wildcard_match_arm: add simple ui test.

tests/ui/wildcard_match_arm.rs

@@ -0,0 +1,36 @@
+#![deny(clippy::wildcard_match_arm)]
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+enum Color {
+	Red,
+	Green,
+	Blue,
+	Rgb(u8, u8, u8),
+	Cyan,
+}
+
+impl Color {
+	fn is_monochrome(self) -> bool {
+		match self {
+			Color::Red | Color::Green | Color::Blue => true,
+			Color::Rgb(r, g, b) => r | g == 0 || r | b == 0 || g | b == 0,
+			Color::Cyan => false,
+		}
+	}
+}
+
+fn main() {
+	let color = Color::Rgb(0, 0, 127);
+	match color {
+		Color::Red => println!("Red"),
+		_ => eprintln!("Not red"),
+	};
+	match color {
+		Color::Red => {},
+		Color::Green => {},
+		Color::Blue => {},
+		Color::Cyan => {},
+		c if c.is_monochrome() => {},
+		Color::Rgb(_, _, _) => {},
+	};
+}

tests/ui/wildcard_match_arm.stderr

@@ -0,0 +1,15 @@
+error: wildcard match will miss any future added variants.
+  --> $DIR/wildcard_match_arm.rs:26:3
+   |
+LL |         _ => eprintln!("Not red"),
+   |         ^
+   |
+note: lint level defined here
+  --> $DIR/wildcard_match_arm.rs:1:9
+   |
+LL | #![deny(clippy::wildcard_match_arm)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: to resolve, match each variant explicitly
+
+error: aborting due to previous error
+