Rollup merge of rust-lang#101410 - dingxiangfei2009:fix-let-else-scop…

…ing, r=jackh726

Reorder nesting scopes and declare bindings without drop schedule

Fix rust-lang#99228
Fix rust-lang#99975

Storages are previously not declared before entering the `else` block of a `let .. else` statement. However, when breaking out of the pattern matching into the `else` block, those storages are recorded as scheduled for drops. This is not expected.

This MR fixes this issue by not scheduling the drops for those storages.

cc `@est31`

compiler/rustc_mir_build/src/build/block.rs

@@ -109,13 +109,170 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                         )
                     );
                 }
+                StmtKind::Let {
+                    remainder_scope,
+                    init_scope,
+                    pattern,
+                    initializer: Some(initializer),
+                    lint_level,
+                    else_block: Some(else_block),
+                } => {
+                    // When lowering the statement `let <pat> = <expr> else { <else> };`,
+                    // the `<else>` block is nested in the parent scope enclosing this statment.
+                    // That scope is usually either the enclosing block scope,
+                    // or the remainder scope of the last statement.
+                    // This is to make sure that temporaries instantiated in `<expr>` are dropped
+                    // as well.
+                    // In addition, even though bindings in `<pat>` only come into scope if
+                    // the pattern matching passes, in the MIR building the storages for them
+                    // are declared as live any way.
+                    // This is similar to `let x;` statements without an initializer expression,
+                    // where the value of `x` in this example may or may be assigned,
+                    // because the storage for their values may not be live after all due to
+                    // failure in pattern matching.
+                    // For this reason, we declare those storages as live but we do not schedule
+                    // any drop yet- they are scheduled later after the pattern matching.
+                    // The generated MIR will have `StorageDead` whenever the control flow breaks out
+                    // of the parent scope, regardless of the result of the pattern matching.
+                    // However, the drops are inserted in MIR only when the control flow breaks out of
+                    // the scope of the remainder scope associated with this `let .. else` statement.
+                    // Pictorial explanation of the scope structure:
+                    // ┌─────────────────────────────────┐
+                    // │  Scope of the enclosing block,  │
+                    // │  or the last remainder scope    │
+                    // │  ┌───────────────────────────┐  │
+                    // │  │  Scope for <else> block   │  │
+                    // │  └───────────────────────────┘  │
+                    // │  ┌───────────────────────────┐  │
+                    // │  │  Remainder scope of       │  │
+                    // │  │  this let-else statement  │  │
+                    // │  │  ┌─────────────────────┐  │  │
+                    // │  │  │ <expr> scope        │  │  │
+                    // │  │  └─────────────────────┘  │  │
+                    // │  │  extended temporaries in  │  │
+                    // │  │  <expr> lives in this     │  │
+                    // │  │  scope                    │  │
+                    // │  │  ┌─────────────────────┐  │  │
+                    // │  │  │ Scopes for the rest │  │  │
+                    // │  │  └─────────────────────┘  │  │
+                    // │  └───────────────────────────┘  │
+                    // └─────────────────────────────────┘
+                    // Generated control flow:
+                    //          │ let Some(x) = y() else { return; }
+                    //          │
+                    // ┌────────▼───────┐
+                    // │ evaluate y()   │
+                    // └────────┬───────┘
+                    //          │              ┌────────────────┐
+                    // ┌────────▼───────┐      │Drop temporaries│
+                    // │Test the pattern├──────►in y()          │
+                    // └────────┬───────┘      │because breaking│
+                    //          │              │out of <expr>   │
+                    // ┌────────▼───────┐      │scope           │
+                    // │Move value into │      └───────┬────────┘
+                    // │binding x       │              │
+                    // └────────┬───────┘      ┌───────▼────────┐
+                    //          │              │Drop extended   │
+                    // ┌────────▼───────┐      │temporaries in  │
+                    // │Drop temporaries│      │<expr> because  │
+                    // │in y()          │      │breaking out of │
+                    // │because breaking│      │remainder scope │
+                    // │out of <expr>   │      └───────┬────────┘
+                    // │scope           │              │
+                    // └────────┬───────┘      ┌───────▼────────┐
+                    //          │              │Enter <else>    ├────────►
+                    // ┌────────▼───────┐      │block           │ return;
+                    // │Continue...     │      └────────────────┘
+                    // └────────────────┘
+
+                    let ignores_expr_result = matches!(pattern.kind, PatKind::Wild);
+                    this.block_context.push(BlockFrame::Statement { ignores_expr_result });
+
+                    // Lower the `else` block first because its parent scope is actually
+                    // enclosing the rest of the `let .. else ..` parts.
+                    let else_block_span = this.thir[*else_block].span;
+                    // This place is not really used because this destination place
+                    // should never be used to take values at the end of the failure
+                    // block.
