Generated code is not public with macro generated impl fn

[hugoduncan]

When generating a public fn with impl code, the pub keyword is ignored, and the resulting function is private (and raises a dead_code warning).

It is also not possible to move the pub keyword into the macro, as it then causes an error:

 expected one of `extern`, `fn`, or `unsafe`, found `pub`

Related issues: #4621, #11403

The workaround suggested in #4621 (generating multiple impl blocks) is unsatisfactory for public impl functions, as it does not allow for documenting the function, and produces doc output with repeated impl Type headers for each function.

The use case for this is generating code for library bindings.

The following code reproduces the issue:

macro_rules! defn {
    ($n:ident) => (
        fn $n (&self) -> i32 { 1 } )
}

#[derive(Copy)]
pub struct S;

impl S {
    pub defn!(f);
}


#[cfg(test)]
mod tests {
    use super::S;

    #[test]
    fn test_f() {
        let s=S;
        assert_eq!(1, s.f());
    }
}

[hugoduncan]

Duplicate of #18317