Initial vendor packages

Signed-off-by: Valentin Popov <valentin@popov.link>

vendor/thiserror/build/probe.rs

@@ -0,0 +1,32 @@
+// This code exercises the surface area that we expect of the Error generic
+// member access API. If the current toolchain is able to compile it, then
+// thiserror is able to provide backtrace support.
+
+#![feature(error_generic_member_access)]
+
+use std::error::{Error, Request};
+use std::fmt::{self, Debug, Display};
+
+struct MyError(Thing);
+struct Thing;
+
+impl Debug for MyError {
+    fn fmt(&self, _formatter: &mut fmt::Formatter) -> fmt::Result {
+        unimplemented!()
+    }
+}
+
+impl Display for MyError {
+    fn fmt(&self, _formatter: &mut fmt::Formatter) -> fmt::Result {
+        unimplemented!()
+    }
+}
+
+impl Error for MyError {
+    fn provide<'a>(&'a self, request: &mut Request<'a>) {
+        request.provide_ref(&self.0);
+    }
+}
+
+// Include in sccache cache key.
+const _: Option<&str> = option_env!("RUSTC_BOOTSTRAP");