+                    let dummy_place = this.temp(this.tcx.types.never, else_block_span);
+                    let failure_entry = this.cfg.start_new_block();
+                    let failure_block;
+                    unpack!(
+                        failure_block = this.ast_block(
+                            dummy_place,
+                            failure_entry,
+                            *else_block,
+                            this.source_info(else_block_span),
+                        )
+                    );
+                    this.cfg.terminate(
+                        failure_block,
+                        this.source_info(else_block_span),
+                        TerminatorKind::Unreachable,
+                    );
+
+                    // Declare the bindings, which may create a source scope.
+                    let remainder_span = remainder_scope.span(this.tcx, this.region_scope_tree);
+                    this.push_scope((*remainder_scope, source_info));
+                    let_scope_stack.push(remainder_scope);
+
+                    let visibility_scope =
+                        Some(this.new_source_scope(remainder_span, LintLevel::Inherited, None));
+
+                    let init = &this.thir[*initializer];
+                    let initializer_span = init.span;
+                    this.declare_bindings(
+                        visibility_scope,
+                        remainder_span,
+                        pattern,
+                        ArmHasGuard(false),
+                        Some((None, initializer_span)),
+                    );
+                    this.visit_primary_bindings(
+                        pattern,
+                        UserTypeProjections::none(),
+                        &mut |this, _, _, _, node, span, _, _| {
+                            this.storage_live_binding(block, node, span, OutsideGuard, false);
+                        },
+                    );
+                    let failure = unpack!(
+                        block = this.in_opt_scope(
+                            opt_destruction_scope.map(|de| (de, source_info)),
+                            |this| {
+                                let scope = (*init_scope, source_info);
+                                this.in_scope(scope, *lint_level, |this| {
+                                    this.ast_let_else(
+                                        block,
+                                        init,
+                                        initializer_span,
+                                        *else_block,
+                                        &last_remainder_scope,
+                                        pattern,
+                                    )
+                                })
+                            }
+                        )
+                    );
+                    this.cfg.goto(failure, source_info, failure_entry);
+
+                    if let Some(source_scope) = visibility_scope {
+                        this.source_scope = source_scope;
+                    }
+                    last_remainder_scope = *remainder_scope;
+                }
+                StmtKind::Let { init_scope, initializer: None, else_block: Some(_), .. } => {
+                    span_bug!(
+                        init_scope.span(this.tcx, this.region_scope_tree),
+                        "initializer is missing, but else block is present in this let binding",
+                    )
+                }
                 StmtKind::Let {
                     remainder_scope,
                     init_scope,
                     ref pattern,
                     initializer,
                     lint_level,
-                    else_block,
+                    else_block: None,
                 } => {
                     let ignores_expr_result = matches!(pattern.kind, PatKind::Wild);
                     this.block_context.push(BlockFrame::Statement { ignores_expr_result });
@@ -141,27 +298,14 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                                 |this| {
                                     let scope = (*init_scope, source_info);
                                     this.in_scope(scope, *lint_level, |this| {
-                                        if let Some(else_block) = else_block {
-                                            this.ast_let_else(
-                                                block,
-                                                init,
-                                                initializer_span,
-                                                *else_block,
-                                                visibility_scope,
-                                                last_remainder_scope,
-                                                remainder_span,
-                                                pattern,
-                                            )
-                                        } else {
-                                            this.declare_bindings(
-                                                visibility_scope,
-                                                remainder_span,
-                                                pattern,
-                                                ArmHasGuard(false),
-                                                Some((None, initializer_span)),
-                                            );
-                                            this.expr_into_pattern(block, pattern, init) // irrefutable pattern
-                                        }
+                                        this.declare_bindings(
+                                            visibility_scope,
+                                            remainder_span,
+                                            pattern,
+                                            ArmHasGuard(false),
+                                            Some((None, initializer_span)),
+                                        );
+                                        this.expr_into_pattern(block, &pattern, init) // irrefutable pattern
                                     })
                                 },
                             )

