Add feature trigger and correct is_struct check

src/test/ui/rfcs/rfc-2528-type-changing-struct-update/feature-gate.rs

@@ -17,10 +17,11 @@ fn update_to_state2() {
         common_field1: "hello",
         common_field2: 2,
     };
-    // FIXME: this should trigger feature gate
     let m2: Machine<State2> = Machine {
         state: State2,
-        ..m1 //~ ERROR mismatched types
+        ..m1
+        //~^ ERROR type changing struct updating is experimental [E0658]
+        //~| ERROR mismatched types [E0308]
     };
     assert_eq!(State2, m2.state);
 }

src/test/ui/rfcs/rfc-2528-type-changing-struct-update/feature-gate.stderr

@@ -1,5 +1,14 @@
+error[E0658]: type changing struct updating is experimental
+  --> $DIR/feature-gate.rs:22:11
+   |
+LL |         ..m1
+   |           ^^
+   |
+   = note: see issue #86555 <https://github.com/rust-lang/rust/issues/86555> for more information
+   = help: add `#![feature(type_changing_struct_update)]` to the crate attributes to enable
+
 error[E0308]: mismatched types
-  --> $DIR/feature-gate.rs:23:11
+  --> $DIR/feature-gate.rs:22:11
    |
 LL |         ..m1
    |           ^^ expected struct `State2`, found struct `State1`
@@ -7,6 +16,7 @@ LL |         ..m1
    = note: expected struct `Machine<State2>`
               found struct `Machine<State1>`
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0308`.
+Some errors have detailed explanations: E0308, E0658.
+For more information about an error, try `rustc --explain E0308`.

src/test/ui/rfcs/rfc-2528-type-changing-struct-update/type-generic-update.rs

@@ -50,7 +50,6 @@ fn fail_update() {
     let m3 = Machine::<i32, i32> {
         ..m1
         //~^ ERROR mismatched types [E0308]
-        //~| ERROR mismatched types [E0308]
     };
 }
 

src/test/ui/rfcs/rfc-2528-type-changing-struct-update/type-generic-update.stderr

@@ -16,15 +16,6 @@ LL |         ..m1
    = note: expected type `i32`
               found type `f64`
 
-error[E0308]: mismatched types
-  --> $DIR/type-generic-update.rs:51:11
-   |
-LL |         ..m1
-   |           ^^ field type mismatch: Machine.message
-   |
-   = note: expected type `i32`
-              found type `f64`
-
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0308`.