl StructuralEq for IntoStringError
impl StructuralEq for NulError
impl StructuralEq for Error
impl StructuralEq for FileType
impl StructuralEq for Permissions
impl StructuralEq for Assume
impl StructuralEq for AddrParseError
impl StructuralEq for Ipv4Addr
impl StructuralEq for Ipv6Addr
impl StructuralEq for SocketAddrV4
impl StructuralEq for SocketAddrV6
impl StructuralEq for NonZeroI8
impl StructuralEq for NonZeroI16
impl StructuralEq for NonZeroI32
impl StructuralEq for NonZeroI64
impl StructuralEq for NonZeroI128
impl StructuralEq for NonZeroIsize
impl StructuralEq for NonZeroU8
impl StructuralEq for NonZeroU16
impl StructuralEq for NonZeroU32
impl StructuralEq for NonZeroU64
impl StructuralEq for NonZeroU128
impl StructuralEq for NonZeroUsize
impl StructuralEq for ParseFloatError
impl StructuralEq for ParseIntError
impl StructuralEq for TryFromIntError
impl StructuralEq for RangeFull
impl StructuralEq for UCred
Available on Unix only.
impl StructuralEq for InvalidHandleError
Available on Windows only.
impl StructuralEq for NullHandleError
Available on Windows only.