Trait frame_support::dispatch::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 frame_support::dispatch::marker::StructuralEq

Background

Implementors§

impl StructuralEq for aho_corasick::ahocorasick::MatchKind

impl StructuralEq for aho_corasick::packed::api::MatchKind

impl StructuralEq for array_bytes::Error

impl StructuralEq for PrintFmt

impl StructuralEq for base16ct::error::Error

impl StructuralEq for DecodeError

impl StructuralEq for byte_slice_cast::Error

impl StructuralEq for byteorder::BigEndian

impl StructuralEq for byteorder::LittleEndian

impl StructuralEq for SecondsFormat

impl StructuralEq for Fixed

impl StructuralEq for Numeric

impl StructuralEq for Pad

impl StructuralEq for ParseErrorKind

impl StructuralEq for Month

impl StructuralEq for RoundingError

impl StructuralEq for Weekday

impl StructuralEq for const_oid::error::Error

impl StructuralEq for ArrayType

impl StructuralEq for BaseUnresolvedName

impl StructuralEq for BuiltinType

impl StructuralEq for CallOffset

impl StructuralEq for ClassEnumType

impl StructuralEq for CtorDtorName

impl StructuralEq for Decltype

impl StructuralEq for DestructorName

impl StructuralEq for cpp_demangle::ast::Encoding

impl StructuralEq for ExprPrimary

impl StructuralEq for cpp_demangle::ast::Expression

impl StructuralEq for GlobalCtorDtor

impl StructuralEq for LocalName

impl StructuralEq for MangledName

impl StructuralEq for Name

impl StructuralEq for NestedName

impl StructuralEq for OperatorName

impl StructuralEq for cpp_demangle::ast::Prefix

impl StructuralEq for PrefixHandle

impl StructuralEq for RefQualifier

impl StructuralEq for SimpleOperatorName

impl StructuralEq for SpecialName

impl StructuralEq for StandardBuiltinType

impl StructuralEq for Substitution

impl StructuralEq for TemplateArg

impl StructuralEq for TemplateTemplateParamHandle

impl StructuralEq for cpp_demangle::ast::Type

impl StructuralEq for TypeHandle

impl StructuralEq for UnqualifiedName

impl StructuralEq for UnresolvedName

impl StructuralEq for UnresolvedType

impl StructuralEq for UnresolvedTypeHandle

impl StructuralEq for UnscopedName

impl StructuralEq for UnscopedTemplateNameHandle

impl StructuralEq for VectorType

impl StructuralEq for WellKnownComponent

impl StructuralEq for DemangleNodeType

impl StructuralEq for cpp_demangle::error::Error

impl StructuralEq for Reloc

impl StructuralEq for CursorPosition

impl StructuralEq for AtomicRmwOp

impl StructuralEq for FloatCC

impl StructuralEq for IntCC

impl StructuralEq for ValueDef

impl StructuralEq for AnyEntity

impl StructuralEq for ArgumentExtension

impl StructuralEq for ArgumentPurpose

impl StructuralEq for ExternalName

impl StructuralEq for UserFuncName

impl StructuralEq for InstructionFormat

impl StructuralEq for Opcode

impl StructuralEq for ResolvedConstraint

impl StructuralEq for KnownSymbol

impl StructuralEq for LibCall

impl StructuralEq for cranelift_codegen::ir::memflags::Endianness

impl StructuralEq for ExpandedProgramPoint

impl StructuralEq for StackSlotKind

impl StructuralEq for cranelift_codegen::ir::trapcode::TrapCode

impl StructuralEq for CallConv