Cannot use non-literal expressions for `name` attribute

[Michał Wawrzyniec Urbańczyk]

Please complete the following tasks

• I have searched the discussions
• I have searched the open and rejected issues

Rust Version

rustc 1.76.0 (07dca489a 2024-02-04)

Clap Version

4.5.3

Minimal reproducible code

use clap::Parser;

pub trait HasName {
    const NAME: &'static str;
}

pub struct Foo;

impl HasName for Foo {
    const NAME: &'static str = "foo";
}

/// Simple program to greet a person
#[derive(Parser, Debug)]
struct Args<T: HasName> {
    /// Name of the person to greet
    #[clap(name = T::NAME, short, long)]
    name: String,

    #[clap(skip)]
    phantom: std::marker::PhantomData<T>,
}

fn main() {
    Args::<Foo>::parse();
}

Steps to reproduce the bug with the above code

cargo build

Actual Behaviour

Build error:

error: expected a literal
  --> src\main.rs:17:19
   |
17 |     #[clap(name = T::NAME, short, long)]
   |                   ^^^^^^^
   |
   = note: only literals (like `"foo"`, `-42` and `3.14`) can be passed to `concat!()`

Expected Behaviour

This code worked in clap 3.2.25 and was not documented as a change, so I think it still should work.

Also, the documentation says that the name attribute argument is an expression, so I would expect it to work. Especially since there is no restriction on the Command::name API. I don't think there are similar restrictions on other attributes.

Additional Context

This blocks me from migrating to clap v4. I relied on this pattern to reuse the same structure definition, while avoiding the name collisions.

Debug Output

No response

[Ed Page]

This was broken in #4947. Looking to see if we can better detect when to use one or the other.