string_add, string_add_assign: split tests, make one rustfixable

tests/ui/string_add.rs

@@ -0,0 +1,19 @@
+#[warn(clippy::string_add)]
+#[allow(clippy::string_add_assign, unused)]
+fn main() {
+    // ignores assignment distinction
+    let mut x = "".to_owned();
+
+    for _ in 1..3 {
+        x = x + ".";
+    }
+
+    let y = "".to_owned();
+    let z = y + "...";
+
+    assert_eq!(&x, &z);
+
+    let mut x = 1;
+    x = x + 1;
+    assert_eq!(2, x);
+}

tests/ui/string_add.stderr

@@ -0,0 +1,30 @@
+error: manual implementation of an assign operation
+  --> $DIR/string_add.rs:8:9
+   |
+LL |         x = x + ".";
+   |         ^^^^^^^^^^^ help: replace it with: `x += "."`
+   |
+   = note: `-D clippy::assign-op-pattern` implied by `-D warnings`
+
+error: you added something to a string. Consider using `String::push_str()` instead
+  --> $DIR/string_add.rs:8:13
+   |
+LL |         x = x + ".";
+   |             ^^^^^^^
+   |
+   = note: `-D clippy::string-add` implied by `-D warnings`
+
+error: you added something to a string. Consider using `String::push_str()` instead
+  --> $DIR/string_add.rs:12:13
+   |
+LL |     let z = y + "...";
+   |             ^^^^^^^^^
+
+error: manual implementation of an assign operation
+  --> $DIR/string_add.rs:17:5
+   |
+LL |     x = x + 1;
+   |     ^^^^^^^^^ help: replace it with: `x += 1`
+
+error: aborting due to 4 previous errors
+

tests/ui/string_add_assign.fixed

@@ -0,0 +1,21 @@
+// run-rustfix
+
+#[allow(clippy::string_add, unused)]
+#[warn(clippy::string_add_assign)]
+fn main() {
+    // ignores assignment distinction
+    let mut x = "".to_owned();
+
+    for _ in 1..3 {
+        x += ".";
+    }
+
+    let y = "".to_owned();
+    let z = y + "...";
+
+    assert_eq!(&x, &z);
+
+    let mut x = 1;
+    x += 1;
+    assert_eq!(2, x);
+}

tests/ui/string_add_assign.rs

@@ -0,0 +1,21 @@
+// run-rustfix
+
+#[allow(clippy::string_add, unused)]
+#[warn(clippy::string_add_assign)]
+fn main() {
+    // ignores assignment distinction
+    let mut x = "".to_owned();
+
+    for _ in 1..3 {
+        x = x + ".";
+    }
+
+    let y = "".to_owned();
+    let z = y + "...";
+
+    assert_eq!(&x, &z);
+
+    let mut x = 1;
+    x = x + 1;
+    assert_eq!(2, x);
+}

tests/ui/string_add_assign.stderr

@@ -0,0 +1,24 @@
+error: you assigned the result of adding something to this string. Consider using `String::push_str()` instead
+  --> $DIR/string_add_assign.rs:10:9
+   |
+LL |         x = x + ".";
+   |         ^^^^^^^^^^^
+   |
+   = note: `-D clippy::string-add-assign` implied by `-D warnings`
+
+error: manual implementation of an assign operation
+  --> $DIR/string_add_assign.rs:10:9
+   |
+LL |         x = x + ".";
+   |         ^^^^^^^^^^^ help: replace it with: `x += "."`
+   |
+   = note: `-D clippy::assign-op-pattern` implied by `-D warnings`
+
+error: manual implementation of an assign operation
+  --> $DIR/string_add_assign.rs:19:5
+   |
+LL |     x = x + 1;
+   |     ^^^^^^^^^ help: replace it with: `x += 1`
+
+error: aborting due to 3 previous errors
+