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Auto merge of #103293 - est31:untwist_and_drop_order, r=nagisa
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Remove drop order twist of && and || and make them associative

Previously a short circuiting binop chain (chain of && or ||s) would drop the temporaries created by the first element after all the other elements, and otherwise follow evaluation order. So `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. In other words, 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". We now also put the lhs into a terminating scope. The drop order of the above expressions becomes `1,2,3,4`.

There might be code relying on the current order, and therefore I'd recommend doing a crater run to gauge the impact. I'd argue that such code is already quite wonky as it is one `foo() &&` addition away from breaking. ~~For the impact, I don't expect any *build* failures, as the compiler gets strictly more tolerant: shortening the lifetime of temporaries only expands the list of programs the compiler accepts as valid. There might be *runtime* failures caused by this change however.~~ Edit: both build and runtime failures are possible, e.g. see the example provided by dtolnay [below](#103293 (comment)). Edit2: the crater run has finished and [results](#103293 (comment)) are that there is only one build failure which is easy to fix with a  /- 1 line diff.

I've included a testcase that now compiles thanks to this patch.

The breakage is also limited to drop order relative to conditionals in the && chain: that is, in code like this:

```Rust
let hello = foo().hi() && bar().world();
println!("hi");
```

we already drop the temporaries of `foo().hi()` before we reach "hi".

I'd ideally have this PR merged before let chains are stabilized. If this PR is taking too long, I'd love to have a more restricted version of this change limited to `&&`'s in let chains: the `&&`'s of such chains are quite special anyways as they accept `let` bindings, in there the `&&` is therefore more a part of the "if let chain" construct than a construct of its own.

Fixes #103107

Status: waiting on [this accepted FCP](#103293 (comment)) finishing.
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@bors
bors committed Dec 4, 2022
2 parents 344889e a59a2d3 commit 19c250a
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47 changes: 38 additions & 9 deletions compiler/rustc_hir_analysis/src/check/region.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -241,17 241,46 @@ 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, .. },
_,
ref r,
)
| hir::ExprKind::Binary(
source_map::Spanned { node: hir::BinOpKind::Or, .. },
_,
source_map::Spanned { node: hir::BinOpKind::And | hir::BinOpKind::Or, .. },
ref l,
ref r,
) => {
// For shortcircuiting operators, mark the RHS as a terminating
// scope since it only executes conditionally.
// expr is a short circuiting operator (|| or &&). As its
// functionality can't be overridden by traits, it always
// processes bool sub-expressions. bools are Copy and thus we
// can drop any temporaries in evaluation (read) order
// (with the exception of potentially failing let expressions).
// We achieve this by enclosing the operands in a terminating
// scope, both the LHS and the RHS.

// We optimize this a little in the presence of chains.
// Chains like a && b && c get lowered to AND(AND(a, b), c).
// In here, b and c are RHS, while a is the only LHS operand in
// that chain. This holds true for longer chains as well: the
// leading operand is always the only LHS operand that is not a
// binop itself. Putting a binop like AND(a, b) into a
// terminating scope is not useful, thus we only put the LHS
// into a terminating scope if it is not a binop.

let terminate_lhs = match l.kind {
// let expressions can create temporaries that live on
hir::ExprKind::Let(_) => false,
// binops already drop their temporaries, so there is no
// need to put them into a terminating scope.
// This is purely an optimization to reduce the number of
// terminating scopes.
hir::ExprKind::Binary(
source_map::Spanned {
node: hir::BinOpKind::And | hir::BinOpKind::Or, ..
},
..,
) => false,
// otherwise: mark it as terminating
_ => 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
Expand Down
97 changes: 90 additions & 7 deletions src/test/ui/drop/drop_order.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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 {
Expand Down Expand Up @@ -118,17 118,85 @@ 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 mixed_and_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_some() // 3
&& self.option_loud_drop(4).is_some() // 4
&& self.option_loud_drop(5).is_none() // 5
|| self.option_loud_drop(6).is_none() // 6
|| self.option_loud_drop(7).is_some() // 7
{
self.print(8); // 8
}
}

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 {
Expand All @@ -152,8 220,8 @@ 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
Expand Down Expand Up @@ -187,6 255,21 @@ 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!("-- mixed and/or chain --");
let collector = DropOrderCollector::default();
collector.mixed_and_or_chain();
collector.assert_sorted();

println!("-- if let --");
let collector = DropOrderCollector::default();
collector.if_let();
Expand Down
37 changes: 37 additions & 0 deletions src/test/ui/drop/issue-103107.rs
View file
Original file line number Diff line number Diff line change
@@ -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();
}

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