reflect 0.0.15

The "but I thought Rust doesn't have reflection?" memorial brand new way of defining procedural macros.
Documentation 
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use crate::{GenericArgument, GenericArguments, Ident, Type};
use syn::ReturnType;

#[derive(Debug, Clone)]
// Consider just using syn::Path
pub struct Path {
    pub(crate) global: bool,
    pub(crate) path: Vec<PathSegment>,
}

#[derive(Debug, Clone)]
pub(crate) struct PathSegment {
    pub(crate) ident: Ident,
    pub(crate) args: PathArguments,
}

#[derive(Debug, Clone)]
pub(crate) enum PathArguments {
    None,
    AngleBracketed(AngleBracketedGenericArguments),
    Parenthesized(ParenthesizedGenericArguments),
}

#[derive(Debug, Clone)]
pub(crate) struct AngleBracketedGenericArguments {
    pub(crate) args: GenericArguments,
}

/// Arguments of a function path segment: the `(A, B) -> C` in `Fn(A, B) -> C`.
#[derive(Debug, Clone)]
pub(crate) struct ParenthesizedGenericArguments {
    /// (A, B)
    pub(crate) inputs: Vec<Type>,
    /// C
    pub(crate) output: Option<Type>,
}

impl Path {
    pub(crate) fn root() -> Self {
        Path {
            global: true,
            path: Vec::new(),
        }
    }

    pub(crate) fn empty() -> Self {
        Path {
            global: false,
            path: Vec::new(),
        }
    }

    pub(crate) fn get_path(&self, segment: &str) -> Self {
        let mut path = self.clone();
        path.path.push(PathSegment {
            ident: Ident::new(segment),
            args: PathArguments::None,
        });
        path
    }

    pub(crate) fn syn_to_path(path: syn::Path) -> Self {
        let syn::Path {
            leading_colon,
            segments,
        } = path;
        let path: Vec<_> = segments
            .into_iter()
            .map(|syn::PathSegment { ident, arguments }| {
                let ident = Ident::from(ident);
                match arguments {
                    syn::PathArguments::None => PathSegment {
                        ident,
                        args: PathArguments::None,
                    },
                    syn::PathArguments::AngleBracketed(generic_args) => PathSegment {
                        ident,
                        args: PathArguments::AngleBracketed(AngleBracketedGenericArguments {
                            args: GenericArguments {
                                args: generic_args
                                    .args
                                    .into_iter()
                                    .map(GenericArgument::syn_to_generic_argument)
                                    .collect(),
                            },
                        }),
                    },

                    syn::PathArguments::Parenthesized(parenthesized) => PathSegment {
                        ident,
                        args: PathArguments::Parenthesized(ParenthesizedGenericArguments {
                            inputs: parenthesized
                                .inputs
                                .into_iter()
                                .map(Type::syn_to_type)
                                .collect(),
                            output: match parenthesized.output {
                                ReturnType::Default => None,
                                ReturnType::Type(_, ty) => Some(Type::syn_to_type(*ty)),
                            },
                        }),
                    },
                }
            })
            .collect();
        Path {
            global: leading_colon.is_some(),
            path,
        }
    }

    pub(crate) fn ident_to_path(ident: Ident) -> Path {
        Path {
            global: false,
            path: vec![PathSegment {
                ident,
                args: PathArguments::None,
            }],
        }
    }
}