clippy-driver: more robust test to see if we're clippy-enabled

Rather than looking for a fixed --emit arg set, just check to see
if we're emitting metadata at all. This makes it more robust to
being invoked by tools other than cargo (or if cargo changes its
invocation).

Issue #3663

Cargo.toml

@@ -34,7 +34,6 @@ path = "src/main.rs"
 
 [[bin]]
 name = "clippy-driver"
-test = false
 path = "src/driver.rs"
 
 [dependencies]

src/driver.rs

@@ -20,6 +20,46 @@ fn show_version() {
     println!(env!("CARGO_PKG_VERSION"));
 }
 
+/// If a command-line option matches `find_arg`, then apply the predicate `pred` on its value. If
+/// true, then return it. The parameter is assumed to be either `--arg=value` or `--arg value`.
+fn arg_value<'a>(
+    args: impl IntoIterator<Item = &'a String>,
+    find_arg: &str,
+    pred: impl Fn(&str) -> bool,
+) -> Option<&'a str> {
+    let mut args = args.into_iter().map(String::as_str);
+
+    while let Some(arg) = args.next() {
+        let arg: Vec<_> = arg.splitn(2, '=').collect();
+        if arg.get(0) != Some(&find_arg) {
+            continue;
+        }
+
+        let value = arg.get(1).cloned().or_else(|| args.next());
+        if value.as_ref().map_or(false, |p| pred(p)) {
+            return value;
+        }
+    }
+    None
+}
+
+#[test]
+fn test_arg_value() {
+    let args: Vec<_> = ["--bar=bar", "--foobar", "123", "--foo"]
+        .iter()
+        .map(|s| s.to_string())
+        .collect();
+
+    assert_eq!(arg_value(None, "--foobar", |_| true), None);
+    assert_eq!(arg_value(&args, "--bar", |_| false), None);
+    assert_eq!(arg_value(&args, "--bar", |_| true), Some("bar"));
+    assert_eq!(arg_value(&args, "--bar", |p| p == "bar"), Some("bar"));
+    assert_eq!(arg_value(&args, "--bar", |p| p == "foo"), None);
+    assert_eq!(arg_value(&args, "--foobar", |p| p == "foo"), None);
+    assert_eq!(arg_value(&args, "--foobar", |p| p == "123"), Some("123"));
+    assert_eq!(arg_value(&args, "--foo", |_| true), None);
+}
+
 #[allow(clippy::too_many_lines)]
 pub fn main() {
     rustc_driver::init_rustc_env_logger();
@@ -79,7 +119,7 @@ pub fn main() {
             // crate is
             // linted but not built
             let clippy_enabled = env::var("CLIPPY_TESTS").ok().map_or(false, |val| val == "true")
-                || orig_args.iter().any(|s| s == "--emit=dep-info,metadata");
+                || arg_value(&orig_args, "--emit", |val| val.split(',').any(|e| e == "metadata")).is_some();
 
             if clippy_enabled {
                 args.extend_from_slice(&["--cfg".to_owned(), r#"feature="cargo-clippy""#.to_owned()]);