Add test for setting RUSTFLAGS

Author: killercup

cargo-fix/tests/all/main.rs

@@ -356,6 +356,7 @@ mod broken_build;
 mod broken_lints;
 mod dependencies;
 mod edition_upgrade;
+mod rust_flags;
 mod smoke;
 mod subtargets;
 mod vcs;

cargo-fix/tests/all/rust_flags.rs

@@ -0,0 +1,34 @@
+use super::project;
+
+#[test]
+fn specify_rustflags() {
+    let p = project()
+        .file(
+            "src/lib.rs",
+            r#"
+                #![allow(unused)]
+                #![feature(rust_2018_preview)]
+
+                mod foo {
+                    pub const FOO: &str = "fooo";
+                }
+
+                fn main() {
+                    let x = ::foo::FOO;
+                }
+            "#,
+        )
+        .build();
+
+    let stderr = "\
+[CHECKING] foo v0.1.0 (CWD)
+[FIXING] src/lib.rs (1 fix)
+[FINISHED] dev [unoptimized + debuginfo]
+";
+
+    p.expect_cmd("cargo-fix fix --prepare-for 2018")
+        .env("RUSTFLAGS", "-C target-cpu=native")
+        .stdout("")
+        .stderr(stderr)
+        .run();
+}