Add testcase for side-effectful reassign

Author: hegza

tests/ui/field_reassign_with_default.rs

@@ -11,6 +11,17 @@ struct B {
     j: i64,
 }
 
+/// Implements .next() that returns a different number each time.
+struct SideEffect(i32);
+
+impl SideEffect {
+    fn new() -> SideEffect { SideEffect(0) }
+    fn next(&mut self) -> i32 {
+        self.0 += 1;
+        self.0
+    }
+}
+
 fn main() {
     // wrong, produces first error in stderr
     let mut a: A = Default::default();
@@ -86,4 +97,11 @@ fn main() {
     let mut x = A::default();
     x.i = 42;
     x.j = 21 + x.i as i64;
+
+    // right, we bail out if there's a reassignment to the same variable, since there is a risk of side-effects affecting the outcome
+    let mut x = A::default();
+    let mut side_effect = SideEffect::new();
+    x.i = side_effect.next();
+    x.j = 2;
+    x.i = side_effect.next();
 }

tests/ui/field_reassign_with_default.stderr

@@ -1,72 +1,72 @@
 error: field assignment outside of initializer for an instance created with Default::default()
-  --> $DIR/field_reassign_with_default.rs:17:5
+  --> $DIR/field_reassign_with_default.rs:28:5
    |
 LL |     a.i = 42;
    |     ^^^^^^^^^
    |
    = note: `-D clippy::field-reassign-with-default` implied by `-D warnings`
 note: consider initializing the variable with `A { i: 42, ..Default::default() }` and removing relevant reassignments
-  --> $DIR/field_reassign_with_default.rs:16:5
+  --> $DIR/field_reassign_with_default.rs:27:5
    |
 LL |     let mut a: A = Default::default();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: field assignment outside of initializer for an instance created with Default::default()
-  --> $DIR/field_reassign_with_default.rs:57:5
+  --> $DIR/field_reassign_with_default.rs:68:5
    |
 LL |     a.j = 43;
    |     ^^^^^^^^^
    |
 note: consider initializing the variable with `A { j: 43, i: 42 }` and removing relevant reassignments
-  --> $DIR/field_reassign_with_default.rs:56:5
+  --> $DIR/field_reassign_with_default.rs:67:5
    |
 LL |     let mut a: A = Default::default();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: field assignment outside of initializer for an instance created with Default::default()
-  --> $DIR/field_reassign_with_default.rs:62:5
+  --> $DIR/field_reassign_with_default.rs:73:5
    |
 LL |     a.i = 42;
    |     ^^^^^^^^^
    |
 note: consider initializing the variable with `A { i: 42, j: 44 }` and removing relevant reassignments
-  --> $DIR/field_reassign_with_default.rs:61:5
+  --> $DIR/field_reassign_with_default.rs:72:5
    |
 LL |     let mut a: A = Default::default();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: field assignment outside of initializer for an instance created with Default::default()
-  --> $DIR/field_reassign_with_default.rs:68:5
+  --> $DIR/field_reassign_with_default.rs:79:5
    |
 LL |     a.i = 42;
    |     ^^^^^^^^^
    |
 note: consider initializing the variable with `A { i: 42, ..Default::default() }` and removing relevant reassignments
-  --> $DIR/field_reassign_with_default.rs:67:5
+  --> $DIR/field_reassign_with_default.rs:78:5
    |
 LL |     let mut a = A::default();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: field assignment outside of initializer for an instance created with Default::default()
-  --> $DIR/field_reassign_with_default.rs:78:5
+  --> $DIR/field_reassign_with_default.rs:89:5
    |
 LL |     a.i = Default::default();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^
    |
 note: consider initializing the variable with `A::default()` and removing relevant reassignments
-  --> $DIR/field_reassign_with_default.rs:77:5
+  --> $DIR/field_reassign_with_default.rs:88:5
    |
 LL |     let mut a: A = Default::default();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: field assignment outside of initializer for an instance created with Default::default()
-  --> $DIR/field_reassign_with_default.rs:82:5
+  --> $DIR/field_reassign_with_default.rs:93:5
    |
 LL |     a.i = Default::default();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^
    |
 note: consider initializing the variable with `A { j: 45, ..Default::default() }` and removing relevant reassignments
-  --> $DIR/field_reassign_with_default.rs:81:5
+  --> $DIR/field_reassign_with_default.rs:92:5
    |
 LL |     let mut a: A = Default::default();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^