Add a fn main() {} to a doctest to prevent the test from being wrapped in a fn main() {} body

compiler/rustc_error_codes/src/error_codes/E0792.md

@@ -39,6 +39,8 @@ type Foo<T> = impl std::fmt::Debug;
 fn foo<U>() -> Foo<U> {
     5u32
 }
+
+fn main() {}
 ```
 
 This means that no matter the generic parameter to `foo`,
@@ -57,4 +59,6 @@ type Foo<T: Debug> = impl Debug;
 fn foo<U: Debug>() -> Foo<U> {
     Vec::<U>::new()
 }
+
+fn main() {}
 ```

compiler/rustc_lint/src/opaque_hidden_inferred_bound.rs

@@ -46,6 +46,8 @@
     /// fn test() -> impl Trait<Assoc = Tait> {
     ///     42
     /// }
+    ///
+    /// fn main() {}
     /// ```
     ///
     /// {{produces}}