src/expand.rs

use crate::lifetime::CollectLifetimes;
use crate::parse::Item;
use crate::receiver::{has_self_in_block, has_self_in_sig, mut_pat, ReplaceSelf};
use proc_macro2::TokenStream;
use quote::{format_ident, quote, quote_spanned, ToTokens};
use std::collections::BTreeSet as Set;
use syn::punctuated::Punctuated;
use syn::visit_mut::{self, VisitMut};
use syn::{
    parse_quote, Attribute, Block, FnArg, GenericParam, Generics, Ident, ImplItem, Lifetime, Pat,
    PatIdent, Receiver, ReturnType, Signature, Stmt, Token, TraitItem, Type, TypeParamBound,
    TypePath, WhereClause,
};

macro_rules! parse_quote_spanned {
    ($span:expr=> $($t:tt)*) => {
        syn::parse2(quote_spanned!($span=> $($t)*)).unwrap()
    };
}

impl ToTokens for Item {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        match self {
            Item::Trait(item) => item.to_tokens(tokens),
            Item::Impl(item) => item.to_tokens(tokens),
        }
    }
}

#[derive(Clone, Copy)]
enum Context<'a> {
    Trait {
        generics: &'a Generics,
        supertraits: &'a Supertraits,
    },
    Impl {
        impl_generics: &'a Generics,
        associated_type_impl_traits: &'a Set<Ident>,
    },
}

impl Context<'_> {
    fn lifetimes<'a>(&'a self, used: &'a [Lifetime]) -> impl Iterator<Item = &'a GenericParam> {
        let generics = match self {
            Context::Trait { generics, .. } => generics,
            Context::Impl { impl_generics, .. } => impl_generics,
        };
        generics.params.iter().filter(move |param| {
            if let GenericParam::Lifetime(param) = param {
                used.contains(&param.lifetime)
            } else {
                false
            }
        })
    }
}

type Supertraits = Punctuated<TypeParamBound, Token![+]>;

pub fn expand(input: &mut Item, is_local: bool) {
    match input {
        Item::Trait(input) => {
            let context = Context::Trait {
                generics: &input.generics,
                supertraits: &input.supertraits,
            };
            for inner in &mut input.items {
                if let TraitItem::Method(method) = inner {
                    let sig = &mut method.sig;
                    if sig.asyncness.is_some() {
                        let block = &mut method.default;
                        let mut has_self = has_self_in_sig(sig);
                        method.attrs.push(parse_quote!(#[must_use]));
                        if let Some(block) = block {
                            has_self |= has_self_in_block(block);
                            transform_block(context, sig, block);
                            method.attrs.push(lint_suppress_with_body());
                        } else {
                            method.attrs.push(lint_suppress_without_body());
                        }
                        let has_default = method.default.is_some();
                        transform_sig(context, sig, has_self, has_default, is_local);
                    }
                }
            }
        }
        Item::Impl(input) => {
            let mut lifetimes = CollectLifetimes::new("'impl", input.impl_token.span);
            lifetimes.visit_type_mut(&mut *input.self_ty);
            lifetimes.visit_path_mut(&mut input.trait_.as_mut().unwrap().1);
            let params = &input.generics.params;
            let elided = lifetimes.elided;
            input.generics.params = parse_quote!(#(#elided,)* #params);

            let mut associated_type_impl_traits = Set::new();
            for inner in &input.items {
                if let ImplItem::Type(assoc) = inner {
                    if let Type::ImplTrait(_) = assoc.ty {
                        associated_type_impl_traits.insert(assoc.ident.clone());
                    }
                }
            }

            let context = Context::Impl {
                impl_generics: &input.generics,
                associated_type_impl_traits: &associated_type_impl_traits,
            };
            for inner in &mut input.items {
                if let ImplItem::Method(method) = inner {
                    let sig = &mut method.sig;
                    if sig.asyncness.is_some() {
                        let block = &mut method.block;
                        let has_self = has_self_in_sig(sig) || has_self_in_block(block);
                        transform_block(context, sig, block);
                        transform_sig(context, sig, has_self, false, is_local);
                        method.attrs.push(lint_suppress_with_body());
                    }
                }
            }
        }
    }
}

fn lint_suppress_with_body() -> Attribute {
    parse_quote! {
        #[allow(
            clippy::let_unit_value,
            clippy::no_effect_underscore_binding,
            clippy::shadow_same,
            clippy::type_complexity,
            clippy::type_repetition_in_bounds,
            clippy::used_underscore_binding
        )]
    }
}

fn lint_suppress_without_body() -> Attribute {
    parse_quote! {
        #[allow(
            clippy::type_complexity,
            clippy::type_repetition_in_bounds
        )]
    }
}

