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.

1 files changed, 11 insertions, 0 deletions

diff --git a/src/test/ui/panic-handler/panic-handler-wrong-location.stderr b/src/test/ui/panic-handler/panic-handler-wrong-location.stderr
new file mode 100644
index 00000000000..f761e26b86e
--- /dev/null
+++ b/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`.