ensure that the proc_macro crate exists in the sysroot

Author: RalfJung

src/tools/miri/test-cargo-miri/subcrate/src/lib.rs

@@ -1,5 +1,16 @@
+// This is a proc-macro crate.
+
+extern crate proc_macro; // make sure proc_macro is in the sysroot
+
 #[cfg(doctest)]
 compile_error!("rustdoc should not touch me");
 
-#[cfg(test)]
+#[cfg(miri)]
 compile_error!("Miri should not touch me");
+
+use proc_macro::TokenStream;
+
+#[proc_macro]
+pub fn make_answer(_item: TokenStream) -> TokenStream {
+    "fn answer() -> u32 { 42 }".parse().unwrap()
+}

src/tools/miri/tests/pass/sysroot.rs

@@ -0,0 +1,9 @@
+//! Just check that some things are available in the sysroot.
+#![feature(test)]
+#![allow(unused)]
+
+extern crate proc_macro;
+extern crate std;
+extern crate test;
+
+fn main() {}