RFC 2302: tuple-struct-self-ctor

lang (syntax | patterns | expressions | resolve)

Summary

Tuple structs can now be constructed and pattern matched with Self(v1, v2, ..). A simple example:

struct TheAnswer(usize);

impl Default for TheAnswer {
    fn default() -> Self { Self(42) }
}

Similarly, unit structs can also be constructed and pattern matched with Self.

Motivation

This RFC proposes a consistency fix allowing Self to be used in more places to better match the users' intuition of the language and to get closer to feature parity between tuple structs and structs with named fields.

Currently, only structs with named fields can be constructed inside impls using Self like so:

struct Mascot { name: String, age: usize }

impl Default for Mascot {
    fn default() -> Self {
        Self {
            name: "Ferris the Crab".into(),
            age: 3
        }
    }
}

while the following is not allowed:

struct Mascot(String, usize);

impl Default for Mascot {
    fn default() -> Self {
        Self("Ferris the Crab".into(), 3)
    }
}

This discrepancy is unfortunate as many users reach for Self(v0, v1, ..) from time to time, only to find that it doesn't work. This creates a break in the users intuition and becomes a papercut. It also has the effect that each user must remember this exception, making the rule-set to remember larger wherefore the language becomes more complex.

There are good reasons why Self { f0: v0, f1: v1, .. } is allowed. Chiefly among those is that it becomes easier to refactor the code when one wants to rename type names. Another important reason is that only having to keep Self in mind means that a developer does not need to keep the type name fresh in their working memory. This is beneficial for users with shorter working memory such as the author of this RFC.

Since Self { f0: v0, .. } is well motivated, those benefits and motivations will also extend to tuple and unit structs. Eliminating this discrepancy between tuple structs and those with named fields will therefore have all the benefits associated with this feature for structs with named fields.

Guide-level explanation

Basic concept

For structs with named fields such as:

struct Person {
    name: String,
    ssn: usize,
    age: usize
}

You may use the syntax Self { field0: value0, .. } as seen below instead of writing TypeName { field0: value0, .. }:

impl Person {
    /// Make a newborn person.
    fn newborn(name: String, ssn: usize) -> Self {
        Self { name, ssn, age: 0 }
    }
}

Through type aliases

This ability does not extend to tuple structs however in current Rust but will with this RFC. To continue on with the previous example, you can now also write:

struct Person(String, usize, usize);

impl Person {
    /// Make a newborn person.
    fn newborn(name: String, ssn: usize) -> Self {
        Self(name, ssn, 0)
    }
}

As with structs with named fields, you may also use Self when you are impling on a type alias of a struct as seen here:

struct FooBar(u8);

type BarFoo = FooBar;

impl Default for BarFoo {
    fn default() -> Self {
        Self(42) // <-- Not allowed before.
    }
}

Patterns

Currently, you can pattern match using Self { .. } on a named struct as in the following example:

struct Person {
    ssn: usize,
    age: usize
}

impl Person {
    /// Make a newborn person.
    fn newborn(ssn: usize) -> Self {
        match { Self { ssn, age: 0 } } {
            Self { ssn, age } // `Self { .. }` is permitted as a pattern!
                => Self { ssn, age }
        }
    }
}

This RFC extends this to tuple structs:

struct Person(usize, usize);

impl Person {
    /// Make a newborn person.
    fn newborn(ssn: usize) -> Self {
        match { Self(ssn, 0) } {
            Self(ssn, age) // `Self(..)` is permitted as a pattern!
                => Self(ssn, age)
        }
    }
}

Of course, this redundant reconstruction is not recommended in actual code, but illustrates what you can do.

Self as a function pointer

When you define a tuple struct today such as:

struct Foo<T>(T);

impl<T> Foo<T> {
    fn fooify_iter(iter: impl Iterator<Item = T>) -> impl Iterator<Item = Foo<T>> {
        iter.map(Foo)
    }
}

you can use Foo as a function pointer typed at: for<T> fn(T) -> T as seen in the example above.

This RFC extends that such that Self can also be used as a function pointer for tuple structs. Modifying the example above gives us:

impl<T> Foo<T> {
    fn fooify_iter(iter: impl Iterator<Item = T>) -> impl Iterator<Item = Foo<T>> {
        iter.map(Self)
    }
}

Unit structs

With this RFC, you can also use Self in pattern and expression contexts when dealing with unit structs. For example:

struct TheAnswer;

impl Default for TheAnswer {
    fn default() -> Self {
        match { Self } { Self => Self }
    }
}

Teaching the contents

This RFC should not require additional effort other than spreading the news that this now is possible as well as the reference. The changes are seen as intuitive enough that it supports what the user already assumes should work and will probably try at some point.

Reference-level explanation

When entering one of the following contexts, a Rust compiler will extend the value namespace with Self which maps to the tuple constructor fn in the case of tuple struct, or a constant, in the case of a unit struct:

As a result, when referring to a tuple struct, Self can be legally coerced into an fn pointer which accepts and returns expressions of the same type as the function pointer Self is referring to accepts.

Another consequence is that Self(p_0, .., p_n) and Self become legal patterns. This works since TupleCtor(p_0, .., p_n) patterns are handled by resolving them in the value namespace and checking that they resolve to a tuple constructor. Since by definition, Self referring to a tuple struct resolves to a tuple constructor, this is OK.

Implementation notes

As an additional check on the sanity of a Rust compiler implementation, a well formed expression Self(v0, v1, ..), must be semantically equivalent to Self { 0: v0, 1: v1, .. } and must also be permitted when the latter would. Likewise the pattern Self(p0, p1, ..) must match exactly the same set of values as Self { 0: p0, 1: p1, .. } would and must be permitted when Self { 0: p0, 1: p1, .. } is well formed.

Furthermore, a well formed expression or pattern Self must be semantically equivalent to Self {} and permitted when Self {} is well formed in the same context.

For example for tuple structs, we have the typing rule:

Δ ⊢ τ_0  type .. Δ ⊢ τ_n  type
Δ ⊢ Self type
Γ ⊢ x_0 : τ_0 .. Γ ⊢ x_n : τ_n
Γ ⊢ Self { 0: x_0, .. n: x_n } : Self
-----------------------------------------
Γ ⊢ Self (    x_0, ..,   x_n ) : Self

and the operational semantics:

Γ ⊢ Self { 0: e_0, .., n: e_n } ⇓ v
-------------------------------------
Γ ⊢ Self {    e_0, ..,    e_n } ⇓ v

for unit structs, the following holds:

Δ ⊢ Self type
Γ ⊢ Self {} : Self
-----------------------------------------
Γ ⊢ Self    : Self

with the operational semantics:

Γ ⊢ Self {} ⇓ v
-------------------------------------
Γ ⊢ Self    ⇓ v

In relation to other RFCs

This RFC expands on RFC 593 and RFC 1647 with respect to where the keyword Self is allowed.

Drawbacks

There are potentially some, but the author could not think of any.

Rationale and alternatives

This is the only design that makes sense in the sense that there really aren't any other. Potentially, Self(v0, ..) should only work when the impled type is not behind a type alias. However, since structs with named fields supports type aliases in this respect, so should tuple structs.

Not providing this feature would preserve papercuts and unintuitive surprises for developers.

Unresolved questions

There are none.

Copyright 2022 Nick Cameron and the RFC authors.