use ::seal_the_deal::with_seals;

#[with_seals]
pub trait SomeTrait {
    /// Shall always return `42`.
    #[sealed]
    fn some_method(&self) -> i32 {
        42
    }
}

expands to:

mod __SomeTraitඞseal_the_deal {
    pub trait Seal<'__> {}
    impl Seal<'_> for () {}
}

pub trait SomeTrait {
    /// Shall always return `42`.
    fn some_method<'sealed>(&self) -> i32
    where
        () : __SomeTraitඞseal_the_deal::Seal<'sealed>,
    {
        42
    }
}

This approach does effectively and nicely seal that method, preventing it from being overridden:

• since the clause is trivially implemented for for at least one lifetime (in practice all of them), callers are not hindered by it.

  • underspecified lifetime params are fine, they do not cause "inference ambiguity errors";
  • this clause is dyn-compatible!
  • since the method stays in the actual trait, it remains visible in the docs;
    • a () : Sealed<'seal> will be visible, which is rather low-noise w.r.t. the semantics involved.

• and yet the clause appears to be complex/convoluted enough for Rust not to allow skipping it, hence the "lifetime mismatch" when people attempt to do actual impls.

• technically-speaking, it is possible for a same-module or submodule-thereof to have enough visibility of __SomeTraitඞseal_the_deal::Sealed to be able to repeat, verbatim, the clause. This ought to be fine since:

  • there is a ඞ in that path!!

  • same-crate code is not really the threat/adversarial model, here, but rather, external code (be it for Semver or unsafety reasons).

  • if this were really deemed a problem, the user could then just further encapsulate the whole thing in a helper private module:

    pub use paranoid::SomeTrait;
    mod paranoid {
        use super::*;
    
        #[::seal_the_deal::with_seals]
        pub trait SomeTrait {
            /// Shall always return `42`.
            #[sealed]
            fn some_method(&self) -> i32 {
                42
            }
        }
    }
    

The usual approach to have a sealed/final method like that is through a blanket-implemented super trait (a.k.a, the "super extension trait").

trait Trait : FinalMethodsOfTrait {
    /* non-final methods here */
}

trait FinalMethodsOfTrait {
    /// Shall always return `42`.
    fn method(&self) -> i32 {
        42
    }
}

impl<T : ?Sized + Trait> FinalMethodsOfTrait for T {}

But this approach has two problems:

• documentation-wise, it is ugly: the final .method() is no longer discoverable on the page for Trait.

• Whilst both properly-Trait-bounded generic parameters and dyn Traits do let one very ergonomically call .method() in a blissfully oblivious-to-the-super-extension-trait way, it turns out that concrete implementors do not let one perform such ergonomic calls directly: the super extension trait is expected to be in scope for the method call to succeed, totally shattering, imho, the illusion and magic of the pattern.

  mod lib {
      pub trait Trait : FinalMethodsOfTrait {
          /* non-final methods here */
      }
  
      pub trait FinalMethodsOfTrait {
          /// Shall always return `42`.
          fn method(&self) -> i32 {
              42
          }
      }
  
      impl<T : ?Sized + Trait> FinalMethodsOfTrait for T {}
  }
  
  use lib::Trait;
  
  struct Foo;
  
  impl Trait for Foo {}
  
  Foo.method(); // Error, `FinalMethodsOfTrait` not in scope! 😭
  

  Error message:

  # /*
  error[E0599]: no method named `method` found for struct `Foo` in the current scope
    --> src/_lib.rs:136:5
     |
  10 |         fn method(&self) -> i32 {
     |            ------ the method is available for `Foo` here
  ...
  20 | struct Foo;
     | ---------- method `method` not found for this struct
  ...
  24 | Foo.method(); // Error, `FinalMethodsOfTrait` not in scope! 😭
     |     ^^^^^^ method not found in `Foo`
     |
     = help: items from traits can only be used if the trait is in scope
  help: trait `FinalMethodsOfTrait` which provides `method` is implemented but not in scope; perhaps you want to import it
     |
  2  + use lib::FinalMethodsOfTrait;
     |
  # */