Auto merge of #9379 - royrustdev:multi_assignments, r=llogiq

new lint

This fixes #6576

If you added a new lint, here's a checklist for things that will be
checked during review or continuous integration.

- \[x] Followed [lint naming conventions][lint_naming]
- \[x] Added passing UI tests (including committed `.stderr` file)
- \[x] `cargo test` passes locally
- \[x] Executed `cargo dev update_lints`
- \[x] Added lint documentation
- \[x] Run `cargo dev fmt`

---

changelog: add [`multi_assignments`] lint

tests/ui/multi_assignments.rs

@@ -0,0 +1,9 @@
+#![warn(clippy::multi_assignments)]
+fn main() {
+    let (mut a, mut b, mut c, mut d) = ((), (), (), ());
+    a = b = c;
+    a = b = c = d;
+    a = b = { c };
+    a = { b = c };
+    a = (b = c);
+}

tests/ui/multi_assignments.stderr

@@ -0,0 +1,40 @@
+error: assignments don't nest intuitively
+  --> $DIR/multi_assignments.rs:4:5
+   |
+LL |     a = b = c;
+   |     ^^^^^^^^^
+   |
+   = note: `-D clippy::multi-assignments` implied by `-D warnings`
+
+error: assignments don't nest intuitively
+  --> $DIR/multi_assignments.rs:5:5
+   |
+LL |     a = b = c = d;
+   |     ^^^^^^^^^^^^^
+
+error: assignments don't nest intuitively
+  --> $DIR/multi_assignments.rs:5:9
+   |
+LL |     a = b = c = d;
+   |         ^^^^^^^^^
+
+error: assignments don't nest intuitively
+  --> $DIR/multi_assignments.rs:6:5
+   |
+LL |     a = b = { c };
+   |     ^^^^^^^^^^^^^
+
+error: assignments don't nest intuitively
+  --> $DIR/multi_assignments.rs:7:5
+   |
+LL |     a = { b = c };
+   |     ^^^^^^^^^^^^^
+
+error: assignments don't nest intuitively
+  --> $DIR/multi_assignments.rs:8:5
+   |
+LL |     a = (b = c);
+   |     ^^^^^^^^^^^
+
+error: aborting due to 6 previous errors
+