getset 0.1.0

Getset, we're ready to go! A procedural macro for generating the most basic getters and setters on fields.
Documentation 
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use proc_macro2::TokenStream as TokenStream2;
use proc_macro2::{Ident, Span};
use proc_macro_error::{abort, ResultExt};
use syn::{self, spanned::Spanned, Field, Lit, Meta, MetaNameValue, Visibility};

use self::GenMode::*;
use super::parse_attr;

pub struct GenParams {
    pub mode: GenMode,
    pub global_attr: Option<Meta>,
}

#[derive(PartialEq, Eq, Copy, Clone)]
pub enum GenMode {
    Get,
    GetCopy,
    Set,
    GetMut,
}

impl GenMode {
    pub fn name(self) -> &'static str {
        match self {
            Get => "get",
            GetCopy => "get_copy",
            Set => "set",
            GetMut => "get_mut",
        }
    }

    pub fn prefix(self) -> &'static str {
        match self {
            Get | GetCopy | GetMut => "",
            Set => "set_",
        }
    }

    pub fn suffix(self) -> &'static str {
        match self {
            Get | GetCopy | Set => "",
            GetMut => "_mut",
        }
    }

    fn is_get(self) -> bool {
        match self {
            GenMode::Get | GenMode::GetCopy | GenMode::GetMut => true,
            GenMode::Set => false,
        }
    }
}

pub fn parse_visibility(attr: Option<&Meta>, meta_name: &str) -> Option<Visibility> {
    match attr {
        // `#[get = "pub"]` or `#[set = "pub"]`
        Some(Meta::NameValue(MetaNameValue {
            lit: Lit::Str(ref s),
            path,
            ..
        })) => {
            if path.is_ident(meta_name) {
                s.value().split(' ').find(|v| *v != "with_prefix").map(|v| {
                    syn::parse_str(v)
                        .map_err(|e| syn::Error::new(s.span(), e))
                        .expect_or_abort("invalid visibility found")
                })
            } else {
                None
            }
        }
        _ => None,
    }
}

/// Some users want legacy/compatability.
/// (Getters are often prefixed with `get_`)
fn has_prefix_attr(f: &Field, mode: GenMode) -> bool {
    let inner = f
        .attrs
        .iter()
        .filter_map(|v| parse_attr(v, mode))
        .filter(|meta| {
            ["get", "get_copy"]
                .iter()
                .any(|ident| meta.path().is_ident(ident))
        })
        .last();
    match inner {
        Some(Meta::NameValue(meta)) => {
            if let Lit::Str(lit) = meta.lit {
                // Naive tokenization to avoid a possible visibility mod named `with_prefix`.
                lit.value().split(' ').any(|v| v == "with_prefix")
            } else {
                false
            }
        }
        _ => false,
    }
}

pub fn implement(field: &Field, params: &GenParams) -> TokenStream2 {
    let field_name = field
        .clone()
        .ident
        .unwrap_or_else(|| abort!(field.span(), "Expected the field to have a name"));

    let fn_name = Ident::new(
        &format!(
            "{}{}{}{}",
            if has_prefix_attr(field, params.mode) && (params.mode.is_get()) {
                "get_"
            } else {
                ""
            },
            params.mode.prefix(),
            field_name,
            params.mode.suffix()
        ),
        Span::call_site(),
    );
    let ty = field.ty.clone();

    let doc = field.attrs.iter().filter(|v| {
        v.parse_meta()
            .map(|meta| meta.path().is_ident("doc"))
            .unwrap_or(false)
    });

    let attr = field
        .attrs
        .iter()
        .filter_map(|v| parse_attr(v, params.mode))
        .last()
        .or_else(|| params.global_attr.clone());

    let visibility = parse_visibility(attr.as_ref(), params.mode.name());
    match attr {
        Some(_) => match params.mode {
            GenMode::Get => {
                quote! {
                    #(#doc)*
                    #[inline(always)]
                    #visibility fn #fn_name(&self) -> &#ty {
                        &self.#field_name
                    }
                }
            }
            GenMode::GetCopy => {
                quote! {
                    #(#doc)*
                    #[inline(always)]
                    #visibility fn #fn_name(&self) -> #ty {
                        self.#field_name
                    }
                }
            }
            GenMode::Set => {
                quote! {
                    #(#doc)*
                    #[inline(always)]
                    #visibility fn #fn_name(&mut self, val: #ty) -> &mut Self {
                        self.#field_name = val;
                        self
                    }
                }
            }
            GenMode::GetMut => {
                quote! {
                    #(#doc)*
                    #[inline(always)]
                    #visibility fn #fn_name(&mut self) -> &mut #ty {
                        &mut self.#field_name
                    }
                }
            }
        },
        // Don't need to do anything.
        None => quote! {},
    }
}