├── .gitignore ├── Cargo.toml ├── LICENSE ├── Makefile ├── README.md └── src └── lib.rs /.gitignore: -------------------------------------------------------------------------------- 1 | /target 2 | /Cargo.lock 3 | -------------------------------------------------------------------------------- /Cargo.toml: -------------------------------------------------------------------------------- 1 | [package] 2 | name = "stack-tokens" 3 | version = "0.1.0" 4 | license = "Apache-2.0" 5 | authors = ["Armin Ronacher "] 6 | description = "stack token implementation for convenient TLS borrowing" 7 | edition = "2018" 8 | rust-version = "1.42.0" 9 | repository = "https://github.com/mitsuhiko/stack-tokens" 10 | keywords = ["tls", "stack-token", "borrow"] 11 | readme = "README.md" -------------------------------------------------------------------------------- /LICENSE: -------------------------------------------------------------------------------- 1 | Apache License 2 | Version 2.0, January 2004 3 | http://www.apache.org/licenses/ 4 | 5 | TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION 6 | 7 | 1. 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This problem however is not always restricted to 16 | thread local storage directly as some APIs are internally constrained by similar 17 | challenges. 18 | 19 | The problem usually appears when a proxy object wants to lend out some memory 20 | but it does not have a better lifetime than itself to constrain the value, but 21 | it does not directly own the value it’s trying to lend out. As a Rust programmer 22 | one is enticed to try to constrain it by the lifetime of &self but thanks to 23 | `Box::leak` that lifetime can become &'static. 24 | 25 | For more information see [the the blog post describing the concept](https://lucumr.pocoo.org/2022/11/23/stack-tokens/). 26 | 27 | ```rust 28 | use stack_tokens::{stack_token, RefCellLocalKeyExt}; 29 | use std::cell::RefCell; 30 | 31 | thread_local! { 32 | static VEC: RefCell> = RefCell::default(); 33 | } 34 | 35 | // places a token on the stack. 36 | stack_token!(scope); 37 | 38 | // you can now directly deref the thread local without closures 39 | VEC.as_mut(scope).push(42); 40 | assert_eq!(VEC.as_ref(scope).len(), 1); 41 | ``` -------------------------------------------------------------------------------- /src/lib.rs: -------------------------------------------------------------------------------- 1 | //! This library implements stack tokens which can be used to safely borrow 2 | //! values with stack-local lifetimes. 3 | //! 4 | //! [`StackToken`]s are zero sized objects that can be placed on the call 5 | //! stack with the [`stack_token!`] macro which then can be used to safely 6 | //! borrow from places such as thread local storage with reduced lifetimes. 7 | //! 8 | //! Without stack tokens the only safe API for such constructs are callback 9 | //! based such as the [`LocalKey::with`](std::thread::LocalKey::with) API. 10 | //! This problem however is not always restricted to thread local storage 11 | //! directly as some APIs are internally constrained by similar challenges. 12 | //! 13 | //! The problem usually appears when a proxy object wants to lend out some 14 | //! memory but it does not have a better lifetime than itself to constrain 15 | //! the value, but it does not directly own the value it's trying to lend 16 | //! out. As a Rust programmer one is enticed to try to constrain it by 17 | //! the lifetime of `&self` but thanks to [`Box::leak`] that lifetime can 18 | //! become `&'static`. 19 | //! 20 | //! For more information see the [the blog post describing the concept](https://lucumr.pocoo.org/2022/11/23/stack-tokens/). 21 | //! 22 | //! # Ref Cells 23 | //! 24 | //! This example shows how stack tokens can be used with the 25 | //! [`RefCellLocalKeyExt`] extension trait to directly borrow into [`RefCell`]s 26 | //! in a thread local. 27 | //! 28 | //! ``` 29 | //! use stack_tokens::{stack_token, RefCellLocalKeyExt}; 30 | //! use std::cell::RefCell; 31 | //! 32 | //! thread_local! { 33 | //! static VEC: RefCell> = RefCell::default(); 34 | //! } 35 | //! 36 | //! // places a token on the stack. 37 | //! stack_token!(scope); 38 | //! 39 | //! // you can now directly deref the thread local without closures 40 | //! VEC.as_mut(scope).push(42); 41 | //! assert_eq!