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add comments from arielb1

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Niko Matsakis 2017-07-31 18:13:49 +03:00
parent 851c77088d
commit 8f67f1efaf
1 changed files with 43 additions and 0 deletions
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43
src/librustc_typeck/check/_match.rs
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@ -410,6 +410,49 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
// is problematic as the HIR is being scraped, but ref bindings may be
// implicit after #42640. We need to make sure that pat_adjustments
// (once introduced) is populated by the time we get here.
//
// arielb1 [writes here in this comment thread][c] that there
// is certainly *some* potential danger, e.g. for an example
// like:
//
// [c]: https://github.com/rust-lang/rust/pull/43399#discussion_r130223956
//
// ```
// let Foo(x) = f()[0];
// ```
//
// Then if the pattern matches by reference, we want to match
// `f()[0]` as a lexpr, so we can't allow it to be
// coerced. But if the pattern matches by value, `f()[0]` is
// still syntactically a lexpr, but we *do* want to allow
// coercions.
//
// However, *likely* we are ok with allowing coercions to
// happen if there are no explicit ref mut patterns - all
// implicit ref mut patterns must occur behind a reference, so
// they will have the "correct" variance and lifetime.
//
// This does mean that the following pattern would be legal:
//
// ```
// struct Foo(Bar);
// struct Bar(u32);
// impl Deref for Foo {
// type Target = Bar;
// fn deref(&self) -> &Bar { &self.0 }
// }
// impl DerefMut for Foo {
// fn deref_mut(&mut self) -> &mut Bar { &mut self.0 }
// }
// fn foo(x: &mut Foo) {
// {
// let Bar(z): &mut Bar = x;
// *z = 42;
// }
// assert_eq!(foo.0.0, 42);
// }
// ```
let contains_ref_bindings = arms.iter()
.filter_map(|a| a.contains_explicit_ref_binding())
.max_by_key(|m| match *m {