Trait scale_info::prelude::marker::StructuralEq

source ·

pub trait StructuralEq { }

🔬This is a nightly-only experimental API. (structural_match)

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.

Trait scale_info::prelude::marker::StructuralEq

Background

Implementors§

impl StructuralEq for byte_slice_cast::Error

impl StructuralEq for PathError

impl StructuralEq for TypeDefPrimitive

impl StructuralEq for MetaForm

impl StructuralEq for PortableForm

impl StructuralEq for scale_info::prelude::cmp::Ordering

impl StructuralEq for TryReserveErrorKind

impl StructuralEq for scale_info::prelude::fmt::Alignment

impl StructuralEq for FpCategory

impl StructuralEq for IntErrorKind

impl StructuralEq for AsciiChar

impl StructuralEq for Which

impl StructuralEq for IpAddr

impl StructuralEq for Ipv6MulticastScope

impl StructuralEq for SocketAddr

impl StructuralEq for SearchStep

impl StructuralEq for core::sync::atomic::Ordering

impl StructuralEq for BacktraceStatus

impl StructuralEq for VarError

impl StructuralEq for SeekFrom

impl StructuralEq for ErrorKind

impl StructuralEq for Shutdown

impl StructuralEq for BacktraceStyle

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 Lsb0

impl StructuralEq for Msb0

impl StructuralEq for OptionBool

impl StructuralEq for parity_scale_codec::error::Error

impl StructuralEq for Const

impl StructuralEq for Mut

impl StructuralEq for NullPtrError

impl StructuralEq for PortableRegistry

impl StructuralEq for Registry

impl StructuralEq for TypeId

impl StructuralEq for TryReserveError

impl StructuralEq for scale_info::prelude::fmt::Error

impl StructuralEq for Assume

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 FromUtf8Error

impl StructuralEq for String

impl StructuralEq for CString

impl StructuralEq for FromVecWithNulError

impl StructuralEq for IntoStringError

impl StructuralEq for NulError

impl StructuralEq for Layout