Correct inline assembly clobber formatting.

src/doc/book/inline-assembly.md

@@ -57,7 +57,7 @@ but you must add the right number of `:` if you skip them:
 asm!("xor %eax, %eax"
     :
     :
-    : "{eax}"
+    : "eax"
    );
 # } }
 ```
@@ -68,7 +68,7 @@ Whitespace also doesn't matter:
 # #![feature(asm)]
 # #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
 # fn main() { unsafe {
-asm!("xor %eax, %eax" ::: "{eax}");
+asm!("xor %eax, %eax" ::: "eax");
 # } }
 ```
 
@@ -127,7 +127,7 @@ stay valid.
 # #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
 # fn main() { unsafe {
 // Put the value 0x200 in eax
-asm!("mov $$0x200, %eax" : /* no outputs */ : /* no inputs */ : "{eax}");
+asm!("mov $$0x200, %eax" : /* no outputs */ : /* no inputs */ : "eax");
 # } }
 ```
 

src/libsyntax_ext/asm.rs

@@ -192,7 +192,10 @@ pub fn expand_asm<'cx>(cx: &'cx mut ExtCtxt, sp: Span, tts: &[tokenstream::Token
 
                     if OPTIONS.iter().any(|&opt| s == opt) {
                         cx.span_warn(p.last_span, "expected a clobber, found an option");
+                    } else if s.starts_with("{") || s.ends_with("}") {
+                        cx.span_err(p.last_span, "clobber should not be surrounded by braces");
                     }
+
                     clobs.push(s);
                 }
             }

src/test/compile-fail/asm-bad-clobber.rs

@@ -0,0 +1,26 @@
+// Copyright 2016 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.
+
+// ignore-android
+// ignore-arm
+// ignore-aarch64
+
+#![feature(asm, rustc_attrs)]
+
+#[cfg(any(target_arch = "x86",
+          target_arch = "x86_64"))]
+#[rustc_error]
+pub fn main() {
+    unsafe {
+        // clobber formatted as register input/output
+        asm!("xor %eax, %eax" : : : "{eax}");
+        //~^ ERROR clobber should not be surrounded by braces
+    }
+}