//! 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_hir::def_id::CRATE_DEF_ID;
use rustc_hir::{Stability, StabilityLevel};

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 DocFolder for StabilityPropagator<'_, '_> {
    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 item_stability = self.cx.tcx.lookup_stability(def_id);
                let inline_stability =
                    item.inline_stmt_id.and_then(|did| self.cx.tcx.lookup_stability(did));
                let is_glob_export = item.inline_stmt_id.map(|id| {
                    let hir_id = self.cx.tcx.local_def_id_to_hir_id(id);
                    matches!(
                        self.cx.tcx.hir_node(hir_id),
                        rustc_hir::Node::Item(rustc_hir::Item {
                            kind: rustc_hir::ItemKind::Use(_, rustc_hir::UseKind::Glob),
                            ..
                        })
                    )
                });
                let own_stability = if let Some(item_stab) = item_stability
                    && let StabilityLevel::Stable { since: _, allowed_through_unstable_modules } =
                        item_stab.level
                    && let Some(mut inline_stab) = inline_stability
                    && let StabilityLevel::Stable {
                        since: inline_since,
                        allowed_through_unstable_modules: _,
                    } = inline_stab.level
                    && let Some(is_global_export) = is_glob_export
                    && !is_global_export
                {
                    inline_stab.level = StabilityLevel::Stable {
                        since: inline_since,
                        allowed_through_unstable_modules,
                    };
                    Some(inline_stab)
                } else {
                    item_stability
                };

                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::RequiredMethodItem(..)
                    | ItemKind::MethodItem(..)
                    | ItemKind::RequiredAssocConstItem(..)
                    | ItemKind::ProvidedAssocConstItem(..)
                    | ItemKind::ImplAssocConstItem(..)
                    | ItemKind::RequiredAssocTypeItem(..)
                    | ItemKind::AssocTypeItem(..)
                    | ItemKind::PrimitiveItem(..)
                    | ItemKind::KeywordItem
                    | ItemKind::AttributeItem => 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: None } =
            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 if let Some(mut own_stab) = own_stability
        && let StabilityLevel::Stable { since, allowed_through_unstable_modules: Some(_) } =
            own_stab.level
        && parent_stability.is_some_and(|stab| stab.is_stable())
    {
        // this property does not apply transitively through re-exports
        own_stab.level = StabilityLevel::Stable { since, allowed_through_unstable_modules: None };
        Some(own_stab)
    } else {
        own_stability
    }
}