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Feature Name: safe_unsafe_trait_methods
Start Date: 2018-01-30
RFC PR: rust-lang/rfcs#2316
Rust Issue: rust-lang/rust#87919

Summary

This RFC allows safe implementations of unsafe trait methods. An impl may implement a trait with methods marked as unsafe without marking the methods in the impl as unsafe. This is referred to as overconstraining the method in the impl. When the trait's unsafe method is called on a specific type where the method is known to be safe, that call does not require an unsafe block.

Motivation

A trait which includes unsafe methods in its definition permits its impls to define methods as unsafe. Safe methods may use unsafe { .. } blocks inside them and so both safe and unsafe methods may use unsafe code internally.

The key difference between safe and unsafe methods is the same as that between safe and unsafe functions. Namely, that calling a safe method with inputs and state produced by other safe methods never leads to memory unsafety, while calling a method marked as unsafe may lead to such unsafety. As such, it is up to the caller of the unsafe method to fulfill a set of invariants as defined by the trait's documentation (the contract).

The safe parts of Rust constitute a language which is a subset of unsafe Rust. As such, it is always permissible to use the safe subset within unsafe contexts. This is currently however not fully recognized by the language as unsafe trait methods must be marked as unsafe in impls even if the method bodies in such an impl uses no unsafe code. This is can currently be overcome by defining a safe free function or inherent method somewhere else and then simply delegate to that function or method. Such a solution, however, has two problems.

1. Needless complexity and poor ergonomics.

When an unsafe method doesn't rely on any unsafe invariants, it still must be marked unsafe. Marking methods as unsafe increases the amount of scrutiny necessary during code-review. Extra care must be given to ensure that uses of the function are correct. Additionally, usage of unsafe functions inside an unsafe method does not require an unsafe block, so the method implementation itself requires extra scrutiny.

One way to avoid this is to break out the internals of the method into a separate safe function. Creating a separate function which is only used at a single place is cumbersome, and does not encourage the keeping of unsafe to a minimum. The edit distance is also somewhat increased.

2. `unsafe` method `impl`s might not require any `unsafe` invariants

The implemented trait method for that specific type, which you know only has a safe implementation and does not really need unsafe, can't be used in a safe context. This invites the use of an unsafe { .. } block in that context, which is unfortunate since the compiler could know that the method is really safe for that specific type.

In summation

The changes proposed in this RFC are intended to increase ergonomics and encourage keeping unsafe to a minimum. By doing so, a small push in favor of correctness is made.

Guide-level explanation

Concretely, this RFC will permit scenarios like the following:

Overconstraining

First consider a trait with one or more unsafe methods. For simplicity, we consider the case with one method as in:

trait Foo {
    unsafe fn foo_computation(&self) -> u8;
}

You now define a type:

struct Bar;

and you implement Foo for Bar like so:

impl Foo for Bar {
    // unsafe <-- Not necessary anymore.
    fn foo_computation(&self) -> u8 { 0 }
}

Before this RFC, you would get the following error message:

error[E0053]: method `foo_computation` has an incompatible type for trait
  --> src/main.rs:11:5
   |
4  |     unsafe fn foo_computation(&self) -> u8;
   |     --------------------------------------- type in trait
...
11 |     fn foo_computation(&self) -> u8 { 0 }
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected unsafe fn, found normal fn
   |
   = note: expected type `unsafe fn(&Bar) -> u8`
              found type `fn(&Bar) -> u8`

But with this RFC implemented, you will no longer get an error in this case.

This general approach of giving up (restricting) capabilities that a trait provides to you, such as the ability to rely on caller-upheld invariants for memory safety, is known as overconstraining.

Taking advantage of overconstraining

You now want to use .foo_computation() for Bar, and proceed to do so as in:

fn main() {
    // unsafe { <-- no unsafe needed!

    let bar = Bar;
    let val = bar.foo_computation();

    // other stuff..

    // }
}

This is permitted since although foo_computation is an unsafe method as specified by Foo, the compiler knows that for the specific concrete type Bar, it is defined as being safe, and may thus be called within a safe context.

Regarding API stability and breaking changes

Note however, that the ability to call overconstrained methods with the absence of unsafe in a safe context means that introducing unsafe later is a breaking change if the type is part of a public API.

Impls for generic types

Consider the type Result<T, E> in the standard library defined as:

pub enum Result<T, E> {
    Ok(T),
    Err(E),
}

Let's now implement Foo for Result<T, E> without using unsafe:

impl<T, E> Foo for Result<T, E> {
    fn foo_computation(&self) -> u8 {
        // Let's assume the implementation does something interesting..
        match *self {
            Ok(_) => 0,
            Err(_) => 1,
        }
    }
}

Since Result<T, E> did not use unsafe in its implementation of Foo, you can still use my_result.foo_computation() in a safe context as shown above.

Recommendations

If you do not plan on introducing unsafe for a trait implementation of your specific type that is part of a public API, you should avoid marking the fn as unsafe. If the type is internal to your crate, you should henceforth never mark it as unsafe unless you need to. If your needs change later, you can always mark impls for internal types as unsafe then.

Tools such as clippy should preferably lint for use of unsafe, where it is not needed, to promote the reduction of needless unsafe.

Reference-level explanation

Assuming a trait which defines some fns marked as unsafe, an impl of that trait for a given type may elect to not mark those fns as unsafe in which case the bodies of those fns in that impl are type checked as safe and not as unsafe. A Rust compiler will keep track of whether the methods were implemented as safe or unsafe.

When a trait method is called for a type in a safe context, the type checker will resolve the impl for a specific known and concrete type. If the impl that was resolved implemented the called method without an unsafe marker, the compiler will permit the call. Otherwise, the compiler will emit an error since it can't guarantee that the implementation was marked as safe.

With respect to a trait bound on a type parameter T: Trait for a trait with unsafe methods, calling any method of Trait marked as unsafe for T is only permitted within an unsafe context such as an unsafe fn or within an unsafe { .. } block.

Drawbacks

While this introduces no additional syntax, it makes the rule-set of the language a bit more complex for both the compiler and the for users of the language. The largest additional complexity is probably for the compiler in this case, as additional state needs to be kept to check if the method was marked as safe or unsafe for an impl.

Rationale and alternatives

This RFC was designed with the goal of keeping the language compatible with potential future effects-polymorphism features. In particular, the discussion and design of RFC 2237 was considered. No issues were found with respect to that RFC.

No other alternatives have been considered. There is always the obvious alternative of not implementing the changes proposed in any RFC. For this RFC, the impact of not accepting it would be too keep the problems as explained in the motivation around.

Unresolved questions

There are currently no unresolved questions.

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