diff options
| -rw-r--r-- | compiler/rustc_hir_analysis/src/check/region.rs | 24 |
1 files changed, 18 insertions, 6 deletions
diff --git a/compiler/rustc_hir_analysis/src/check/region.rs b/compiler/rustc_hir_analysis/src/check/region.rs index 8c255746478..6221b1b9937 100644 --- a/compiler/rustc_hir_analysis/src/check/region.rs +++ b/compiler/rustc_hir_analysis/src/check/region.rs @@ -250,13 +250,23 @@ fn resolve_expr<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, expr: &'tcx h 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. - // 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, .. }, ..) @@ -264,6 +274,8 @@ fn resolve_expr<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, expr: &'tcx h { false } + // If the LHS is not another binop itself of the same kind as + // the current binop, mark it as terminating. _ => true, }; if terminate_lhs { |