user0/rust
1
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

Explain the fake read injection better

Browse source
This commit is contained in:
Remy Rakic 2018-09-14 18:07:17 +02:00
parent c3c7a5bafe
commit 52b5362990
1 changed files with 17 additions and 6 deletions
Download patch file Download diff file Expand all files Collapse all files

23
src/librustc_mir/build/matches/mod.rs
View file

@ -265,9 +265,20 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard);
unpack!(block = self.into(&place, block, initializer));
// Inject a fake read of the newly created binding
// to test the fallout of fixing issue #53695 where NLL
// allows creating unused variables that are effectively unusable.
// Officially, the semantics of
//
// `let pattern = <expr>;`
//
// is that `<expr>` is evaluated into a temporary and then this temporary is
// into the pattern.
//
// However, if we see the simple pattern `let var = <expr>`, we optimize this to
// evaluate `<expr>` directly into the variable `var`. This is mostly unobservable,
// but in some cases it can affect the borrow checker, as in #53695.
// Therefore, we insert a "fake read" here to ensure that we get
// appropriate errors.
//
let source_info = self.source_info(irrefutable_pat.span);
self.cfg.push(
block,
@ -318,9 +329,9 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
},
);
// Inject a fake read of the newly created binding
// to test the fallout of fixing issue #53695 where NLL
// allows creating unused variables that are effectively unusable.
// Similarly to the `let var = <expr>` case, we insert a "fake read" here to
// ensure that we get appropriate errors when this usually unobservable
// optimization affects the borrow checker.
self.cfg.push(
block,
Statement {