compiler/rustc_mir_build/src/build/matches/mod.rs

@@ -699,7 +699,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         self.cfg.push(block, Statement { source_info, kind: StatementKind::StorageLive(local_id) });
         // Although there is almost always scope for given variable in corner cases
         // like #92893 we might get variable with no scope.
-        if let Some(region_scope) = self.region_scope_tree.var_scope(var.0.local_id) && schedule_drop{
+        if let Some(region_scope) = self.region_scope_tree.var_scope(var.0.local_id) && schedule_drop {
             self.schedule_drop(span, region_scope, local_id, DropKind::Storage);
         }
         Place::from(local_id)
@@ -2274,23 +2274,14 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         init: &Expr<'tcx>,
         initializer_span: Span,
         else_block: BlockId,
-        visibility_scope: Option<SourceScope>,
-        remainder_scope: region::Scope,
-        remainder_span: Span,
+        let_else_scope: &region::Scope,
         pattern: &Pat<'tcx>,
-    ) -> BlockAnd<()> {
+    ) -> BlockAnd<BasicBlock> {
         let else_block_span = self.thir[else_block].span;
-        let (matching, failure) = self.in_if_then_scope(remainder_scope, |this| {
+        let (matching, failure) = self.in_if_then_scope(*let_else_scope, |this| {
             let scrutinee = unpack!(block = this.lower_scrutinee(block, init, initializer_span));
             let pat = Pat { ty: init.ty, span: else_block_span, kind: PatKind::Wild };
             let mut wildcard = Candidate::new(scrutinee.clone(), &pat, false);
-            this.declare_bindings(
-                visibility_scope,
-                remainder_span,
-                pattern,
-                ArmHasGuard(false),
-                Some((None, initializer_span)),
-            );
             let mut candidate = Candidate::new(scrutinee.clone(), pattern, false);
             let fake_borrow_temps = this.lower_match_tree(
                 block,
@@ -2321,28 +2312,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 None,
                 None,
             );
-            this.break_for_else(failure, remainder_scope, this.source_info(initializer_span));
+            this.break_for_else(failure, *let_else_scope, this.source_info(initializer_span));
             matching.unit()
         });
-
-        // This place is not really used because this destination place
-        // should never be used to take values at the end of the failure
-        // block.
-        let dummy_place = self.temp(self.tcx.types.never, else_block_span);
-        let failure_block;
-        unpack!(
-            failure_block = self.ast_block(
-                dummy_place,
-                failure,
-                else_block,
-                self.source_info(else_block_span),
-            )
-        );
-        self.cfg.terminate(
-            failure_block,
-            self.source_info(else_block_span),
-            TerminatorKind::Unreachable,
-        );
-        matching.unit()
+        matching.and(failure)
     }
 }

compiler/rustc_typeck/src/check/region.rs

@@ -126,6 +126,29 @@ fn resolve_block<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, blk: &'tcx h
 
