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Remove drop order twist of && and || and make them associative 

Previously a short circuiting && chain would drop the
first element after all the other elements, and otherwise
follow evaluation order, so code like:

f(1).g() && f(2).g() && f(3).g() && f(4).g()

would drop the temporaries in the order 2,3,4,1. This made
&& and || non-associative regarding drop order, so
adding ()'s to the expression would change drop order:

f(1).g() && (f(2).g() && f(3).g()) && f(4).g()

for example would drop in the order 3,2,4,1.

As, except for the bool result, there is no data returned
by the sub-expressions of the short circuiting binops,
we can safely discard of any temporaries created by the
sub-expr. Previously, code was already putting the rhs's
into terminating scopes, but missed it for the lhs's.

This commit addresses this "twist". In the expression,
we now also put the lhs into a terminating scope.
The drop order for the above expressions is 1,2,3,4
now.
This commit is contained in:
est31 2022-10-20 09:50:32 +02:00
parent cab4fd678c
commit 8cf521d80e
3 changed files with 129 additions and 11 deletions
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25
compiler/rustc_hir_analysis/src/check/region.rs
View File

@ -241,18 +241,35 @@ fn resolve_expr<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, expr: &'tcx h
// scopes, meaning that temporaries cannot outlive them.
// This ensures fixed size stacks.
hir::ExprKind::Binary(
source_map::Spanned { node: hir::BinOpKind::And, .. },
_,
source_map::Spanned { node: outer @ hir::BinOpKind::And, .. },
ref l,
ref r,
)
| hir::ExprKind::Binary(
source_map::Spanned { node: hir::BinOpKind::Or, .. },
_,
source_map::Spanned { node: outer @ hir::BinOpKind::Or, .. },
ref l,
ref r,
) => {
// For shortcircuiting operators, mark the RHS as a terminating
// scope since it only executes conditionally.
// If the LHS is not another binop itself of the same kind as ours,
// we also mark it as terminating, so that in && or || chains,
// the temporaries are dropped in order instead of the very first
// being dropped last. For the Let exception, see below.
let terminate_lhs = match l.kind {
hir::ExprKind::Let(_) => false,
hir::ExprKind::Binary(source_map::Spanned { node, .. }, ..)
if node == outer =>
{
false
}
_ => true,
};
if terminate_lhs {
terminating(l.hir_id.local_id);
}
// `Let` expressions (in a let-chain) shouldn't be terminating, as their temporaries
// should live beyond the immediate expression
if !matches!(r.kind, hir::ExprKind::Let(_)) {

78
src/test/ui/drop/drop_order.rs
View File

@ -43,7 +43,7 @@ impl DropOrderCollector {
}
if {
if self.option_loud_drop(7).is_some() && self.option_loud_drop(6).is_some() {
if self.option_loud_drop(6).is_some() && self.option_loud_drop(7).is_some() {
self.loud_drop(8);
true
} else {
@ -118,17 +118,71 @@ impl DropOrderCollector {
}
}
fn and_chain(&self) {
// issue-103107
if self.option_loud_drop(1).is_some() // 1
&& self.option_loud_drop(2).is_some() // 2
&& self.option_loud_drop(3).is_some() // 3
&& self.option_loud_drop(4).is_some() // 4
&& self.option_loud_drop(5).is_some() // 5
{
self.print(6); // 6
}
let _ = self.option_loud_drop(7).is_some() // 1
&& self.option_loud_drop(8).is_some() // 2
&& self.option_loud_drop(9).is_some(); // 3
self.print(10); // 4
// Test associativity
if self.option_loud_drop(11).is_some() // 1
&& (self.option_loud_drop(12).is_some() // 2
&& self.option_loud_drop(13).is_some() // 3
&& self.option_loud_drop(14).is_some()) // 4
&& self.option_loud_drop(15).is_some() // 5
{
self.print(16); // 6
}
}
fn or_chain(&self) {
// issue-103107
if self.option_loud_drop(1).is_none() // 1
|| self.option_loud_drop(2).is_none() // 2
|| self.option_loud_drop(3).is_none() // 3
|| self.option_loud_drop(4).is_none() // 4
|| self.option_loud_drop(5).is_some() // 5
{
self.print(6); // 6
}
let _ = self.option_loud_drop(7).is_none() // 1
|| self.option_loud_drop(8).is_none() // 2
|| self.option_loud_drop(9).is_none(); // 3
self.print(10); // 4
// Test associativity
if self.option_loud_drop(11).is_none() // 1
|| (self.option_loud_drop(12).is_none() // 2
|| self.option_loud_drop(13).is_none() // 3
|| self.option_loud_drop(14).is_none()) // 4
|| self.option_loud_drop(15).is_some() // 5
{
self.print(16); // 6
}
}
fn let_chain(&self) {
// take the "then" branch
if self.option_loud_drop(2).is_some() // 2
&& self.option_loud_drop(1).is_some() // 1
if self.option_loud_drop(1).is_some() // 1
&& self.option_loud_drop(2).is_some() // 2
&& let Some(_d) = self.option_loud_drop(4) { // 4
self.print(3); // 3
}
// take the "else" branch
if self.option_loud_drop(6).is_some() // 2
&& self.option_loud_drop(5).is_some() // 1
if self.option_loud_drop(5).is_some() // 1
&& self.option_loud_drop(6).is_some() // 2
&& let None = self.option_loud_drop(8) { // 4
unreachable!();
} else {
@ -152,8 +206,8 @@ impl DropOrderCollector {
}
// let exprs last
if self.option_loud_drop(20).is_some() // 2
&& self.option_loud_drop(19).is_some() // 1
if self.option_loud_drop(19).is_some() // 1
&& self.option_loud_drop(20).is_some() // 2
&& let Some(_d) = self.option_loud_drop(23) // 5
&& let Some(_e) = self.option_loud_drop(22) { // 4
self.print(21); // 3
@ -187,6 +241,16 @@ fn main() {
collector.if_();
collector.assert_sorted();
println!("-- and chain --");
let collector = DropOrderCollector::default();
collector.and_chain();
collector.assert_sorted();
println!("-- or chain --");
let collector = DropOrderCollector::default();
collector.or_chain();
collector.assert_sorted();
println!("-- if let --");
let collector = DropOrderCollector::default();
collector.if_let();

37
src/test/ui/drop/issue-103107.rs Normal file
View File

@ -0,0 +1,37 @@
// check-pass
// compile-flags: -Z validate-mir
struct Foo<'a>(&'a mut u32);
impl<'a> Drop for Foo<'a> {
fn drop(&mut self) {
*self.0 = 0;
}
}
fn and() {
let mut foo = 0;
// This used to compile also before the fix
if true && *Foo(&mut foo).0 == 0 && ({ foo = 0; true}) {}
// This used to fail before the fix
if *Foo(&mut foo).0 == 0 && ({ foo = 0; true}) {}
println!("{foo}");
}
fn or() {
let mut foo = 0;
// This used to compile also before the fix
if false || *Foo(&mut foo).0 == 1 || ({ foo = 0; true}) {}
// This used to fail before the fix
if *Foo(&mut foo).0 == 1 || ({ foo = 0; true}) {}
println!("{foo}");
}
fn main() {
and();
or();
}