Add test for incomplete derive macro output parse.

Author: m-ou-se

src/test/ui/proc-macro/auxiliary/derive-bad.rs

@@ -11,3 +11,8 @@ use proc_macro::TokenStream;
 pub fn derive_a(_input: TokenStream) -> TokenStream {
     "struct A { inner }".parse().unwrap()
 }
+
+#[proc_macro_derive(B)]
+pub fn derive_b(_input: TokenStream) -> TokenStream {
+    "const _: () = (); { HEY! }".parse().unwrap()
+}

src/test/ui/proc-macro/derive-bad.rs

@@ -8,4 +8,8 @@ extern crate derive_bad;
 //~| ERROR expected `:`, found `}`
 struct A; //~ ERROR the name `A` is defined multiple times
 
+#[derive(B)]
+//~^ ERROR proc-macro derive produced unparseable tokens
+struct B;
+
 fn main() {}

src/test/ui/proc-macro/derive-bad.stderr

@@ -23,6 +23,12 @@ LL | struct A;
    |
    = note: `A` must be defined only once in the type namespace of this module
 
-error: aborting due to 3 previous errors
+error: proc-macro derive produced unparseable tokens
+  --> $DIR/derive-bad.rs:11:10
+   |
+LL | #[derive(B)]
+   |          ^
+
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0428`.