(VEC.as_ref(scope).len(), 1); 42 | //! ``` 43 | //! 44 | //! # Basic Use 45 | //! 46 | //! This example shows how stack tokens can be used with the [`LocalKeyExt`] 47 | //! extension trait to get stack local borrows from a standard library thread 48 | //! local. 49 | //! 50 | //! ``` 51 | //! use stack_tokens::{stack_token, LocalKeyExt}; 52 | //! use std::sync::atomic::{AtomicUsize, Ordering}; 53 | //! 54 | //! thread_local! { 55 | //! static COUNTER: AtomicUsize = AtomicUsize::new(0); 56 | //! } 57 | //! 58 | //! // places a token on the stack 59 | //! stack_token!(scope); 60 | //! 61 | //! // borrow can be used to get a stack local reference 62 | //! COUNTER.borrow(scope).fetch_add(1, Ordering::Acquire); 63 | //! assert_eq!(COUNTER.borrow(scope).load(Ordering::Acquire), 1); 64 | //! ``` 65 | //! 66 | //! # Implementing Stack Local APIs 67 | //! 68 | //! To implement your own methods that use stack tokens introduce a new lifetime 69 | //! (eg: `'stack`) and constrain both `&self` and the passed token with it: 70 | //! 71 | //! ``` 72 | //! use stack_tokens::StackToken; 73 | //! use std::marker::PhantomData; 74 | //! 75 | //! struct MyTls(PhantomData::); 76 | //! 77 | //! impl MyTls { 78 | //! pub fn get<'stack>(&'stack self, token: &'stack StackToken) -> &'stack T { 79 | //! let _ = token; 80 | //! todo!() 81 | //! } 82 | //! } 83 | //! ``` 84 | use std::cell::{Ref, RefCell, RefMut}; 85 | use std::marker::PhantomData; 86 | use std::mem::transmute; 87 | use std::thread::LocalKey; 88 | 89 | /// A token to bind lifetimes to a specific stack. 90 | /// 91 | /// For more information see [`stack_token`]. 92 | pub struct StackToken { 93 | _marker: PhantomData<*const ()>, 94 | } 95 | 96 | impl StackToken { 97 | #[doc(hidden)] 98 | pub unsafe fn __private_new() -> StackToken { 99 | StackToken { 100 | _marker: PhantomData, 101 | } 102 | } 103 | } 104 | 105 | /// Creates a new [`StackToken`] with a given name on the stack. 106 | #[macro_export] 107 | macro_rules! stack_token { 108 | ($name:ident) => { 109 | #[allow(unsafe_code)] 110 | let $name = &unsafe { $crate::StackToken::__private_new() }; 111 | }; 112 | } 113 | 114 | /// Adds [`StackToken`] support to the standard library's [`LocalKey`]. 115 | pub trait LocalKeyExt { 116 | /// Borrows the value from the TLS with a [`StackToken`]. 117 | fn borrow<'stack>(&'static self, token: &'stack StackToken) -> &'stack T; 118 | } 119 | 120 | impl LocalKeyExt for LocalKey { 121 | fn borrow<'stack>(&'static self, token: &'stack StackToken) -> &'stack T { 122 | let _ = token; 123 | self.with(|value| unsafe { transmute::<&T, &'stack T>(value) }) 124 | } 125 | } 126 | 127 | /// Additional utility methods to [`LocalKey`]s holding [`RefCell`] values. 128 | /// 129 | /// This extension traits provides the two methods [`as_ref`](Self::as_ref) 130 | /// and [`as_mut`](Self::as_mut) that let you directly borrow into the 131 | /// contained [`RefCell`] with a [`StackToken`]. 132 | pub trait RefCellLocalKeyExt { 133 | /// Acquires a reference to the contained value. 134 | fn as_ref<'stack>(&'static self, token: &'stack StackToken) -> Ref<'stack, T>; 135 | 136 | /// Acquires a mutable reference to the contained value. 137 | fn as_mut<'stack>(&'static self, token: &'stack StackToken) -> RefMut<'stack, T>; 138 | } 139 | 140 | impl RefCellLocalKeyExt for LocalKey> { 141 | fn as_ref<'stack>(&'static self, token: &'stack StackToken) -> Ref<'stack, T> { 142 | self.borrow(token).borrow() 143 | } 144 | 145 | fn as_mut<'stack>(&'static self, token: &'stack StackToken) -> RefMut<'stack, T> { 146 | self.borrow(token).borrow_mut() 147 | } 148 | } 149 | 150 | #[test] 151 | fn test_tls_basic() { 152 | use crate::stack_token; 153 | use std::cell::RefCell; 154 | 155 | thread_local! { static FOO: RefCell = RefCell::default(); } 156 | 157 | stack_token!(scope); 158 | *FOO.borrow(scope).borrow_mut() += 1; 159 | assert_eq!(*FOO.borrow(scope).borrow(), 1); 160 | } 161 | 162 | #[test] 163 | fn test_tls_ref_cell() { 164 | use crate::stack_token; 165 | use std::cell::RefCell; 166 | 167 | thread_local! { static FOO: RefCell = RefCell::default(); } 168 | 169 | stack_token!(scope); 170 | *FOO.as_mut(scope) += 1; 171 | assert_eq!(*FOO.as_ref(scope), 1); 172 | } 173 | --------------------------------------------------------------------------------