Auto merge of #103293 - est31:untwist_and_drop_order, r=nagisa

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.

compiler/rustc_hir_analysis/src/check/region.rs

@@ -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

src/test/ui/drop/drop_order.rs

@@ -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 +118,85 @@ 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 {
@@ -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
@@ -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();

src/test/ui/drop/issue-103107.rs

@@ -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();
+}