Initial commit

.gitignore

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+/target

Cargo.toml

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+[package]
+name = "null_fn"
+version = "0.1.0"
+edition = "2018"
+authors = ["William Venner <[email protected]>"]
+license = "MIT"
+repository = "https://github.com/WilliamVenner/null_fn"
+description = "A proc attribute macro that allows for creating null function pointers in statics"
+categories = ["development-tools::ffi"]
+keywords = ["unsafe", "ffi", "null", "pointer", "static"]
+
+[lib]
+proc-macro = true
+
+[dependencies]
+proc-macro2 = "1"
+syn = { version = "1", features = ["full"] }
+quote = "1"

LICENSE

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+MIT License
+
+Copyright (c) 2021 William Venner
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

README.md

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+# ✨ `null_fn`
+
+A proc attribute macro that allows for creating null function pointers in `static`s.
+
+## Example
+
+```rust
+static mut UTIL_PlayerByUserId: unsafe extern "C" fn(userid: i32) -> *mut c_void = unsafe { std::mem::transmute::<*const (), _>(std::ptr::null()) }; // error[E0080]: it is undefined behavior to use this value
+
+#[null_fn]
+static mut UTIL_PlayerByUserId: unsafe extern "C" fn(userid: i32) -> *mut c_void = std::ptr::null(); // works!
+
+fn main() {
+	unsafe {
+		UTIL_PlayerByUserId(20); // This would panic, as the pointer is NULL.
+
+		let is_null = UTIL_PlayerByUserId.is_null(); // It's just a normal pointer, so we can call pointer-related functions on it.
+
+		UTIL_PlayerByUserId = /* magically find the pointer to the function */;
+		let player = UTIL_PlayerByUserId(20); // Now that we set the function pointer, we can call the function without panicking.
+	}
+}
+```

src/lib.rs

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+#![doc = include_str!("../README.md")]
+
+use proc_macro::TokenStream;
+use proc_macro2::Ident;
+use quote::{quote, format_ident};
+use syn::{Generics, ItemFn, ItemStatic, Signature, Token, parse_macro_input, spanned::Spanned};
+
+#[proc_macro_attribute]
+#[doc = include_str!("../README.md")]
+pub fn null_fn(_: TokenStream, input: TokenStream) -> TokenStream {
+	let mut static_item = parse_macro_input!(input as ItemStatic);
+
+	let null_fn = ItemFn {
+		attrs: vec![],
+		vis: syn::parse_str("pub(super)").unwrap(),
+		block: syn::parse_str("{ panic!(\"This function has not been initialized yet\") }").unwrap(),
+		sig: Signature {
+			constness: None,
+			asyncness: None,
+			unsafety: Some(Token![unsafe](static_item.span())),
+			abi: None,
+			fn_token: Default::default(),
+			ident: Ident::new("null_fn", static_item.span()),
+			generics: Generics::default(),
+			paren_token: Default::default(),
+			inputs: Default::default(),
+			variadic: Default::default(),
+			output: syn::ReturnType::Default
+		},
+	};
+
+	static_item.expr = syn::parse_str(&format!("unsafe{{std::mem::transmute(__null_fn__{}::null_fn as *const ())}}", static_item.ident)).unwrap();
+
+	let mod_name = format_ident!("__null_fn__{}", static_item.ident);
+	quote!(
+		#[allow(non_snake_case)]
+		mod #mod_name {
+			#[allow(unused_variables)]
+			#null_fn
+		}
+		#static_item
+	).into()
+}