Add basic test

Author: LeSeulArtichaut

src/test/ui/structs/struct-fn-in-definition.rs

@@ -0,0 +1,14 @@
+// It might be intuitive for a user coming from languages like Java
+// to declare a method directly in a struct's definition. Make sure
+// rustc can give a helpful suggestion.
+// Suggested in issue #76421
+
+struct S {
+    field: usize,
+    fn do_something() {}
+    //~^ ERROR functions are not allowed in struct definitions
+    //~| HELP unlike in C++, Java, and C#, functions are declared in `impl` blocks
+    //~| HELP see https://doc.rust-lang.org/book/ch05-03-method-syntax.html for more information
+}
+
+fn main() {}

src/test/ui/structs/struct-fn-in-definition.stderr

@@ -0,0 +1,11 @@
+error: functions are not allowed in struct definitions
+  --> $DIR/struct-fn-in-definition.rs:5:5
+   |
+LL |     fn do_something() {}
+   |     ^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: unlike in C++, Java, and C#, functions are declared in `impl` blocks
+   = help: see https://doc.rust-lang.org/book/ch05-03-method-syntax.html for more information
+
+error: aborting due to previous error
+