// Input:
//     async fn f<T>(&self, x: &T) -> Ret;
//
// Output:
//     fn f<'life0, 'life1, 'async_trait, T>(
//         &'life0 self,
//         x: &'life1 T,
//     ) -> Pin<Box<dyn Future<Output = Ret> + Send + 'async_trait>>
//     where
//         'life0: 'async_trait,
//         'life1: 'async_trait,
//         T: 'async_trait,
//         Self: Sync + 'async_trait;
fn transform_sig(
    context: Context,
    sig: &mut Signature,
    has_self: bool,
    has_default: bool,
    is_local: bool,
) {
    sig.fn_token.span = sig.asyncness.take().unwrap().span;

    let ret = match &sig.output {
        ReturnType::Default => quote!(()),
        ReturnType::Type(_, ret) => quote!(#ret),
    };

    let default_span = sig
        .ident
        .span()
        .join(sig.paren_token.span)
        .unwrap_or_else(|| sig.ident.span());

    let mut lifetimes = CollectLifetimes::new("'life", default_span);
    for arg in sig.inputs.iter_mut() {
        match arg {
            FnArg::Receiver(arg) => lifetimes.visit_receiver_mut(arg),
            FnArg::Typed(arg) => lifetimes.visit_type_mut(&mut arg.ty),
        }
    }

    for param in sig
        .generics
        .params
        .iter()
        .chain(context.lifetimes(&lifetimes.explicit))
    {
        match param {
            GenericParam::Type(param) => {
                let param = &param.ident;
                let span = param.span();
                where_clause_or_default(&mut sig.generics.where_clause)
                    .predicates
                    .push(parse_quote_spanned!(span=> #param: 'async_trait));
            }
            GenericParam::Lifetime(param) => {
                let param = &param.lifetime;
                let span = param.span();
                where_clause_or_default(&mut sig.generics.where_clause)
                    .predicates
                    .push(parse_quote_spanned!(span=> #param: 'async_trait));
            }
            GenericParam::Const(_) => {}
        }
    }

    if sig.generics.lt_token.is_none() {
        sig.generics.lt_token = Some(Token![<](sig.ident.span()));
    }
    if sig.generics.gt_token.is_none() {
        sig.generics.gt_token = Some(Token![>](sig.paren_token.span));
    }

    for elided in lifetimes.elided {
        sig.generics.params.push(parse_quote!(#elided));
        where_clause_or_default(&mut sig.generics.where_clause)
            .predicates
            .push(parse_quote_spanned!(elided.span()=> #elided: 'async_trait));
    }

    sig.generics
        .params
        .push(parse_quote_spanned!(default_span=> 'async_trait));

    if has_self {
        let bound_span = sig.ident.span();
        let bound = match sig.inputs.iter().next() {
            Some(FnArg::Receiver(Receiver {
                reference: Some(_),
                mutability: None,
                ..
            })) => Ident::new("Sync", bound_span),
            Some(FnArg::Typed(arg))
                if match (arg.pat.as_ref(), arg.ty.as_ref()) {
                    (Pat::Ident(pat), Type::Reference(ty)) => {
                        pat.ident == "self" && ty.mutability.is_none()
                    }
                    _ => false,
                } =>
            {
                Ident::new("Sync", bound_span)
            }
            _ => Ident::new("Send", bound_span),
        };

        let assume_bound = match context {
            Context::Trait { supertraits, .. } => !has_default || has_bound(supertraits, &bound),
            Context::Impl { .. } => true,
        };

        let where_clause = where_clause_or_default(&mut sig.generics.where_clause);
        where_clause.predicates.push(if assume_bound || is_local {
            parse_quote_spanned!(bound_span=> Self: 'async_trait)
        } else {
            parse_quote_spanned!(bound_span=> Self: ::core::marker::#bound + 'async_trait)
        });
    }

    for (i, arg) in sig.inputs.iter_mut().enumerate() {
        match arg {
            FnArg::Receiver(Receiver {
                reference: Some(_), ..
            }) => {}
            FnArg::Receiver(arg) => arg.mutability = None,
            FnArg::Typed(arg) => {
                if let Pat::Ident(ident) = &mut *arg.pat {
                    ident.by_ref = None;
                    ident.mutability = None;
                } else {
                    let positional = positional_arg(i, &arg.pat);
                    let m = mut_pat(&mut arg.pat);
                    arg.pat = parse_quote!(#m #positional);
                }
            }
        }
    }

    let ret_span = sig.ident.span();
    let bounds = if is_local {
        quote_spanned!(ret_span=> 'async_trait)
    } else {
        quote_spanned!(ret_span=> ::core::marker::Send + 'async_trait)
    };
    sig.output = parse_quote_spanned! {ret_span=>
        -> ::core::pin::Pin<Box<
            dyn ::core::future::Future<Output = #ret> + #bounds
        >>
    };
}

