Warn on pointless #[derive] in more places

This fixes the regression in #49934 and ensures that unused `#[derive]`s on statements, expressions and generic type parameters survive to trip the `unused_attributes` lint. For `#[derive]` on macro invocations it has a hardcoded warning since linting occurs after expansion. This also adds regression testing for some nodes that were already warning properly.

closes #49934

src/test/ui/issue-49934.rs

@@ -0,0 +1,52 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-pass
+
+#![feature(stmt_expr_attributes)]
+#![warn(unused_attributes)] //~ NOTE lint level defined here
+
+fn foo<#[derive(Debug)] T>() { //~ WARN unused attribute
+    match 0 {
+        #[derive(Debug)] //~ WARN unused attribute
+        _ => (),
+    }
+}
+
+fn main() {
+    // fold_stmt (Item)
+    #[allow(dead_code)]
+    #[derive(Debug)] // should not warn
+    struct Foo;
+
+    // fold_stmt (Mac)
+    #[derive(Debug)]
+    //~^ WARN `#[derive]` does nothing on macro invocations
+    //~| NOTE this may become a hard error in a future release
+    println!("Hello, world!");
+
+    // fold_stmt (Semi)
+    #[derive(Debug)] //~ WARN unused attribute
+    "Hello, world!";
+
+    // fold_stmt (Local)
+    #[derive(Debug)] //~ WARN unused attribute
+    let _ = "Hello, world!";
+
+    // fold_expr
+    let _ = #[derive(Debug)] "Hello, world!";
+    //~^ WARN unused attribute
+
+    let _ = [
+        // fold_opt_expr
+        #[derive(Debug)] //~ WARN unused attribute
+        "Hello, world!"
+    ];
+}

src/test/ui/issue-49934.stderr

@@ -0,0 +1,50 @@
+warning: `#[derive]` does nothing on macro invocations
+  --> $DIR/issue-49934.rs:30:5
+   |
+LL |     #[derive(Debug)]
+   |     ^^^^^^^^^^^^^^^^
+   |
+   = note: this may become a hard error in a future release
+
+warning: unused attribute
+  --> $DIR/issue-49934.rs:16:8
+   |
+LL | fn foo<#[derive(Debug)] T>() { //~ WARN unused attribute
+   |        ^^^^^^^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/issue-49934.rs:14:9
+   |
+LL | #![warn(unused_attributes)] //~ NOTE lint level defined here
+   |         ^^^^^^^^^^^^^^^^^
+
+warning: unused attribute
+  --> $DIR/issue-49934.rs:18:9
+   |
+LL |         #[derive(Debug)] //~ WARN unused attribute
+   |         ^^^^^^^^^^^^^^^^
+
+warning: unused attribute
+  --> $DIR/issue-49934.rs:36:5
+   |
+LL |     #[derive(Debug)] //~ WARN unused attribute
+   |     ^^^^^^^^^^^^^^^^
+
+warning: unused attribute
+  --> $DIR/issue-49934.rs:40:5
+   |
+LL |     #[derive(Debug)] //~ WARN unused attribute
+   |     ^^^^^^^^^^^^^^^^
+
+warning: unused attribute
+  --> $DIR/issue-49934.rs:44:13
+   |
+LL |     let _ = #[derive(Debug)] "Hello, world!";
+   |             ^^^^^^^^^^^^^^^^
+
+warning: unused attribute
+  --> $DIR/issue-49934.rs:49:9
+   |
+LL |         #[derive(Debug)] //~ WARN unused attribute
+   |         ^^^^^^^^^^^^^^^^
+