         for (i, statement) in blk.stmts.iter().enumerate() {
             match statement.kind {
+                hir::StmtKind::Local(hir::Local { els: Some(els), .. }) => {
+                    // Let-else has a special lexical structure for variables.
+                    // First we take a checkpoint of the current scope context here.
+                    let mut prev_cx = visitor.cx;
+
+                    visitor.enter_scope(Scope {
+                        id: blk.hir_id.local_id,
+                        data: ScopeData::Remainder(FirstStatementIndex::new(i)),
+                    });
+                    visitor.cx.var_parent = visitor.cx.parent;
+                    visitor.visit_stmt(statement);
+                    // We need to back out temporarily to the last enclosing scope
+                    // for the `else` block, so that even the temporaries receiving
+                    // extended lifetime will be dropped inside this block.
+                    // We are visiting the `else` block in this order so that
+                    // the sequence of visits agree with the order in the default
+                    // `hir::intravisit` visitor.
+                    mem::swap(&mut prev_cx, &mut visitor.cx);
+                    visitor.terminating_scopes.insert(els.hir_id.local_id);
+                    visitor.visit_block(els);
+                    // From now on, we continue normally.
+                    visitor.cx = prev_cx;
+                }
                 hir::StmtKind::Local(..) | hir::StmtKind::Item(..) => {
                     // Each declaration introduces a subscope for bindings
                     // introduced by the declaration; this subscope covers a
@@ -138,10 +161,10 @@ fn resolve_block<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, blk: &'tcx h
                         data: ScopeData::Remainder(FirstStatementIndex::new(i)),
                     });
                     visitor.cx.var_parent = visitor.cx.parent;
+                    visitor.visit_stmt(statement)
                 }
-                hir::StmtKind::Expr(..) | hir::StmtKind::Semi(..) => {}
+                hir::StmtKind::Expr(..) | hir::StmtKind::Semi(..) => visitor.visit_stmt(statement),
             }
-            visitor.visit_stmt(statement)
         }
         walk_list!(visitor, visit_expr, &blk.expr);
     }
@@ -460,7 +483,6 @@ fn resolve_local<'tcx>(
     visitor: &mut RegionResolutionVisitor<'tcx>,
     pat: Option<&'tcx hir::Pat<'tcx>>,
     init: Option<&'tcx hir::Expr<'tcx>>,
-    els: Option<&'tcx hir::Block<'tcx>>,
 ) {
     debug!("resolve_local(pat={:?}, init={:?})", pat, init);
 
@@ -547,9 +569,6 @@ fn resolve_local<'tcx>(
     if let Some(pat) = pat {
         visitor.visit_pat(pat);
     }
-    if let Some(els) = els {
-        visitor.visit_block(els);
-    }
 
     /// Returns `true` if `pat` match the `P&` non-terminal.
     ///
@@ -766,7 +785,7 @@ impl<'tcx> Visitor<'tcx> for RegionResolutionVisitor<'tcx> {
             // (i.e., `'static`), which means that after `g` returns, it drops,
             // and all the associated destruction scope rules apply.
             self.cx.var_parent = None;
-            resolve_local(self, None, Some(&body.value), None);
+            resolve_local(self, None, Some(&body.value));
         }
 
         if body.generator_kind.is_some() {
@@ -793,7 +812,7 @@ impl<'tcx> Visitor<'tcx> for RegionResolutionVisitor<'tcx> {
         resolve_expr(self, ex);
     }
     fn visit_local(&mut self, l: &'tcx Local<'tcx>) {
-        resolve_local(self, Some(&l.pat), l.init, l.els)
+        resolve_local(self, Some(&l.pat), l.init)
     }
 }
 

src/test/ui/async-await/async-await-let-else.stderr

@@ -35,7 +35,7 @@ LL |         bar2(Rc::new(())).await
    |              |
    |              has type `Rc<()>` which is not `Send`
 LL |     };
-   |      - `Rc::new(())` is later dropped here
+   |     - `Rc::new(())` is later dropped here
 note: required by a bound in `is_send`
   --> $DIR/async-await-let-else.rs:16:15
    |