Function rustc_trait_selection::traits::structural_match::search_for_structural_match_violation

pub fn search_for_structural_match_violation<'tcx>(
    span: Span,
    tcx: TyCtxt<'tcx>,
    ty: Ty<'tcx>
) -> Option<Ty<'tcx>>

This method traverses the structure of ty, trying to find an instance of an ADT (i.e. struct or enum) that doesn’t implement the structural-match traits, or a generic type parameter (which cannot be determined to be structural-match).

The “structure of a type” includes all components that would be considered when doing a pattern match on a constant of that type.

• This means this method descends into fields of structs/enums, and also descends into the inner type T of &T and &mut T

• The traversal doesn’t dereference unsafe pointers (*const T, *mut T), and it does not visit the type arguments of an instantiated generic like PhantomData<T>.

The reason we do this search is Rust currently require all ADTs reachable from a constant’s type to implement the structural-match traits, which essentially say that the implementation of PartialEq::eq behaves equivalently to a comparison against the unfolded structure.

For more background on why Rust has this requirement, and issues that arose when the requirement was not enforced completely, see Rust RFC 1445, rust-lang/rust#61188, and rust-lang/rust#62307.