//! Propagates stability to child items.
//!
//! The purpose of this pass is to make items whose parents are "more unstable"
//! than the item itself inherit the parent's stability.
//! For example, [`core::error::Error`] is marked as stable since 1.0.0, but the
//! [`core::error`] module is marked as stable since 1.81.0, so we want to show
//! [`core::error::Error`] as stable since 1.81.0 as well.

use rustc_attr::{Stability, StabilityLevel};
use rustc_hir::def_id::CRATE_DEF_ID;

use crate::clean::{Crate, Item, ItemId, ItemKind};
use crate::core::DocContext;
use crate::fold::DocFolder;
use crate::passes::Pass;

pub(crate) const PROPAGATE_STABILITY: Pass = Pass {
    name: "propagate-stability",
    run: Some(propagate_stability),
    description: "propagates stability to child items",
};

pub(crate) fn propagate_stability(cr: Crate, cx: &mut DocContext<'_>) -> Crate {
    let crate_stability = cx.tcx.lookup_stability(CRATE_DEF_ID);
    StabilityPropagator { parent_stability: crate_stability, cx }.fold_crate(cr)
}

struct StabilityPropagator<'a, 'tcx> {
    parent_stability: Option<Stability>,
    cx: &'a mut DocContext<'tcx>,
}

impl<'a, 'tcx> DocFolder for StabilityPropagator<'a, 'tcx> {
    fn fold_item(&mut self, mut item: Item) -> Option<Item> {
        let parent_stability = self.parent_stability;

        let stability = match item.item_id {
            ItemId::DefId(def_id) => {
                let own_stability = self.cx.tcx.lookup_stability(def_id);

                let (ItemKind::StrippedItem(box kind) | kind) = &item.kind;
                match kind {
                    ItemKind::ExternCrateItem { .. }
                    | ItemKind::ImportItem(..)
                    | ItemKind::StructItem(..)
                    | ItemKind::UnionItem(..)
                    | ItemKind::EnumItem(..)
                    | ItemKind::FunctionItem(..)
                    | ItemKind::ModuleItem(..)
                    | ItemKind::TypeAliasItem(..)
                    | ItemKind::StaticItem(..)
                    | ItemKind::TraitItem(..)
                    | ItemKind::TraitAliasItem(..)
                    | ItemKind::StructFieldItem(..)
                    | ItemKind::VariantItem(..)
                    | ItemKind::ForeignFunctionItem(..)
                    | ItemKind::ForeignStaticItem(..)
                    | ItemKind::ForeignTypeItem
                    | ItemKind::MacroItem(..)
                    | ItemKind::ProcMacroItem(..)
                    | ItemKind::ConstantItem(..) => {
                        // If any of the item's parents was stabilized later or is still unstable,
                        // then use the parent's stability instead.
                        merge_stability(own_stability, parent_stability)
                    }

                    // Don't inherit the parent's stability for these items, because they
                    // are potentially accessible even if the parent is more unstable.
                    ItemKind::ImplItem(..)
                    | ItemKind::TyMethodItem(..)
                    | ItemKind::MethodItem(..)
                    | ItemKind::TyAssocConstItem(..)
                    | ItemKind::AssocConstItem(..)
                    | ItemKind::TyAssocTypeItem(..)
                    | ItemKind::AssocTypeItem(..)
                    | ItemKind::PrimitiveItem(..)
                    | ItemKind::KeywordItem => own_stability,

                    ItemKind::StrippedItem(..) => unreachable!(),
                }
            }
            ItemId::Auto { .. } | ItemId::Blanket { .. } => {
                // For now, we do now show stability for synthesized impls.
                None
            }
        };

        item.inner.stability = stability;
        self.parent_stability = stability;
        let item = self.fold_item_recur(item);
        self.parent_stability = parent_stability;

        Some(item)
    }
}

fn merge_stability(
    own_stability: Option<Stability>,
    parent_stability: Option<Stability>,
) -> Option<Stability> {
    if let Some(own_stab) = own_stability
        && let StabilityLevel::Stable { since: own_since, allowed_through_unstable_modules: false } =
            own_stab.level
        && let Some(parent_stab) = parent_stability
        && (parent_stab.is_unstable()
            || parent_stab.stable_since().is_some_and(|parent_since| parent_since > own_since))
    {
        parent_stability
    } else {
        own_stability
    }
}