de-duplicate condition scoping logic

Author: dianne

compiler/rustc_ast_lowering/src/expr.rs

@@ -532,7 +532,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         then: &Block,
         else_opt: Option<&Expr>,
     ) -> hir::ExprKind<'hir> {
-        let lowered_cond = self.lower_cond(cond);
+        let lowered_cond = self.lower_expr(cond);
         let then_expr = self.lower_block_expr(then);
         if let Some(rslt) = else_opt {
             hir::ExprKind::If(
@@ -545,44 +545,6 @@ impl<'hir> LoweringContext<'_, 'hir> {
         }
     }
 
-    // Lowers a condition (i.e. `cond` in `if cond` or `while cond`), wrapping it in a terminating scope
-    // so that temporaries created in the condition don't live beyond it.
-    fn lower_cond(&mut self, cond: &Expr) -> &'hir hir::Expr<'hir> {
-        fn has_let_expr(expr: &Expr) -> bool {
-            match &expr.kind {
-                ExprKind::Binary(_, lhs, rhs) => has_let_expr(lhs) || has_let_expr(rhs),
-                ExprKind::Let(..) => true,
-                _ => false,
-            }
-        }
-
-        // We have to take special care for `let` exprs in the condition, e.g. in
-        // `if let pat = val` or `if foo && let pat = val`, as we _do_ want `val` to live beyond the
-        // condition in this case.
-        //
-        // In order to maintain the drop behavior for the non `let` parts of the condition,
-        // we still wrap them in terminating scopes, e.g. `if foo && let pat = val` essentially
-        // gets transformed into `if { let _t = foo; _t } && let pat = val`
-        match &cond.kind {
-            ExprKind::Binary(op @ Spanned { node: ast::BinOpKind::And, .. }, lhs, rhs)
-                if has_let_expr(cond) =>
-            {
-                let op = self.lower_binop(*op);
-                let lhs = self.lower_cond(lhs);
-                let rhs = self.lower_cond(rhs);
-
-                self.arena.alloc(self.expr(cond.span, hir::ExprKind::Binary(op, lhs, rhs)))
-            }
-            ExprKind::Let(..) => self.lower_expr(cond),
-            _ => {
-                let cond = self.lower_expr(cond);
-                let reason = DesugaringKind::CondTemporary;
-                let span_block = self.mark_span_with_reason(reason, cond.span, None);
-                self.expr_drop_temps(span_block, cond)
-            }
-        }
-    }
-
     // We desugar: `'label: while $cond $body` into:
     //
     // ```
@@ -606,7 +568,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         body: &Block,
         opt_label: Option<Label>,
     ) -> hir::ExprKind<'hir> {
-        let lowered_cond = self.with_loop_condition_scope(|t| t.lower_cond(cond));
+        let lowered_cond = self.with_loop_condition_scope(|t| t.lower_expr(cond));
         let then = self.lower_block_expr(body);
         let expr_break = self.expr_break(span);
         let stmt_break = self.stmt_expr(span, expr_break);
@@ -2095,7 +2057,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
     /// In terms of drop order, it has the same effect as wrapping `expr` in
     /// `{ let _t = $expr; _t }` but should provide better compile-time performance.
     ///
-    /// The drop order can be important in e.g. `if expr { .. }`.
+    /// The drop order can be important, e.g. to drop temporaries from an `async fn`
+    /// body before its parameters.
     pub(super) fn expr_drop_temps(
         &mut self,
         span: Span,

