config proc macro doesn't support generic parameters with inline bounds

[luketpeterson]

The following code won't compile:

#[config]
#[derive(clap::Parser, Debug, Default)]
pub struct FrameworkConfig<AppArgsT: clap::Args + std::fmt::Debug + Default + DeserializeOwned + Serialize> {

    #[command(flatten)]
    #[serde(skip)]
    pub app_args: AppArgsT,

    #[serde(rename = "scratchPath")]
    pub scratch_path: Option<PathBuf>,
}

I get the following error:

struct takes 1 generic argument but 0 generic arguments were supplied
expected 1 generic argument

[luketpeterson]

Investigating further, The issue is due to the fact that I inlined the bounds for the type parameter. The problem is that it's not valid Rust syntax to include the bounds in the type instantiation part of the impl. See the second use of #struct_gen here:

twelf/config-derive/src/lib.rs

Line 155 in 7ad38f3

impl #struct_gen #struct_name #struct_gen #struct_where {

Unfortunately there isn't an easy fix offered by the syn crate. The params field of https://docs.rs/syn/latest/syn/struct.Generics.html needs to be parsed manually to separate the type identifiers from the bounds.

Luckily, using a where clause in the struct definition instead of inline bounds gets past this problem, but it appears there is a problem onion.

[luketpeterson]

So this code gets further:

#[config]
#[derive(Parser, Debug, Default)]
pub struct FrameworkConfig <AppArgsT>
    where AppArgsT: Args + Clone + Sync + Send + std::fmt::Debug + Default + DeserializeOwned + Serialize
{
    #[command(flatten)]
    #[serde(skip)]
    pub app_args: AppArgsT,

    #[serde(rename = "scratchPath")]
    pub scratch_path: Option<PathBuf>,
}

But it still hits the rocks in the clap::Parser derive because the #[config] macro eats the attribute intended for clap. But that's a separate issue.