Lint path statements to use drop for drop types

src/test/ui/warn-path-statement.rs

@@ -1,6 +1,17 @@
 // compile-flags: -D path-statements
-fn main() {
+struct Droppy;
 
+impl Drop for Droppy {
+    fn drop(&mut self) {}
+}
+
+fn main() {
     let x = 10;
     x; //~ ERROR path statement with no effect
+
+    let y = Droppy;
+    y; //~ ERROR path statement drops value
+
+    let z = (Droppy,);
+    z; //~ ERROR path statement drops value
 }

src/test/ui/warn-path-statement.stderr

@@ -1,10 +1,22 @@
 error: path statement with no effect
-  --> $DIR/warn-path-statement.rs:5:5
+  --> $DIR/warn-path-statement.rs:10:5
    |
 LL |     x;
    |     ^^
    |
    = note: requested on the command line with `-D path-statements`
 
-error: aborting due to previous error
+error: path statement drops value
+  --> $DIR/warn-path-statement.rs:13:5
+   |
+LL |     y;
+   |     ^^ help: use `drop` to clarify the intent: `drop(y);`
+
+error: path statement drops value
+  --> $DIR/warn-path-statement.rs:16:5
+   |
+LL |     z;
+   |     ^^ help: use `drop` to clarify the intent: `drop(z);`
+
+error: aborting due to 3 previous errors