//! Propagates [`#[doc(cfg(...))]`](https://github.com/rust-lang/rust/issues/43781) to child items.

use rustc_ast::token::{Token, TokenKind};
use rustc_ast::tokenstream::{TokenStream, TokenTree};
use rustc_hir::{AttrArgs, Attribute};
use rustc_span::symbol::sym;

use crate::clean::inline::{load_attrs, merge_attrs};
use crate::clean::{CfgInfo, Crate, Item, ItemKind};
use crate::core::DocContext;
use crate::fold::DocFolder;
use crate::passes::Pass;

pub(crate) const PROPAGATE_DOC_CFG: Pass = Pass {
    name: "propagate-doc-cfg",
    run: Some(propagate_doc_cfg),
    description: "propagates `#[doc(cfg(...))]` to child items",
};

pub(crate) fn propagate_doc_cfg(cr: Crate, cx: &mut DocContext<'_>) -> Crate {
    if cx.tcx.features().doc_cfg() {
        CfgPropagator { cx, cfg_info: CfgInfo::default() }.fold_crate(cr)
    } else {
        cr
    }
}

struct CfgPropagator<'a, 'tcx> {
    cx: &'a mut DocContext<'tcx>,
    cfg_info: CfgInfo,
}

/// Returns true if the provided `token` is a `cfg` ident.
fn is_cfg_token(token: &TokenTree) -> bool {
    // We only keep `doc(cfg)` items.
    matches!(token, TokenTree::Token(Token { kind: TokenKind::Ident(sym::cfg, _,), .. }, _,),)
}

/// We only want to keep `#[cfg()]` and `#[doc(cfg())]` attributes so we rebuild a vec of
/// `TokenTree` with only the tokens we're interested into.
fn filter_non_cfg_tokens_from_list(args_tokens: &TokenStream) -> Vec<TokenTree> {
    let mut tokens = Vec::with_capacity(args_tokens.len());
    let mut skip_next_delimited = false;
    for token in args_tokens.iter() {
        match token {
            TokenTree::Delimited(..) => {
                if !skip_next_delimited {
                    tokens.push(token.clone());
                }
                skip_next_delimited = false;
            }
            token if is_cfg_token(token) => {
                skip_next_delimited = false;
                tokens.push(token.clone());
            }
            _ => {
                skip_next_delimited = true;
            }
        }
    }
    tokens
}

/// This function goes through the attributes list (`new_attrs`) and extract the `cfg` tokens from
/// it and put them into `attrs`.
fn add_only_cfg_attributes(attrs: &mut Vec<Attribute>, new_attrs: &[Attribute]) {
    for attr in new_attrs {
        if attr.is_doc_comment() {
            continue;
        }
        let mut attr = attr.clone();
        if let Attribute::Unparsed(ref mut normal) = attr
            && let [ident] = &*normal.path.segments
        {
            let ident = ident.name;
            if ident == sym::doc
                && let AttrArgs::Delimited(args) = &mut normal.args
            {
                let tokens = filter_non_cfg_tokens_from_list(&args.tokens);
                args.tokens = TokenStream::new(tokens);
                attrs.push(attr);
            } else if ident == sym::cfg_trace {
                // If it's a `cfg()` attribute, we keep it.
                attrs.push(attr);
            }
        }
    }
}

impl CfgPropagator<'_, '_> {
    // Some items need to merge their attributes with their parents' otherwise a few of them
    // (mostly `cfg` ones) will be missing.
    fn merge_with_parent_attributes(&mut self, item: &mut Item) {
        let mut attrs = Vec::new();
        // We only need to merge an item attributes with its parent's in case it's an impl as an
        // impl might not be defined in the same module as the item it implements.
        //
        // Otherwise, `cfg_info` already tracks everything we need so nothing else to do!
        if matches!(item.kind, ItemKind::ImplItem(_))
            && let Some(def_id) = item.item_id.as_def_id().and_then(|def_id| def_id.as_local())
        {
            let mut next_def_id = def_id;
            while let Some(parent_def_id) = self.cx.tcx.opt_local_parent(next_def_id) {
                let x = load_attrs(self.cx, parent_def_id.to_def_id());
                add_only_cfg_attributes(&mut attrs, x);
                next_def_id = parent_def_id;
            }
        }

        let (_, cfg) = merge_attrs(
            self.cx,
            item.attrs.other_attrs.as_slice(),
            Some((&attrs, None)),
            &mut self.cfg_info,
        );
        item.inner.cfg = cfg;
    }
}

impl DocFolder for CfgPropagator<'_, '_> {
    fn fold_item(&mut self, mut item: Item) -> Option<Item> {
        let old_cfg_info = self.cfg_info.clone();

        self.merge_with_parent_attributes(&mut item);

        let result = self.fold_item_recur(item);
        self.cfg_info = old_cfg_info;

        Some(result)
    }
}