Help: surround the struct literal with parentheses

help
1

The project structure and code looks like below:

.
├── Cargo.toml
├── reflect-derive
│   ├── Cargo.toml
│   └── src
│       └── lib.rs
└── src
    ├── lib.rs
    └── main.rs

// reflect-derive/src/lib.rs
use proc_macro::TokenStream;
use quote::quote;
use syn::{parse_macro_input, DeriveInput};

#[proc_macro_derive(Reflective)]
pub fn reflect_derive_fn(input: TokenStream) -> TokenStream {
    let input = parse_macro_input!(input as DeriveInput);

    let name = input.ident;

    let output = quote! {
        impl Reflective for #name {
            fn name(&self) -> String {
                String::from(#name)
            }
        }
    };

    TokenStream::from(output)
}

// src/lib.rs
pub use reflect_derive::Reflective;

pub trait Reflective {
    fn name(&self) -> String;
}

// src/main.rs

use reflect::Reflective;

#[derive(Reflective)]
struct Foo {}

fn main() {
    println!("Hello, world!");
}

# Cargo.toml
[package]
name = "reflect"
version = "0.1.0"
edition = "2021"

[dependencies]
reflect-derive = { path = "reflect-derive" }

# reflect-derive/Cargo.toml
[package]
name = "reflect-derive"
version = "0.1.0"
edition = "2021"

[dependencies]
quote = "1.0.26"
syn = "2.0.15"

[lib]
proc-macro = true

This is not a serious project, I just want to understand procedural macros, that's all. What I want to do is to write a derive macro. The problem is, compiler complains saying:

error[E0423]: expected value, found struct `Foo`
 --> src/main.rs:4:8
  |
4 | struct Foo {}
  |        ^^^ not a value
  |
help: surround the struct literal with parentheses
  |
4 | struct (Foo {})
  |        +      +

For more information about this error, try `rustc --explain E0423`.
error: could not compile `reflect` due to previous error

I think I'm missing something. Thanks in advance.

2

This line looks fishy. Could you try adding quotation marks around #name? Like String::from("#name").

1 Like
3

I don't exactly know why, but I always find these error messages... hilarious. Not at all in a negative way, sometimes it just leads to a really good laugh. I remember experiencing #106720 first-hand, and it really made my day.

Still (and somewhat unfortunately), they are bugs and thus must be fixed.

4

You're on the right track. Idk how I've missed it. This time, the problem is this:

// src/main.rs
use reflect::Reflective;

#[derive(Reflective)]
struct Foo {}

fn main() {
    let foo = Foo {};
    println!("{}", foo.name());
}

This will print out #name instead of Foo. I'm looking for a way to use those markers (?) inside string literals. If you know a way, let me know.

5

An identifier will print as an identifier, and a string literal will be interpolated as a string literal. "#name" is just a string literal. There's no chance quote! will look into it – if it did, then you couldn't spell a literal "#name" (and it would be surprising in a bad way, anyway).

Just convert the name to a string upfront, and interpolate that:

let name_str = name.to_string();
quote! {
    ...
    String::from(#name_str)
    ...
}
2 Likes
6

and

...solved the issue. Thanks.

The final code is:

use proc_macro::TokenStream;
use quote::quote;
use syn::{parse_macro_input, DeriveInput};

#[proc_macro_derive(Reflective)]
pub fn reflect_derive_fn(input: TokenStream) -> TokenStream {
    let input = parse_macro_input!(input as DeriveInput);

    let ident = input.ident;
    let ident_name = ident.to_string();

    let output = quote! {
        impl Reflective for #ident {
            fn name(&self) -> String {
                String::from(#ident_name)
            }
        }
    };

    TokenStream::from(output)
}

Inside quote!, using #ident_name does not work for some reason despite its type being String, I needed to do either String::from(#ident_name) or #ident_name.to_string(). I kinda understand that I guess? #ident is type Ident and inside quote!, it resolves into Foo identifier. I don't know what #ident_name expands into, though. Whatever, it works.

7

Again, it expands to a string literal. A string literal does not have type String. It's &'static str.

1 Like
8

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