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Allow proc macro to initialize a private field with a def_site value #47311
Description
In a struct initializer expression S { k: v } generated by a procedural macro where the field k is private, it seems both k and v need to be spanned with call_site in order for the generated code to compile. I believe only k should be required to be call_site, and v should be allowed to be def_site or anything else.
This currently blocks a correct implementation of derive(Deserialize).
#![feature(proc_macro)]
extern crate mac;
struct S {
unit: (),
}
fn main() {
// Expands to `S { unit: () }`.
mac::init_field_unit!(S unit);
}#![feature(proc_macro)]
extern crate proc_macro;
use proc_macro::{TokenStream, TokenTree, TokenNode, Delimiter, Spacing, Span};
#[proc_macro]
pub fn init_field_unit(input: TokenStream) -> TokenStream {
let mut iter = input.into_iter();
let struct_name = iter.next().unwrap();
let field_name = iter.next().unwrap();
assert!(iter.next().is_none());
// Expands to `$struct_name { $field_name: () }`.
vec![
struct_name,
TokenTree {
span: Span::call_site(),
kind: TokenNode::Group(Delimiter::Brace, vec![
field_name,
TokenTree {
span: Span::call_site(),
kind: TokenNode::Op(':', Spacing::Alone),
},
TokenTree {
// Works if this is call_site, but in derive(Deserialize) I
// need to be able to use def_site variables here.
span: Span::def_site(),
kind: TokenNode::Group(Delimiter::Parenthesis, TokenStream::empty()),
},
].into_iter().collect()),
},
].into_iter().collect()
}error[E0451]: field `unit` of struct `S` is private
--> src/main.rs:49:5
|
49 | mac::init_field_unit!(S unit);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ field `unit` is private
Reacted by Jeffrey Seyfried
Activity
dtolnay commentedon Jan 10, 2018
Found a workaround: wrap the def_site value in a set of call_site parentheses. It should work without this workaround.
Rewrite `#[derive(Queryable)]` in derives2
Rewrite `#[derive(Queryable)]` in derives2
Rewrite `#[derive(QueryableByName)]` in derives2
Rewrite `#[derive(QueryableByName)]` in derives2
Rewrite `#[derive(QueryableByName)]` in derives2
Rewrite `#[derive(QueryableByName)]` in derives2
Rewrite `#[derive(QueryableByName)]` in derives2
jseyfried commentedon Feb 9, 2018
@dtolnay Sorry for the delay!
Fixed in #48082.
Rollup merge of rust-lang#48082 - jseyfried:improve_struct_field_hygi…
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