Reuse the P in InvocationCollector::fold_{,opt_}expr.

This requires adding a new method, `P::filter_map`.

This commit reduces instruction counts for various benchmarks by up to
0.7%.

src/libsyntax/ext/expand.rs

@@ -1201,50 +1201,62 @@ impl<'a, 'b> InvocationCollector<'a, 'b> {
 
 impl<'a, 'b> Folder for InvocationCollector<'a, 'b> {
     fn fold_expr(&mut self, expr: P<ast::Expr>) -> P<ast::Expr> {
-        let mut expr = self.cfg.configure_expr(expr).into_inner();
-        expr.node = self.cfg.configure_expr_kind(expr.node);
-
-        // ignore derives so they remain unused
-        let (attr, expr, after_derive) = self.classify_nonitem(expr);
-
-        if attr.is_some() {
-            // collect the invoc regardless of whether or not attributes are permitted here
-            // expansion will eat the attribute so it won't error later
-            attr.as_ref().map(|a| self.cfg.maybe_emit_expr_attr_err(a));
-
-            // AstFragmentKind::Expr requires the macro to emit an expression
-            return self.collect_attr(attr, vec![], Annotatable::Expr(P(expr)),
-                                     AstFragmentKind::Expr, after_derive).make_expr();
-        }
+        let expr = self.cfg.configure_expr(expr);
+        expr.map(|mut expr| {
+            expr.node = self.cfg.configure_expr_kind(expr.node);
+
+            // ignore derives so they remain unused
+            let (attr, expr, after_derive) = self.classify_nonitem(expr);
+
+            if attr.is_some() {
+                // Collect the invoc regardless of whether or not attributes are permitted here
+                // expansion will eat the attribute so it won't error later.
+                attr.as_ref().map(|a| self.cfg.maybe_emit_expr_attr_err(a));
+
+                // AstFragmentKind::Expr requires the macro to emit an expression.
+                return self.collect_attr(attr, vec![], Annotatable::Expr(P(expr)),
+                                         AstFragmentKind::Expr, after_derive)
+                    .make_expr()
+                    .into_inner()
+            }
 
-        if let ast::ExprKind::Mac(mac) = expr.node {
-            self.check_attributes(&expr.attrs);
-            self.collect_bang(mac, expr.span, AstFragmentKind::Expr).make_expr()
-        } else {
-            P(noop_fold_expr(expr, self))
-        }
+            if let ast::ExprKind::Mac(mac) = expr.node {
+                self.check_attributes(&expr.attrs);
+                self.collect_bang(mac, expr.span, AstFragmentKind::Expr)
+                    .make_expr()
+                    .into_inner()
+            } else {
+                noop_fold_expr(expr, self)
+            }
+        })
     }
 
     fn fold_opt_expr(&mut self, expr: P<ast::Expr>) -> Option<P<ast::Expr>> {
-        let mut expr = configure!(self, expr).into_inner();
-        expr.node = self.cfg.configure_expr_kind(expr.node);
+        let expr = configure!(self, expr);
+        expr.filter_map(|mut expr| {
+            expr.node = self.cfg.configure_expr_kind(expr.node);
 
-        // ignore derives so they remain unused
-        let (attr, expr, after_derive) = self.classify_nonitem(expr);
+            // Ignore derives so they remain unused.
+            let (attr, expr, after_derive) = self.classify_nonitem(expr);
 
-        if attr.is_some() {
-            attr.as_ref().map(|a| self.cfg.maybe_emit_expr_attr_err(a));
+            if attr.is_some() {
+                attr.as_ref().map(|a| self.cfg.maybe_emit_expr_attr_err(a));
 
-            return self.collect_attr(attr, vec![], Annotatable::Expr(P(expr)),
-                                     AstFragmentKind::OptExpr, after_derive).make_opt_expr();
-        }
+                return self.collect_attr(attr, vec![], Annotatable::Expr(P(expr)),
+                                         AstFragmentKind::OptExpr, after_derive)
+                    .make_opt_expr()
+                    .map(|expr| expr.into_inner())
+            }
 
-        if let ast::ExprKind::Mac(mac) = expr.node {
-            self.check_attributes(&expr.attrs);
-            self.collect_bang(mac, expr.span, AstFragmentKind::OptExpr).make_opt_expr()
-        } else {
-            Some(P(noop_fold_expr(expr, self)))
-        }
+            if let ast::ExprKind::Mac(mac) = expr.node {
+                self.check_attributes(&expr.attrs);
+                self.collect_bang(mac, expr.span, AstFragmentKind::OptExpr)
+                    .make_opt_expr()
+                    .map(|expr| expr.into_inner())
+            } else {
+                Some(noop_fold_expr(expr, self))
+            }
+        })
     }
 
     fn fold_pat(&mut self, pat: P<ast::Pat>) -> P<ast::Pat> {

src/libsyntax/ptr.rs

@@ -72,7 +72,7 @@ impl<T: 'static> P<T> {
         *self.ptr
     }
 
-    /// Transform the inner value, consuming `self` and producing a new `P<T>`.
+    /// Produce a new `P<T>` from `self` without reallocating.
     pub fn map<F>(mut self, f: F) -> P<T> where
         F: FnOnce(T) -> T,
     {
@@ -88,8 +88,30 @@ impl<T: 'static> P<T> {
             ptr::write(p, f(ptr::read(p)));
 
             // Recreate self from the raw pointer.
-            P {
-                ptr: Box::from_raw(p)
+            P { ptr: Box::from_raw(p) }
+        }
+    }
+
+    /// Optionally produce a new `P<T>` from `self` without reallocating.
+    pub fn filter_map<F>(mut self, f: F) -> Option<P<T>> where
+        F: FnOnce(T) -> Option<T>,
+    {
+        let p: *mut T = &mut *self.ptr;
+
+        // Leak self in case of panic.
+        // FIXME(eddyb) Use some sort of "free guard" that
+        // only deallocates, without dropping the pointee,
+        // in case the call the `f` below ends in a panic.
+        mem::forget(self);
+
+        unsafe {
+            if let Some(v) = f(ptr::read(p)) {
+                ptr::write(p, v);
+
+                // Recreate self from the raw pointer.
+                Some(P { ptr: Box::from_raw(p) })
+            } else {
+                None
             }
         }
     }