// Input:
//     async fn f<T>(&self, x: &T, (a, b): (A, B)) -> Ret {
//         self + x + a + b
//     }
//
// Output:
//     Box::pin(async move {
//         let ___ret: Ret = {
//             let __self = self;
//             let x = x;
//             let (a, b) = __arg1;
//
//             __self + x + a + b
//         };
//
//         ___ret
//     })
fn transform_block(context: Context, sig: &mut Signature, block: &mut Block) {
    if let Some(Stmt::Item(syn::Item::Verbatim(item))) = block.stmts.first() {
        if block.stmts.len() == 1 && item.to_string() == ";" {
            return;
        }
    }

    let mut self_span = None;
    let decls = sig
        .inputs
        .iter()
        .enumerate()
        .map(|(i, arg)| match arg {
            FnArg::Receiver(Receiver {
                self_token,
                mutability,
                ..
            }) => {
                let ident = Ident::new("__self", self_token.span);
                self_span = Some(self_token.span);
                quote!(let #mutability #ident = #self_token;)
            }
            FnArg::Typed(arg) => {
                if let Pat::Ident(PatIdent {
                    ident, mutability, ..
                }) = &*arg.pat
                {
                    if ident == "self" {
                        self_span = Some(ident.span());
                        let prefixed = Ident::new("__self", ident.span());
                        quote!(let #mutability #prefixed = #ident;)
                    } else {
                        quote!(let #mutability #ident = #ident;)
                    }
                } else {
                    let pat = &arg.pat;
                    let ident = positional_arg(i, pat);
                    quote!(let #pat = #ident;)
                }
            }
        })
        .collect::<Vec<_>>();

    if let Some(span) = self_span {
        let mut replace_self = ReplaceSelf(span);
        replace_self.visit_block_mut(block);
    }

    let stmts = &block.stmts;
    let let_ret = match &mut sig.output {
        ReturnType::Default => quote_spanned! {block.brace_token.span=>
            #(#decls)*
            let _: () = { #(#stmts)* };
        },
        ReturnType::Type(_, ret) => {
            if contains_associated_type_impl_trait(context, ret) {
                if decls.is_empty() {
                    quote!(#(#stmts)*)
                } else {
                    quote!(#(#decls)* { #(#stmts)* })
                }
            } else {
                quote_spanned! {block.brace_token.span=>
                    if let ::core::option::Option::Some(__ret) = ::core::option::Option::None::<#ret> {
                        return __ret;
                    }
                    #(#decls)*
                    let __ret: #ret = { #(#stmts)* };
                    #[allow(unreachable_code)]
                    __ret
                }
            }
        }
    };
    let box_pin = quote_spanned!(block.brace_token.span=>
        Box::pin(async move { #let_ret })
    );
    block.stmts = parse_quote!(#box_pin);
}

fn positional_arg(i: usize, pat: &Pat) -> Ident {
    use syn::spanned::Spanned;
    format_ident!("__arg{}", i, span = pat.span())
}

fn has_bound(supertraits: &Supertraits, marker: &Ident) -> bool {
    for bound in supertraits {
        if let TypeParamBound::Trait(bound) = bound {
            if bound.path.is_ident(marker)
                || bound.path.segments.len() == 3
                    && (bound.path.segments[0].ident == "std"
                        || bound.path.segments[0].ident == "core")
                    && bound.path.segments[1].ident == "marker"
                    && bound.path.segments[2].ident == *marker
            {
                return true;
            }
        }
    }
    false
}

fn contains_associated_type_impl_trait(context: Context, ret: &mut Type) -> bool {
    struct AssociatedTypeImplTraits<'a> {
        set: &'a Set<Ident>,
        contains: bool,
    }

    impl<'a> VisitMut for AssociatedTypeImplTraits<'a> {
        fn visit_type_path_mut(&mut self, ty: &mut TypePath) {
            if ty.qself.is_none()
                && ty.path.segments.len() == 2
                && ty.path.segments[0].ident == "Self"
                && self.set.contains(&ty.path.segments[1].ident)
            {
                self.contains = true;
            }
            visit_mut::visit_type_path_mut(self, ty);
        }
    }

    match context {
        Context::Trait { .. } => false,
        Context::Impl {
            associated_type_impl_traits,
            ..
        } => {
            let mut visit = AssociatedTypeImplTraits {
                set: associated_type_impl_traits,
                contains: false,
            };
            visit.visit_type_mut(ret);
            visit.contains
        }
    }
}

fn where_clause_or_default(clause: &mut Option<WhereClause>) -> &mut WhereClause {
    clause.get_or_insert_with(|| WhereClause {
        where_token: Default::default(),
        predicates: Punctuated::new(),
    })
}