tests(compile_fail): add test for IsVariant must_use

Cargo.toml

@@ -234,7 +234,7 @@ required-features = ["try_unwrap"]
 [[test]]
 name = "compile_fail"
 path = "tests/compile_fail/mod.rs"
-required-features = ["as_ref", "debug", "display", "from", "into"]
+required-features = ["as_ref", "debug", "display", "from", "into", "try_from", "is_variant"]
 
 [[test]]
 name = "no_std"

tests/compile_fail/is_variant/must_use.rs

@@ -0,0 +1,10 @@
+#[derive(derive_more::IsVariant)]
+enum MustUse {
+    Yes,
+}
+
+#[forbid(unused_must_use)]
+fn main() {
+    let must_use = MustUse::Yes;
+    must_use.is_yes();
+}

tests/compile_fail/is_variant/must_use.stderr

@@ -0,0 +1,15 @@
+error: unused return value of `MustUse::is_yes` that must be used
+ --> tests/compile_fail/is_variant/must_use.rs:9:5
+  |
+9 |     must_use.is_yes();
+  |     ^^^^^^^^^^^^^^^^^
+  |
+note: the lint level is defined here
+ --> tests/compile_fail/is_variant/must_use.rs:6:10
+  |
+6 | #[forbid(unused_must_use)]
+  |          ^^^^^^^^^^^^^^^
+help: use `let _ = ...` to ignore the resulting value
+  |
+9 |     let _ = must_use.is_yes();
+  |     +++++++