bjoernager/rust
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Move comments for fake reads where the causes are defined

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This commit is contained in:
Remy Rakic 2018-09-14 21:54:45 +02:00
parent ab236dfc86
commit ae6479c13b
2 changed files with 25 additions and 24 deletions
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18
src/librustc/mir/mod.rs
View file

@ -1667,7 +1667,25 @@ pub enum StatementKind<'tcx> {
/// The `FakeReadCause` describes the type of pattern why a `FakeRead` statement exists.
#[derive(Copy, Clone, RustcEncodable, RustcDecodable, Debug)]
pub enum FakeReadCause {
/// Inject a fake read of the borrowed input at the start of each arm's
/// pattern testing code.
///
/// This should ensure that you cannot change the variant for an enum
/// while you are in the midst of matching on it.
ForMatch,
/// 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.
ForLet,
}

31
src/librustc_mir/build/matches/mod.rs
View file

@ -145,19 +145,16 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
if let (true, Some(borrow_temp)) =
(tcx.emit_read_for_match(), borrowed_input_temp.clone())
{
// inject a fake read of the borrowed input at
// the start of each arm's pattern testing
// code.
//
// This should ensure that you cannot change
// the variant for an enum while you are in
// the midst of matching on it.
// Inject a fake read, see comments on `FakeReadCause::ForMatch`.
let pattern_source_info = self.source_info(pattern.span);
self.cfg.push(
*pre_binding_block,
Statement {
source_info: pattern_source_info,
kind: StatementKind::FakeRead(FakeReadCause::ForMatch, borrow_temp.clone()),
kind: StatementKind::FakeRead(
FakeReadCause::ForMatch,
borrow_temp.clone(),
),
},
);
}
@ -266,19 +263,7 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
unpack!(block = self.into(&place, block, initializer));
// 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.
//
// Inject a fake read, see comments on `FakeReadCause::ForLet`.
let source_info = self.source_info(irrefutable_pat.span);
self.cfg.push(
block,
@ -329,9 +314,7 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
},
);
// 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.
// Inject a fake read, see comments on `FakeReadCause::ForLet`.
self.cfg.push(
block,
Statement {