Struct Request

pub struct Request<'a>(/* private fields */);

🔬This is a nightly-only experimental API. (error_generic_member_access)

Request supports generic, type-driven access to data. Its use is currently restricted to the standard library in cases where trait authors wish to allow trait implementors to share generic information across trait boundaries. The motivating and prototypical use case is core::error::Error which would otherwise require a method per concrete type (eg. std::backtrace::Backtrace instance that implementors want to expose to users).

Data flow

To describe the intended data flow for Request objects, let’s consider two conceptual users separated by API boundaries:

• Consumer - the consumer requests objects using a Request instance; eg a crate that offers fancy Error/Result reporting to users wants to request a Backtrace from a given dyn Error.

• Producer - the producer provides objects when requested via Request; eg. a library with an an Error implementation that automatically captures backtraces at the time instances are created.

The consumer only needs to know where to submit their request and are expected to handle the request not being fulfilled by the use of Option<T> in the responses offered by the producer.

• A Producer initializes the value of one of its fields of a specific type. (or is otherwise prepared to generate a value requested). eg, backtrace::Backtrace or std::backtrace::Backtrace
• A Consumer requests an object of a specific type (say std::backtrace::Backtrace). In the case of a dyn Error trait object (the Producer), there are functions called request_ref and request_value to simplify obtaining an Option<T> for a given type.
• The Producer, when requested, populates the given Request object which is given as a mutable reference.
• The Consumer extracts a value or reference to t