address review comments

tests/ui/missing_const_for_fn/could_be_const.fixed

@@ -104,15 +104,18 @@ fn main() {}
 
 struct D;
 
+/* FIXME(effects)
 impl const Drop for D {
     fn drop(&mut self) {
         todo!();
     }
 }
+*/
 
 // Lint this, since it can be dropped in const contexts
 // FIXME(effects)
-fn d(this: D) {}
+const fn d(this: D) {}
+//~^ ERROR: this could be a `const fn`
 
 mod msrv {
     struct Foo(*const u8, &'static u8);

tests/ui/missing_const_for_fn/could_be_const.stderr

@@ -161,6 +161,11 @@ error: this could be a `const fn`
    |
 LL | fn d(this: D) {}
    | ^^^^^^^^^^^^^^^^
+   |
+help: make the function `const`
+   |
+LL | const fn d(this: D) {}
+   | +++++
 
 error: this could be a `const fn`
   --> tests/ui/missing_const_for_fn/could_be_const.rs:125:9