capnproto-rust | Cap'n Proto for Rust

Creating a Minimal, Complete, and Verifiable example that reproduces the problem is a useful step:

Rust will not infer types in function parameter position. This is by design, as the Rust language FAQ states:

Why aren't function signatures inferred?

In Rust, declarations tend to come with explicit types, while actual code has its types inferred. There are several reasons for this design:

• Mandatory declaration signatures help enforce interface stability at both the module and crate level.

• Signatures improve code comprehension for the programmer, eliminating the need for an IDE running an inference algorithm across an entire crate to be able to guess at a function’s argument types; it’s always explicit and nearby.

• Mechanically, it simplifies the inference algorithm, as inference only requires looking at one function at a time.

Since capnp::message::Builder takes a type parameter A, you need to qualify the type of the parameter by giving A a value: