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);

#[allow(pointer_structural_match)]
fn main() {
    match CFN {
        CFN => {}
        _ => {}
    }
}

Run

(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.

Trait std::marker::StructuralEq

§Background

Implementors§

impl StructuralEq for AsciiChar

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 bool

impl StructuralEq for char

impl StructuralEq for i8

impl StructuralEq for i16

impl StructuralEq for i32

impl StructuralEq for i64

impl StructuralEq for i128

impl StructuralEq for isize

impl StructuralEq for str

impl StructuralEq for u8

impl StructuralEq for u16

impl StructuralEq for u32

impl StructuralEq for u64

impl StructuralEq for u128

impl StructuralEq for ()

impl StructuralEq for usize

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 FromBytesUntilNulError

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