don't panic on #[derive] placed on statements/expressions

closes rust-lang#49934

src/libsyntax/ext/base.rs

@@ -118,6 +118,20 @@ impl Annotatable {
         }
     }
 
+    pub fn expect_stmt(self) -> ast::Stmt {
+        match self {
+            Annotatable::Stmt(stmt) => stmt.into_inner(),
+            _ => panic!("expected statement"),
+        }
+    }
+
+    pub fn expect_expr(self) -> P<ast::Expr> {
+        match self {
+            Annotatable::Expr(expr) => expr,
+            _ => panic!("expected expression"),
+        }
+    }
+
     pub fn derive_allowed(&self) -> bool {
         match *self {
             Annotatable::Item(ref item) => match item.node {

src/libsyntax/ext/expand.rs

@@ -143,7 +143,7 @@ impl ExpansionKind {
     }
 
     fn expect_from_annotatables<I: IntoIterator<Item = Annotatable>>(self, items: I) -> Expansion {
-        let items = items.into_iter();
+        let mut items = items.into_iter();
         match self {
             ExpansionKind::Items =>
                 Expansion::Items(items.map(Annotatable::expect_item).collect()),
@@ -153,7 +153,14 @@ impl ExpansionKind {
                 Expansion::TraitItems(items.map(Annotatable::expect_trait_item).collect()),
             ExpansionKind::ForeignItems =>
                 Expansion::ForeignItems(items.map(Annotatable::expect_foreign_item).collect()),
-            _ => unreachable!(),
+            ExpansionKind::Stmts => Expansion::Stmts(items.map(Annotatable::expect_stmt).collect()),
+            ExpansionKind::Expr => Expansion::Expr(
+                items.next().expect("expected exactly one expression").expect_expr()
+            ),
+            ExpansionKind::OptExpr =>
+                Expansion::OptExpr(items.next().map(Annotatable::expect_expr)),
+            ExpansionKind::Pat | ExpansionKind::Ty =>
+                panic!("patterns and types aren't annotatable"),
         }
     }
 }

src/test/compile-fail/issue-49934.rs

@@ -0,0 +1,23 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+#![feature(stmt_expr_attributes)]
+
+fn main() {
+    #[derive(Debug)] //~ ERROR `derive`
+    println!("Hello, world!");
+
+    let _ = #[derive(Debug)] "Hello, world!";
+    //~^ ERROR `derive`
+
+    let _ = [
+        #[derive(Debug)] //~ ERROR `derive`
+        "Hello, world!"
+    ];
+}