Auto merge of #3518 - sinkuu:redundant_clone_tw, r=phansch

Lint redundant clone of fields

Makes `redundant_clone` warn on unnecessary `foo.field.clone()` sometimes (it can detect an unnecessary clone only if the base of projection, `foo` in this case, is not used at all after that). This is enough for cases like `returns_tuple().0.clone()`.

tests/ui/redundant_clone.rs

@@ -34,14 +34,35 @@ fn main() {
 
     // Check that lint level works
     #[allow(clippy::redundant_clone)] let _ = String::new().to_string();
+
+    let tup = (String::from("foo"),);
+    let _ = tup.0.clone();
+
+    let tup_ref = &(String::from("foo"),);
+    let _s = tup_ref.0.clone(); // this `.clone()` cannot be removed
 }
 
 #[derive(Clone)]
 struct Alpha;
-fn double(a: Alpha) -> (Alpha, Alpha) {
-    if true {
+fn with_branch(a: Alpha, b: bool) -> (Alpha, Alpha) {
+    if b {
         (a.clone(), a.clone())
     } else {
         (Alpha, a)
     }
 }
+
+struct TypeWithDrop {
+    x: String,
+}
+
+impl Drop for TypeWithDrop {
+    fn drop(&mut self) {}
+}
+
+fn cannot_move_from_type_with_drop() -> String {
+    let s = TypeWithDrop {
+        x: String::new()
+    };
+    s.x.clone() // removing this `clone()` summons E0509
+}

tests/ui/redundant_clone.stderr

@@ -96,16 +96,28 @@ note: this value is dropped without further use
    |             ^^^^^^^^^^^^^^^
 
 error: redundant clone
-  --> $DIR/redundant_clone.rs:43:22
+  --> $DIR/redundant_clone.rs:39:18
    |
-43 |         (a.clone(), a.clone())
+39 |     let _ = tup.0.clone();
+   |                  ^^^^^^^^ help: remove this
+   |
+note: this value is dropped without further use
+  --> $DIR/redundant_clone.rs:39:13
+   |
+39 |     let _ = tup.0.clone();
+   |             ^^^^^
+
+error: redundant clone
+  --> $DIR/redundant_clone.rs:49:22
+   |
+49 |         (a.clone(), a.clone())
    |                      ^^^^^^^^ help: remove this
    |
 note: this value is dropped without further use
-  --> $DIR/redundant_clone.rs:43:21
+  --> $DIR/redundant_clone.rs:49:21
    |
-43 |         (a.clone(), a.clone())
+49 |         (a.clone(), a.clone())
    |                     ^
 
-error: aborting due to 9 previous errors
+error: aborting due to 10 previous errors