bjoernager/rust
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elide noop coercions

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This commit is contained in:
Niko Matsakis 2016-03-18 17:19:52 -04:00
parent bca07b5ebb
commit 25988b645d
1 changed files with 16 additions and 2 deletions
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18
src/librustc_typeck/check/coercion.rs
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@ -218,10 +218,10 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
// to type check, we will construct the type that `&M*expr` would
// yield.
let r_a = match a.sty {
let (r_a, mt_a) = match a.sty {
ty::TyRef(r_a, mt_a) => {
try!(coerce_mutbls(mt_a.mutbl, mt_b.mutbl));
r_a
(r_a, mt_a)
}
_ => return self.unify_and_identity(a, b)
};
@ -355,6 +355,20 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
// Now apply the autoref. We have to extract the region out of
// the final ref type we got.
if ty == a && mt_a.mutbl == hir::MutImmutable && autoderefs == 1 {
// As a special case, if we would produce `&'a *x`, that's
// a total no-op. We end up with the type `&'a T` just as
// we started with. In that case, just skip it
// altogether. This is just an optimization.
//
// Note that for `&mut`, we DO want to reborrow --
// otherwise, this would be a move, which might be an
// error. For example `foo(self.x)` where `self` and
// `self.x` both have `&mut `type would be a move of
// `self.x`, but we auto-coerce it to `foo(&mut *self.x)`,
// which is a borrow.
return self.identity(ty);
}
let r_borrow = match ty.sty {
ty::TyRef(r_borrow, _) => r_borrow,
_ => self.tcx().sess.span_bug(span,