Test cross-crate runtime contract checks

These tests capture the behaviour that the decision to include/exclude
runtime contract assertions is determined on a per-crate basis,
i.e. by the flags used to compile each crate.

Author: dawidl022

tests/ui/contracts/cross-crate-checks/auxiliary/id.rs

@@ -0,0 +1,16 @@
+//@ [unchk_pass] compile-flags: -Zcontract-checks=no
+//@ [unchk_fail] compile-flags: -Zcontract-checks=yes
+//@ [chk_pass] compile-flags: -Zcontract-checks=no
+//@ [chk_fail] compile-flags: -Zcontract-checks=yes
+
+#![crate_type = "lib"]
+#![feature(contracts)]
+
+extern crate core;
+use core::contracts::requires;
+
+/// Example function with a spec to be called across a crate boundary.
+#[requires(x > 0)]
+pub fn id_if_positive(x: u32) -> u32 {
+    x
+}

tests/ui/contracts/cross-crate-checks/cross-crate.rs

@@ -0,0 +1,26 @@
+//@ aux-build:id.rs
+//@ revisions: unchk_pass unchk_fail chk_pass chk_fail
+//
+// The dependency crate `id` can be compiled with runtime contract checking
+// enabled independently of whether this crate is compiled with contract checks
+// or not.
+//
+// chk/unchk indicates whether this crate is compiled with contracts or not
+// and pass/fail indicates whether the `id` crate is compiled with contract checks.
+//
+//@ [unchk_pass] run-pass
+//@ [unchk_fail] run-crash
+//@ [chk_pass] run-pass
+//@ [chk_fail] run-crash
+//
+//
+//@ [unchk_pass] compile-flags: -Zcontract-checks=no
+//@ [unchk_fail] compile-flags: -Zcontract-checks=no
+//@ [chk_pass] compile-flags: -Zcontract-checks=yes
+//@ [chk_fail] compile-flags: -Zcontract-checks=yes
+
+extern crate id;
+
+fn main() {
+    id::id_if_positive(0);
+}