add test case for issue rust-lang#42008

Author: JeanMertz

src/test/ui-fulldeps/proc-macro/auxiliary/issue-42008.rs

@@ -0,0 +1,16 @@
+//! Proc macro crate
+
+// no-prefer-dynamic
+
+#![deny(missing_docs)]
+#![crate_type = "proc-macro"]
+
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+
+/// foo is documented
+#[proc_macro]
+pub fn foo(input: TokenStream) -> TokenStream {
+    input
+}

src/test/ui-fulldeps/proc-macro/issue-42008.rs

@@ -0,0 +1,10 @@
+// aux-build:issue-42008.rs
+
+#![feature(proc_macro_non_items)]
+
+#[macro_use]
+extern crate issue_42008;
+
+fn main() {
+    let _ = foo!("");
+}

src/test/ui-fulldeps/proc-macro/issue-42008.stderr

@@ -0,0 +1,12 @@
+error: missing documentation for a module
+   |
+note: lint level defined here
+  --> $DIR/auxiliary/issue-42008.rs:4:9
+   |
+LL | #![deny(missing_docs)]
+   |         ^^^^^^^^^^^^
+
+error: missing documentation for a function
+
+error: aborting due to 2 previous errors
+