Patterns of empty types

Motivation

The story about pattern-matching is incomplete with regards to empty types: users sometimes have to write unreachable!() for cases they know to be impossible. This is a papercut that we can solve, and would make for a more consistent pattern-matching story.

This is particularly salient as the never type ! is planned to be stabilized in edition 2024.

The status quo

Empty types are used commonly to indicate cases that can't happen, e.g. in error-generic interfaces:

#![allow(unused)]
fn main() {
impl TryFrom<X> for Y {
    type Error = Infallible;
    ...
}
// or
impl SomeAST {
    pub fn map_nodes<E>(self, f: impl FnMut(Node) -> Result<Node, E>) -> Result<Self, E> { ... }
}
// used in the infallible case like:
let Ok(new_ast) = ast.map_nodes::<!>(|node| node) else { unreachable!() };
// or:
let new_ast = match ast.map_nodes::<!>(|node| node) {
    Ok(new_ast) => new_ast,
    Err(never) => match never {},
}
}

and conditional compilation:

#![allow(unused)]
fn main() {
pub struct PoisonError<T> {
    guard: T,
    #[cfg(not(panic = "unwind"))]
    _never: !,
}
pub enum TryLockError<T> {
    Poisoned(PoisonError<T>),
    WouldBlock,
}
}

For the most part, pattern-matching today treats empty types as if they were non-empty. E.g. in the above example, both the else { unreachable!() } above and the Err branch are required.

The unstable exhaustive_patterns allows all patterns of empty type to be omitted. It has never been stabilized because it goes against design axiom n.1 "Pattern semantics are predictable" when interacting to possibly-uninitialized data.

The next six months

The first step is already about to complete: the min_exhaustive_patterns feature is in FCP and about to be stabilized. This covers a large number of use-cases.

After min_exhaustive_patterns, there remains the case of empty types behind references, pointers and union fields. The current proposal for these is never_patterns; next steps are to submit the RFC and then finish the implementation according to the RFC outcome.

The "shiny future" we are working towards

The ideal endpoint is that users never have code to handle a pattern of empty type.

Design axioms

• Pattern semantics are predictable: users should be able to tell what data a pattern touches by looking at it. This is crucial when matching on partially-initialized data.
• Impossible cases can be omitted: users shouldn't have to write code for cases that are statically impossible.

Ownership and other resources

Owner: Nadrieril

I (Nadrieril) am putting forward my own contribution for driving this forward, both on the RFC and implementation sides. I am an experienced compiler contributor and have been driving this forward already for several months.

Support needed from the project

• I expect to be authoring one RFC, on never patterns (unless it gets rejected and we need a different approach).
  • The feature may require one design meeting.
• Implementation work is 80% done, which leaves about 80% more to do. This will require reviews from the compiler team, but not more than the ordinary.

Outputs and milestones

Outputs

• Stabilized language features that make pattern-matching on empty types more convenient;
• Deprecation of the unstable exhaustive_patterns feature.

Milestones

Frequently asked questions