tests: Check stderr when trying to mutate inside Option::inspect()

Author: Enselic

tests/ui/borrowck/option-inspect-mutation.rs

@@ -0,0 +1,18 @@
+//! Regression test for <https://github.com/rust-lang/rust/issues/128381>.
+
+struct Struct {
+    field: u32,
+}
+
+fn main() {
+    let mut some_struct = Some(Struct { field: 42 });
+    some_struct.as_mut().inspect(|some_struct| {
+        some_struct.field *= 10; //~ ERROR cannot assign to `some_struct.field`, which is behind a `&` reference
+        // Users can't change type of `some_struct` param, so above error must not suggest it.
+    });
+
+    // Same check as above but using `hir::ExprKind::Call` instead of `hir::ExprKind::MethodCall`.
+    Option::inspect(some_struct.as_mut(), |some_struct| {
+        some_struct.field *= 20; //~ ERROR cannot assign to `some_struct.field`, which is behind a `&` reference
+    });
+}

tests/ui/borrowck/option-inspect-mutation.stderr

@@ -0,0 +1,19 @@
+error[E0594]: cannot assign to `some_struct.field`, which is behind a `&` reference
+  --> $DIR/option-inspect-mutation.rs:10:9
+   |
+LL |     some_struct.as_mut().inspect(|some_struct| {
+   |                                   ----------- consider changing this binding's type to be: `&mut &mut Struct`
+LL |         some_struct.field *= 10;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^ `some_struct` is a `&` reference, so it cannot be written to
+
+error[E0594]: cannot assign to `some_struct.field`, which is behind a `&` reference
+  --> $DIR/option-inspect-mutation.rs:16:9
+   |
+LL |     Option::inspect(some_struct.as_mut(), |some_struct| {
+   |                                            ----------- consider changing this binding's type to be: `&mut &mut Struct`
+LL |         some_struct.field *= 20;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^ `some_struct` is a `&` reference, so it cannot be written to
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0594`.