use std::iter;

use super::prelude::*;
use crate::session_diagnostics;

pub(crate) struct AllowInternalUnstableParser;
impl<S: Stage> CombineAttributeParser<S> for AllowInternalUnstableParser {
    const PATH: &[Symbol] = &[sym::allow_internal_unstable];
    type Item = (Symbol, Span);
    const CONVERT: ConvertFn<Self::Item> =
        |items, span| AttributeKind::AllowInternalUnstable(items, span);
    const ALLOWED_TARGETS: AllowedTargets = AllowedTargets::AllowList(&[
        Allow(Target::MacroDef),
        Allow(Target::Fn),
        Warn(Target::Field),
        Warn(Target::Arm),
    ]);
    const TEMPLATE: AttributeTemplate = template!(Word, List: &["feat1, feat2, ..."]);

    fn extend(
        cx: &mut AcceptContext<'_, '_, S>,
        args: &ArgParser,
    ) -> impl IntoIterator<Item = Self::Item> {
        parse_unstable(cx, args, <Self as CombineAttributeParser<S>>::PATH[0])
            .into_iter()
            .zip(iter::repeat(cx.attr_span))
    }
}

pub(crate) struct UnstableFeatureBoundParser;
impl<S: Stage> CombineAttributeParser<S> for UnstableFeatureBoundParser {
    const PATH: &[rustc_span::Symbol] = &[sym::unstable_feature_bound];
    type Item = (Symbol, Span);
    const CONVERT: ConvertFn<Self::Item> = |items, _| AttributeKind::UnstableFeatureBound(items);
    const ALLOWED_TARGETS: AllowedTargets = AllowedTargets::AllowList(&[
        Allow(Target::Fn),
        Allow(Target::Impl { of_trait: true }),
        Allow(Target::Trait),
    ]);
    const TEMPLATE: AttributeTemplate = template!(Word, List: &["feat1, feat2, ..."]);

    fn extend(
        cx: &mut AcceptContext<'_, '_, S>,
        args: &ArgParser,
    ) -> impl IntoIterator<Item = Self::Item> {
        if !cx.features().staged_api() {
            cx.emit_err(session_diagnostics::StabilityOutsideStd { span: cx.attr_span });
        }
        parse_unstable(cx, args, <Self as CombineAttributeParser<S>>::PATH[0])
            .into_iter()
            .zip(iter::repeat(cx.attr_span))
    }
}

pub(crate) struct RustcAllowConstFnUnstableParser;
impl<S: Stage> CombineAttributeParser<S> for RustcAllowConstFnUnstableParser {
    const PATH: &[Symbol] = &[sym::rustc_allow_const_fn_unstable];
    type Item = Symbol;
    const CONVERT: ConvertFn<Self::Item> =
        |items, first_span| AttributeKind::RustcAllowConstFnUnstable(items, first_span);
    const ALLOWED_TARGETS: AllowedTargets = AllowedTargets::AllowList(&[
        Allow(Target::Fn),
        Allow(Target::Method(MethodKind::Inherent)),
        Allow(Target::Method(MethodKind::Trait { body: true })),
        Allow(Target::Method(MethodKind::TraitImpl)),
    ]);
    const TEMPLATE: AttributeTemplate = template!(Word, List: &["feat1, feat2, ..."]);

    fn extend(
        cx: &mut AcceptContext<'_, '_, S>,
        args: &ArgParser,
    ) -> impl IntoIterator<Item = Self::Item> {
        parse_unstable(cx, args, <Self as CombineAttributeParser<S>>::PATH[0])
    }
}

fn parse_unstable<S: Stage>(
    cx: &AcceptContext<'_, '_, S>,
    args: &ArgParser,
    symbol: Symbol,
) -> impl IntoIterator<Item = Symbol> {
    let mut res = Vec::new();

    let Some(list) = args.list() else {
        cx.emit_err(session_diagnostics::ExpectsFeatureList {
            span: cx.attr_span,
            name: symbol.to_ident_string(),
        });
        return res;
    };

    for param in list.mixed() {
        let param_span = param.span();
        if let Some(ident) = param.meta_item().and_then(|i| i.path().word()) {
            res.push(ident.name);
        } else {
            cx.emit_err(session_diagnostics::ExpectsFeatures {
                span: param_span,
                name: symbol.to_ident_string(),
            });
        }
    }

    res
}
