CEP-0038: Expected-Type Dyn Conversions¶

Summary¶

Catalyst should eventually decide whether owned dynamic conversions can infer their target contract from an expected type.

V1 spells destination contracts explicitly for into_dyn(C) and upcast(C).

Example¶

Possible future shape:

fn make_drawable(shape: Circle, allocator: *std.mem.Allocator) Box(dyn Draw) {
  return shape.into_dyn(allocator)
}

V1 requires the erased target to remain explicit:

return shape.into_dyn(Draw, allocator)

Motivation¶

Expected-type inference can reduce repetition in return statements, assignments, and argument positions. It can also hide important erased-surface decisions, especially when multiple dyn-safe targets are plausible.

Proposed Direction¶

The proposal should cover:

• expected-type sources that may infer C;
• ambiguity diagnostics;
• interaction with overload resolution and generic parameter inference;
• whether inference applies to borrowed dyn pointer coercions, owned Box conversions, or both;
• display of inferred erased surfaces in diagnostics.

V1 Compatibility¶

V1 keeps into_dyn(C) and upcast(C) explicit. Boxed dyn upcasts remain explicit consuming operations.

Related Details¶

• Box Resource
• Expected-Type Shorthand
• CEP-0016: Generic Parameter Inference