Extend lang items to assert correct target.

This commit extends the existing lang items functionality to assert
that the `#[lang_item]` attribute is only found on the appropriate item
for any given lang item. That is, language items representing traits
must only ever have their corresponding attribute placed on a trait, for
example.

src/test/ui/error-codes/E0152.rs

@@ -10,7 +10,7 @@
 
 #![feature(lang_items)]
 
-#[lang = "panic_impl"]
+#[lang = "arc"]
 struct Foo; //~ ERROR E0152
 
 fn main() {

src/test/ui/error-codes/E0152.stderr

@@ -1,10 +1,10 @@
-error[E0152]: duplicate lang item found: `panic_impl`.
+error[E0152]: duplicate lang item found: `arc`.
   --> $DIR/E0152.rs:14:1
    |
 LL | struct Foo; //~ ERROR E0152
    | ^^^^^^^^^^^
    |
-   = note: first defined in crate `std`.
+   = note: first defined in crate `alloc`.
 
 error: aborting due to previous error
 

src/test/ui/error-codes/E0718.rs

@@ -0,0 +1,17 @@
+// Copyright 2013 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(lang_items)]
+
+// Arc is expected to be a struct, so this will error.
+#[lang = "arc"]
+static X: u32 = 42;
+
+fn main() {}

src/test/ui/error-codes/E0718.stderr

@@ -0,0 +1,9 @@
+error[E0718]: `arc` language item must be applied to a struct
+  --> $DIR/E0718.rs:14:1
+   |
+LL | #[lang = "arc"]
+   | ^^^^^^^^^^^^^^^ attribute should be applied to a struct, not a static item
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0718`.

src/test/ui/panic-handler/panic-handler-wrong-location.rs

@@ -0,0 +1,18 @@
+// 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.
+
+// compile-flags:-C panic=abort
+
+#![no_std]
+#![no_main]
+
+#[panic_handler]
+#[no_mangle]
+static X: u32 = 42;

src/test/ui/panic-handler/panic-handler-wrong-location.stderr

@@ -0,0 +1,11 @@
+error[E0718]: `panic_impl` language item must be applied to a function
+  --> $DIR/panic-handler-wrong-location.rs:16:1
+   |
+LL | #[panic_handler]
+   | ^^^^^^^^^^^^^^^^ attribute should be applied to a function, not a static item
+
+error: `#[panic_handler]` function required, but not found
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0718`.