Add regression test for useless_attribute lint

Author: flip1995

tests/ui/useless_attribute.fixed

@@ -0,0 +1,57 @@
+// run-rustfix
+// aux-build:proc_macro_derive.rs
+
+#![warn(clippy::useless_attribute)]
+#![warn(unreachable_pub)]
+#![feature(rustc_private)]
+
+#![allow(dead_code)]
+#![cfg_attr(feature = "cargo-clippy", allow(dead_code))]
+#[rustfmt::skip]
+#[allow(unused_imports)]
+#[allow(unused_extern_crates)]
+#[macro_use]
+extern crate rustc;
+
+#[macro_use]
+extern crate proc_macro_derive;
+
+// don't lint on unused_import for `use` items
+#[allow(unused_imports)]
+use std::collections;
+
+// don't lint on deprecated for `use` items
+mod foo {
+    #[deprecated]
+    pub struct Bar;
+}
+#[allow(deprecated)]
+pub use foo::Bar;
+
+// This should not trigger the lint. There's lint level definitions inside the external derive
+// that would trigger the useless_attribute lint.
+#[derive(DeriveSomething)]
+struct Baz;
+
+// don't lint on unreachable_pub for `use` items
+mod a {
+    mod b {
+        #[allow(dead_code)]
+        #[allow(unreachable_pub)]
+        pub struct C {}
+    }
+
+    #[allow(unreachable_pub)]
+    pub use self::b::C;
+}
+
+fn test_indented_attr() {
+    #![allow(clippy::almost_swapped)]
+    use std::collections::HashSet;
+
+    let _ = HashSet::<u32>::default();
+}
+
+fn main() {
+    test_indented_attr();
+}

tests/ui/useless_attribute.rs

@@ -1,3 +1,4 @@
+// run-rustfix
 // aux-build:proc_macro_derive.rs
 
 #![warn(clippy::useless_attribute)]
@@ -44,4 +45,13 @@ mod a {
     pub use self::b::C;
 }
 
-fn main() {}
+fn test_indented_attr() {
+    #[allow(clippy::almost_swapped)]
+    use std::collections::HashSet;
+
+    let _ = HashSet::<u32>::default();
+}
+
+fn main() {
+    test_indented_attr();
+}

tests/ui/useless_attribute.stderr

@@ -1,16 +1,22 @@
 error: useless lint attribute
-  --> $DIR/useless_attribute.rs:7:1
+  --> $DIR/useless_attribute.rs:8:1
    |
 LL | #[allow(dead_code)]
    | ^^^^^^^^^^^^^^^^^^^ help: if you just forgot a `!`, use: `#![allow(dead_code)]`
    |
    = note: `-D clippy::useless-attribute` implied by `-D warnings`
 
 error: useless lint attribute
-  --> $DIR/useless_attribute.rs:8:1
+  --> $DIR/useless_attribute.rs:9:1
    |
 LL | #[cfg_attr(feature = "cargo-clippy", allow(dead_code))]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: if you just forgot a `!`, use: `#![cfg_attr(feature = "cargo-clippy", allow(dead_code)`
 
-error: aborting due to 2 previous errors
+error: useless lint attribute
+  --> $DIR/useless_attribute.rs:49:5
+   |
+LL |     #[allow(clippy::almost_swapped)]
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: if you just forgot a `!`, use: `#![allow(clippy::almost_swapped)]`
+
+error: aborting due to 3 previous errors