compiler/rustc_hir_analysis/src/check/region.rs

@@ -176,23 +176,39 @@ fn resolve_block<'tcx>(
     visitor.cx = prev_cx;
 }
 
-fn resolve_arm<'tcx>(visitor: &mut ScopeResolutionVisitor<'tcx>, arm: &'tcx hir::Arm<'tcx>) {
-    fn has_let_expr(expr: &Expr<'_>) -> bool {
-        match &expr.kind {
-            hir::ExprKind::Binary(_, lhs, rhs) => has_let_expr(lhs) || has_let_expr(rhs),
-            hir::ExprKind::Let(..) => true,
-            _ => false,
-        }
-    }
+/// Resolve a condition from an `if` expression or match guard so that it is a terminating scope
+/// if it doesn't contain `let` expressions.
+fn resolve_cond<'tcx>(visitor: &mut ScopeResolutionVisitor<'tcx>, cond: &'tcx hir::Expr<'tcx>) {
+    let terminate = match cond.kind {
+        // Temporaries for `let` expressions must live into the success branch.
+        hir::ExprKind::Let(_) => false,
+        // Logical operator chains are handled in `resolve_expr`. Since logical operator chains in
+        // conditions are lowered to control-flow rather than boolean temporaries, there's no
+        // temporary to drop for logical operators themselves. `resolve_expr` will also recursively
+        // wrap any operands in terminating scopes, other than `let` expressions (which we shouldn't
+        // terminate) and other logical operators (which don't need a terminating scope, since their
+        // operands will be terminated). Any temporaries that would need to be dropped will be
+        // dropped before we leave this operator's scope; terminating them here would be redundant.
+        hir::ExprKind::Binary(
+            source_map::Spanned { node: hir::BinOpKind::And | hir::BinOpKind::Or, .. },
+            _,
+            _,
+        ) => false,
+        // Otherwise, conditions should always drop their temporaries.
+        _ => true,
+    };
+    resolve_expr(visitor, cond, terminate);
+}
 
+fn resolve_arm<'tcx>(visitor: &mut ScopeResolutionVisitor<'tcx>, arm: &'tcx hir::Arm<'tcx>) {
     let prev_cx = visitor.cx;
 
     visitor.enter_node_scope_with_dtor(arm.hir_id.local_id, true);
     visitor.cx.var_parent = visitor.cx.parent;
 
     resolve_pat(visitor, arm.pat);
     if let Some(guard) = arm.guard {
-        resolve_expr(visitor, guard, !has_let_expr(guard));
+        resolve_cond(visitor, guard);
     }
     resolve_expr(visitor, arm.body, false);
 
@@ -420,7 +436,7 @@ fn resolve_expr<'tcx>(
             };
             visitor.enter_scope(Scope { local_id: then.hir_id.local_id, data });
             visitor.cx.var_parent = visitor.cx.parent;
-            visitor.visit_expr(cond);
+            resolve_cond(visitor, cond);
             resolve_expr(visitor, then, true);
             visitor.cx = expr_cx;
             resolve_expr(visitor, otherwise, true);
@@ -435,7 +451,7 @@ fn resolve_expr<'tcx>(
             };
             visitor.enter_scope(Scope { local_id: then.hir_id.local_id, data });
             visitor.cx.var_parent = visitor.cx.parent;
-            visitor.visit_expr(cond);
+            resolve_cond(visitor, cond);
             resolve_expr(visitor, then, true);
             visitor.cx = expr_cx;
         }

compiler/rustc_hir_typeck/src/lib.rs

@@ -428,11 +428,9 @@ fn report_unexpected_variant_res(
                 }
                 hir::Node::Expr(hir::Expr { kind: hir::ExprKind::Binary(..), hir_id, .. }) => {
                     suggestion.push((expr.span.shrink_to_lo(), "(".to_string()));
-                    if let hir::Node::Expr(drop_temps) = tcx.parent_hir_node(*hir_id)
-                        && let hir::ExprKind::DropTemps(_) = drop_temps.kind
-                        && let hir::Node::Expr(parent) = tcx.parent_hir_node(drop_temps.hir_id)
+                    if let hir::Node::Expr(parent) = tcx.parent_hir_node(*hir_id)
                         && let hir::ExprKind::If(condition, block, None) = parent.kind
-                        && condition.hir_id == drop_temps.hir_id
+                        && condition.hir_id == *hir_id
                         && let hir::ExprKind::Block(block, _) = block.kind
                         && block.stmts.is_empty()
                         && let Some(expr) = block.expr