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Added comment explaining purpose of `search_for_adt_without_structura…

…l_match`.
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pnkfelix committed Jul 9, 2019
1 parent 02af3ca commit b0b64ddcd171be6e460b2d7c1ada4f98a3cdb026
Showing with 24 additions and 0 deletions.
  1. +24 −0 src/librustc_mir/hair/pattern/mod.rs
@@ -1145,6 +1145,30 @@ impl<'a, 'tcx> PatternContext<'a, 'tcx> {
}
}

/// This method traverses the structure of `ty`, trying to find an
/// instance of an ADT (i.e. struct or enum) that was declared without
/// the `#[structural_match]` attribute.
///
/// 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 ADT's
/// reachable from a constant's type to be annotated with
/// `#[structural_match]`, an attribute which essentially says 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.
fn search_for_adt_without_structural_match<'tcx>(tcx: TyCtxt<'tcx>,
ty: Ty<'tcx>)
-> Option<&'tcx AdtDef>

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