Add ui test for mixed_attributes_style

tests/ui/mixed_attributes_style.rs

@@ -0,0 +1,39 @@
+#![warn(clippy::mixed_attributes_style)]
+
+#[allow(unused)] //~ ERROR: item has both inner and outer attributes
+fn foo1() {
+    #![allow(unused)]
+}
+
+#[allow(unused)]
+#[allow(unused)]
+fn foo2() {}
+
+fn foo3() {
+    #![allow(unused)]
+    #![allow(unused)]
+}
+
+/// linux
+//~^ ERROR: item has both inner and outer attributes
+fn foo4() {
+    //! windows
+}
+
+/// linux
+/// windows
+fn foo5() {}
+
+fn foo6() {
+    //! linux
+    //! windows
+}
+
+#[allow(unused)] //~ ERROR: item has both inner and outer attributes
+mod bar {
+    #![allow(unused)]
+}
+
+fn main() {
+    // test code goes here
+}

tests/ui/mixed_attributes_style.stderr

@@ -0,0 +1,30 @@
+error: item has both inner and outer attributes
+  --> tests/ui/mixed_attributes_style.rs:3:1
+   |
+LL | / #[allow(unused)]
+LL | | fn foo1() {
+LL | |     #![allow(unused)]
+   | |_____________________^
+   |
+   = note: `-D clippy::mixed-attributes-style` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::mixed_attributes_style)]`
+
+error: item has both inner and outer attributes
+  --> tests/ui/mixed_attributes_style.rs:17:1
+   |
+LL | / /// linux
+LL | |
+LL | | fn foo4() {
+LL | |     //! windows
+   | |_______________^
+
+error: item has both inner and outer attributes
+  --> tests/ui/mixed_attributes_style.rs:32:1
+   |
+LL | / #[allow(unused)]
+LL | | mod bar {
+LL | |     #![allow(unused)]
+   | |_____________________^
+
+error: aborting due to 3 previous errors
+