Rewrite stmt processing not to recurse to avoid stack overflow if there

are tons of let statements. Fixes #29466.

Author: nikomatsakis

src/librustc_mir/build/scope.rs

@@ -135,8 +135,7 @@ impl<'a,'tcx> Builder<'a,'tcx> {
         debug!("in_scope(extent={:?}, block={:?})", extent, block);
         self.push_scope(extent, block);
         let rv = unpack!(block = f(self));
-        assert_eq!(self.extent_of_innermost_scope(), extent);
-        self.pop_scope(block);
+        self.pop_scope(extent, block);
         debug!("in_scope: exiting extent={:?} block={:?}", extent, block);
         block.and(rv)
     }
@@ -156,13 +155,15 @@ impl<'a,'tcx> Builder<'a,'tcx> {
         });
     }
 
-    /// Pops the innermost scope, adding any drops onto the end of
-    /// `block` that are needed.  This must match 1-to-1 with
-    /// `push_scope`.
-    pub fn pop_scope(&mut self, block: BasicBlock) {
-        debug!("pop_scope({:?})", block);
+    /// Pops a scope, which should have extent `extent`, adding any
+    /// drops onto the end of `block` that are needed.  This must
+    /// match 1-to-1 with `push_scope`.
+    pub fn pop_scope(&mut self, extent: CodeExtent, block: BasicBlock) {
+        debug!("pop_scope({:?}, {:?})", extent, block);
         let scope = self.scopes.pop().unwrap();
 
+        assert_eq!(scope.extent, extent);
+
         // add in any drops needed on the fallthrough path (any other
         // exiting paths, such as those that arise from `break`, will
         // have drops already)

src/librustc_mir/build/stmt.rs

@@ -14,48 +14,70 @@ use repr::*;
 
 impl<'a,'tcx> Builder<'a,'tcx> {
     pub fn stmts(&mut self, mut block: BasicBlock, stmts: Vec<StmtRef<'tcx>>) -> BlockAnd<()> {
-        for stmt in stmts {
-            unpack!(block = self.stmt(block, stmt));
-        }
-        block.unit()
-    }
-
-    pub fn stmt(&mut self, mut block: BasicBlock, stmt: StmtRef<'tcx>) -> BlockAnd<()> {
+        // This convoluted structure is to avoid using recursion as we walk down a list
+        // of statements. Basically, the structure we get back is something like:
+        //
+        //    let x = <init> in {
+        //       let y = <init> in {
+        //           expr1;
+        //           expr2;
+        //       }
+        //    }
+        //
+        // To process this, we keep a stack of (Option<CodeExtent>,
+        // vec::IntoIter<Stmt>) pairs.  At each point we pull off the
+        // top most pair and extract one statement from the
+        // iterator. Once it's complete, we pop the scope from the
+        // first half the pair.
         let this = self;
-        let Stmt { span, kind } = this.hir.mirror(stmt);
-        match kind {
-            StmtKind::Let { remainder_scope,
-                            init_scope,
-                            pattern,
-                            initializer: Some(initializer),
-                            stmts } => {
-                this.in_scope(remainder_scope, block, |this| {
-                    unpack!(block = this.in_scope(init_scope, block, |this| {
-                        this.expr_into_pattern(block, remainder_scope, pattern, initializer)
+        let mut stmt_lists = vec![(None, stmts.into_iter())];
+        while !stmt_lists.is_empty() {
+            let stmt = {
+                let &mut (_, ref mut stmts) = stmt_lists.last_mut().unwrap();
+                stmts.next()
+            };
+
+            let stmt = match stmt {
+                Some(stmt) => stmt,
+                None => {
+                    let (extent, _) = stmt_lists.pop().unwrap();
+                    if let Some(extent) = extent {
+                        this.pop_scope(extent, block);
+                    }
+                    continue
+                }
+            };
+
+            let Stmt { span, kind } = this.hir.mirror(stmt);
+            match kind {
+                StmtKind::Let { remainder_scope, init_scope, pattern, initializer, stmts } => {
+                    this.push_scope(remainder_scope, block);
+                    stmt_lists.push((Some(remainder_scope), stmts.into_iter()));
+                    unpack!(block = this.in_scope(init_scope, block, move |this| {
+                        // FIXME #30046                              ^~~~
+                        match initializer {
+                            Some(initializer) => {
+                                this.expr_into_pattern(block, remainder_scope, pattern, initializer)
+                            }
+                            None => {
+                                this.declare_bindings(remainder_scope, &pattern);
+                                block.unit()
+                            }
+                        }
                     }));
-                    this.stmts(block, stmts)
-                })
-            }
+                }
 
-            StmtKind::Let { remainder_scope, init_scope, pattern, initializer: None, stmts } => {
-                this.in_scope(remainder_scope, block, |this| {
-                    unpack!(block = this.in_scope(init_scope, block, |this| {
-                        this.declare_bindings(remainder_scope, &pattern);
+                StmtKind::Expr { scope, expr } => {
+                    unpack!(block = this.in_scope(scope, block, |this| {
+                        let expr = this.hir.mirror(expr);
+                        let temp = this.temp(expr.ty.clone());
+                        unpack!(block = this.into(&temp, block, expr));
+                        this.cfg.push_drop(block, span, DropKind::Deep, &temp);
                         block.unit()
                     }));
-                    this.stmts(block, stmts)
-                })
-            }
-
-            StmtKind::Expr { scope, expr } => {
-                this.in_scope(scope, block, |this| {
-                    let expr = this.hir.mirror(expr);
-                    let temp = this.temp(expr.ty.clone());
-                    unpack!(block = this.into(&temp, block, expr));
-                    this.cfg.push_drop(block, span, DropKind::Deep, &temp);
-                    block.unit()
-                })
+                }
             }
         }
+        block.unit()
     }
 }