Field accessor traits,and emulation of structural types

12 releases (4 breaking)

new 0.4.3 Aug 1, 2020
0.4.2 Jul 30, 2020
0.4.1 May 16, 2020
0.3.0-alpha Mar 25, 2020
0.2.1 Nov 28, 2019

#124 in Rust patterns

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This library provides field accessor traits,and emulation of structural types.


These are some of the features this library provides:


For examples you can look at the examples section of the documentation for the root module of the structural crate


The way that this library emulates structural types is by using traits as bounds or trait objects.

All the structural traits are dyn-compatible(also known as object-safe), and no change will be made to make them not dyn-compatible.

By default all structural types are open, structs and enums can have more variants and or fields than are required.
The only exception to this is exhaustive enums, in which the variant count and names must match exactly, this is useful for exhaustive matching of variants (in the switch macro).

Every trait with the _SI/_ESI/_VSI suffixes in the examples are traits generated by the Structural derive macro. These traits alias the accessor traits implemented by the type they're named after.

Required macros

The only macros that are required to use this crate are the ones for TStr, every other macro expands to code that can be written manually (except for the __TS type, that is an implementation detail that only macros from this crate should use by name).


The changelog is in the "Changelog.md" file.

Future plans

Making the FromStructural and TryFromStructural traits derivable.

no-std support

To use structural in no_std contexts disable the default-feature.


This crate has few items that require the std crate (instead of core).

The "std" and "alloc" features are enabled by default so that users that are not aware of the core/alloc crates don't have to pass a feature to enable std support.

Cargo Features

These are the cargo features in structural:

  • std: Enables std support,this is enabled by default.

  • alloc: Enables alloc crate support,this is enabled by default.

  • specialization: Enables specialization inside structural,without enabling the nightly feature flag. This is for the case that specialization is stabilized after the last update to this library.

  • nightly_specialization: Enables specialization inside structural, requires nightly because it enables the nightly feature.

  • impl_fields: This allows using field_name: impl Foo fields in the structural_alias macro, which as of 2020-03-21 requires the associated_type_bounds Rust nightly feature.
    If this doesn't work,try using the "nightly_impl_fields" feature in Rust nightly instead.

  • nightly_impl_fields Equivalent to the impl_fields feature, as well as enabling the associated_type_bounds nightly features required for using the feature as of 2020-03-21.

  • use_const_str: Changes the internal implementation of TStr (the type level string type) to use a &'static str const parameter instead of types.
    Use this if const generics (eg:struct Foo<const S: &'static str>;) are usable on stable.

  • nightly_use_const_str: Equivalent to the use_const_str feature, which also enables the nightly Rust features required for const generics as of 2020-03-21.

  • disable_const_str: Disables const generics, useful if other crates enabling const generics causes internal errors in this Rust version.

Specialization is used inside structural for performance reasons. There are no benchmarks comparing when specialization is enabled and disabled yet.

Minimum Rust version

This crate support Rust back to 1.40, and uses a build script to automatically enable features from newer versions.

It requires Rust 1.40 to use proc macros in type position,eg: TS and FP.


Licensed under either of Apache License, Version 2.0 or MIT license at your option. Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in Structural by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.


~23K SLoC