从 dyn Any 到其他 trait 对象的转换

• no_std no alloc 支持
• 无 proc macros
• 无不安全代码

用法

[dependencies]
crosstrait = "0.1"

然后使用 register!{ 类型 => 特性 } 声明性宏和 Cast 特性。

对于嵌入式，链接器需要被告知类型注册表。

示例

use core::any::Any;
use crosstrait::{Cast, Castable, CastableRef, entry, register, REGISTRY, Registry};

// Some example traits to play with
use core::{fmt::{Debug, Formatter, Write}, ops::{AddAssign, SubAssign}};

// Add types and trait implementations to the default registry
// Implementation status is verified at compile time
register! { i32 => dyn Debug }

// Registering foreign types and traits works fine
// Serialization/deserialization of `dyn Any` is a major use case
// register! { i32 => dyn erased_serde::Serialize }

// If a type is not Send + Sync, it can't cast as Arc.
// `no_arc` accounts for that
register! { Formatter => dyn Write, no_arc }

// Check for trait impl registration on concrete type
assert!(i32::castable::<dyn Debug>());

// Check for trait impl registration on Any
let any: &dyn Any = &42i32;
assert!(any.castable::<dyn Debug>());

// SubAssign<i32> is impl'd for i32 but not registered
assert!(!any.castable::<dyn SubAssign<i32>>());

// Cast ref
let a: &dyn Debug = any.cast().unwrap();
println!("42 = {a:?}");

// Cast mut
let mut value = 5i32;
let any: &mut dyn Any = &mut value;
let v: &mut dyn AddAssign<i32> = any.cast().unwrap();
*v += 3;
assert_eq!(value, 5 + 3);

// Cast Box
let any: Box<dyn Any> = Box::new(0i32);
let _: Box<dyn Debug> = any.cast().unwrap();

// Cast Rc
use std::rc::Rc;
let any: Rc<dyn Any> = Rc::new(0i32);
let _: Rc<dyn Debug> = any.cast().unwrap();

// Cast Arc
use std::sync::Arc;
let any: Arc<dyn Any + Sync + Send> = Arc::new(0i32);
let _: Arc<dyn Debug> = any.cast().unwrap();

// Explicit registry usage
let any: &dyn Any = &0i32;
let _: &dyn Debug = REGISTRY.cast_ref(any).unwrap();

// Custom non-static registry
let myreg = Registry::new(&[entry!(i32 => dyn Debug)]);
let _: &dyn Debug = myreg.cast_ref(any).unwrap();

// Autotraits and type/const generics are distinct
let a: Option<&(dyn Debug + Sync)> = any.cast();
assert!(a.is_none());

// Registration in the default registry can happen anywhere
// in any order in any downstream crate
register! { i32 => dyn AddAssign<i32> }

相关软件包

• intertrait: crosstrait 的想法来源，类似目标，类似功能，std，proc macros
• miniconf: 提供了从异构嵌套数据结构中的节点获取 dyn Any 的多种方法，no_std，no alloc
• erased_serde: Serialize/Serializer/Deserializer 特性对象
• downcast/downcast-rs: 支持 dyn Trait -> 类型
• linkme: 用于构建分布式静态类型注册表的链接器魔法