Decision on "must define before use" for opaque types · Issue #117866 · rust-lang/rust (original) (raw)

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Description

Nominating for T-lang to decide whether the following rule should hold for opaque types:

If the body of an item that may define the hidden type of some opaque does define that hidden type, it must do so syntactically before using the opaque type in a non-defining way.

This restriction is called "must define before use."

Consider:

use core::convert::identity;

struct I; struct IShow; impl I { fn show(&self) -> IShow { IShow } }

struct OnIShow; trait OnI { fn show(&self) -> OnIShow { OnIShow } } impl OnI for I {}

fn test(n: bool) -> impl OnI { let true = n else { loop {} }; let x = test(!n); //~ NOTE this is the opaque type let _: OnIShow = x.show(); //~ NOTE this is a non-defining use //^ ERROR if the body registers a hidden type for the opaque, it // must do so before using it opaquely let _: IShow = identity::(x).show(); //^ NOTE this registers a hidden type for the opaque, but does so // too late loop {} }

fn main() {}

Because the code above works today, this would be a breaking change for RPIT.