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Document pitfall with impl PartialEq<B> for A

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Fixes #66476 by turning the violating example into an explicit
counterexample.
This commit is contained in:
Robert Bamler
2019-11-19 19:57:03 -08:00
committed by Robert Bamler
parent 618b01f9fa
commit 5028fd8ab9
1 changed files with 18 additions and 14 deletions
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30
src/libcore/cmp.rs
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@@ -135,10 +135,15 @@ use self::Ordering::*;
/// By changing `impl PartialEq for Book` to `impl PartialEq<BookFormat> for Book`,
/// we allow `BookFormat`s to be compared with `Book`s.
///
/// You can also combine these implementations to let the `==` operator work with
/// two different types:
/// A comparison like the one above, which ignores some fields of the struct,
/// can be dangerous. It can easily lead to an unintended violation of the
/// requirements for a partial equivalence relation. For example, if we kept
/// the above implementation of `PartialEq<Book>` for `BookFormat` and added an
/// implementation of `PartialEq<Book>` for `Book` (either via a `#[derive]` or
/// via the manual implementation from the first example) then the result would
/// violate transitivity:
///
/// ```
/// ```should_panic
/// #[derive(PartialEq)]
/// enum BookFormat {
/// Paperback,
@@ -146,6 +151,7 @@ use self::Ordering::*;
/// Ebook,
/// }
///
/// #[derive(PartialEq)]
/// struct Book {
/// isbn: i32,
/// format: BookFormat,
@@ -163,18 +169,16 @@ use self::Ordering::*;
/// }
/// }
///
/// impl PartialEq for Book {
/// fn eq(&self, other: &Book) -> bool {
/// self.isbn == other.isbn
/// }
/// }
///
/// let b1 = Book { isbn: 3, format: BookFormat::Paperback };
/// let b2 = Book { isbn: 3, format: BookFormat::Ebook };
/// fn main() {
/// let b1 = Book { isbn: 1, format: BookFormat::Paperback };
/// let b2 = Book { isbn: 2, format: BookFormat::Paperback };
///
/// assert!(b1 == BookFormat::Paperback);
/// assert!(BookFormat::Ebook != b1);
/// assert!(b1 == b2);
/// assert!(BookFormat::Paperback == b2);
///
/// // The following should hold by transitivity but doesn't.
/// assert!(b1 == b2); // <-- PANICS
/// }
/// ```
///
/// # Examples