working

.gitignore

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+/target
+/Cargo.lock

Cargo.toml

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+[package]
+name = "pub-fields"
+version = "0.1.0"
+edition = "2021"
+authors = ["sam0x17"]
+description = "Provides a proc macro attribute that defaults all struct fields to public unless private is prepended."
+
+[lib]
+proc-macro = true
+
+[dependencies]
+syn = { version = "2", features = ["full"]}
+quote = "1"

src/lib.rs

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+use proc_macro::TokenStream;
+use quote::ToTokens;
+use syn::{parse::Nothing, parse_macro_input, parse_quote, ItemStruct};
+
+#[proc_macro_attribute]
+pub fn pub_fields(attr: TokenStream, tokens: TokenStream) -> TokenStream {
+    parse_macro_input!(attr as Nothing);
+    let mut item_struct = parse_macro_input!(tokens as ItemStruct);
+    for field in &mut item_struct.fields {
+        field.vis = match &field.vis {
+            syn::Visibility::Public(p) => syn::Visibility::Public(*p),
+            syn::Visibility::Restricted(res) => syn::Visibility::Restricted(res.clone()),
+            syn::Visibility::Inherited => parse_quote!(pub),
+        };
+    }
+    item_struct.to_token_stream().into()
+}

tests/tests.rs

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+mod submod {
+    use pub_fields::pub_fields;
+
+    #[pub_fields]
+    pub struct MyStruct {
+        a: usize,
+        b: usize,
+        c: usize,
+    }
+}
+
+#[test]
+fn test_it() {
+    let x = submod::MyStruct { a: 3, b: 4, c: 5 };
+}