Add tests

src/test/ui/meta/cfg-ignore-with-target.rs

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+// This test checks that the "ignore-cfg" compiletest option is evaluated
+// based on the "--target" option in "compile-flags", rather than
+// the default platform.
+//
+// compile-flags: --target x86_64-unknown-linux-gnu
+// needs-llvm-components: x86
+// ignore-cfg: target_os=linux
+// check-pass
+
+compile_error!("this should never be compiled");
+
+fn main() {}

src/test/ui/meta/cfg-ignore.rs

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+// This test checks that the "ignore-cfg" compiletest option matches
+// up with rustc's builtin evaluation of cfgs.
+//
+// ignore-cfg: target_family=unix
+// check-pass
+
+#[cfg(target_family = "unix")]
+compile_error!("this shouldn't be compiled on unix");
+
+fn main() {}

src/test/ui/meta/cfg-only.rs

@@ -0,0 +1,10 @@
+// This test checks that the "ignore-cfg" compiletest option matches
+// up with rustc's builtin evaluation of cfgs.
+//
+// only-cfg: target_family=unix
+// check-pass
+
+#[cfg(not(target_family = "unix"))]
+compile_error!("this should only be compiled on unix");
+
+fn main() {}