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//! Procedural macro support crate for the `illicit` crate.

#![deny(missing_docs)]

extern crate proc_macro;
use proc_macro::TokenStream;
use proc_macro_error::{abort, abort_call_site, proc_macro_error};
use quote::quote;
use syn::{
    parse::Parser, parse_macro_input, parse_quote, punctuated::Punctuated, spanned::Spanned,
    Attribute, FnArg, ItemFn, Local, PatType, Stmt, Token, Type, TypeReference,
};

/// Defines required `illicit::get` values for a function. Binds the provided
/// types as if references to them were implicit function arguments.
///
/// # Panics
///
/// Will cause the annotated function to panic if it is invoked without the
/// requested type in its environment. This attribute adds an `Environment
/// Expectations` section to the doc comments of the annotated function.
#[proc_macro_attribute]
#[proc_macro_error]
pub fn from_env(args: TokenStream, input: TokenStream) -> TokenStream {
    let mut input_fn: ItemFn = parse_macro_input!(input);

    let args = Punctuated::<FnArg, Token![,]>::parse_terminated.parse(args).unwrap();
    if args.is_empty() {
        abort_call_site!("must specify >=1 one argument");
    }

    let doc_prelude = "
# Environment Expectations

This function requires the following types to be visible to [`illicit::get`] and will
panic otherwise:
";

    for line in doc_prelude.lines() {
        input_fn.attrs.push(parse_quote!(#[doc = #line]));
    }

    for arg in args {
        let arg = match arg {
            FnArg::Receiver(rec) => abort!(rec.span(), "can't receive self by-environment"),
            FnArg::Typed(pt) => pt,
        };
        let ([first_stmt, second_stmt], doc_attr) = bind_env_reference(&arg);
        input_fn.block.stmts.insert(0, Stmt::Local(first_stmt));
        input_fn.block.stmts.insert(1, Stmt::Local(second_stmt));
        input_fn.attrs.push(doc_attr);
    }

    quote::quote!(#input_fn).into()
}

/// Create a pair of local assignment expressions from the `pattern: &type`
/// pair which is passed.
fn bind_env_reference(arg: &PatType) -> ([Local; 2], Attribute) {
    let arg_span = arg.span();

    let ty = match &*arg.ty {
        Type::Reference(TypeReference { lifetime, mutability, elem, .. }) => {
            if mutability.is_some() {
                abort!(mutability.span(), "mutable references cannot be passed by environment");
            }

            if lifetime.is_some() {
                abort!(
                    lifetime.span(),
                    "cannot bind to concrete lifetimes for environment references"
                );
            }

            quote!(#elem)
        }

        ty => abort!(ty.span(), "arguments must be references"),
    };

    let name = *arg.pat.clone();
    let init_expr = parse_quote!(illicit::expect::<#ty>());
    let deref_expr = parse_quote!(&*#name);

    let shadowed = Local {
        attrs: vec![],
        let_token: Token![let](arg_span),
        pat: name.clone(),
        init: Some((Token![=](arg_span), Box::new(init_expr))),
        semi_token: Token![;](arg_span),
    };
    let derefd = Local {
        attrs: vec![],
        let_token: Token![let](arg_span),
        pat: name,
        init: Some((Token![=](arg_span), Box::new(deref_expr))),
        semi_token: Token![;](arg_span),
    };

    let ty_bullet = format!("* `{}`", ty);
    let doc_attr = parse_quote!(#[doc = #ty_bullet]);

    ([shadowed, derefd], doc_attr)
}