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//! Things related to generics in the next-trait-solver.

use hir_def::{
    ConstParamId, GenericDefId, GenericParamId, LifetimeParamId, TypeOrConstParamId, TypeParamId,
    hir::generics::{GenericParams, TypeOrConstParamData},
};
use rustc_type_ir::inherent::GenericsOf;

use crate::generics::parent_generic_def;

use super::SolverDefId;

use super::DbInterner;

pub(crate) fn generics(interner: DbInterner<'_>, def: SolverDefId) -> Generics {
    let mk_lt = |parent, index, local_id| {
        let id = GenericParamId::LifetimeParamId(LifetimeParamId { parent, local_id });
        GenericParamDef { index, id }
    };
    let mk_ty = |parent, index, local_id, p: &TypeOrConstParamData| {
        let id = TypeOrConstParamId { parent, local_id };
        let id = match p {
            TypeOrConstParamData::TypeParamData(_) => {
                GenericParamId::TypeParamId(TypeParamId::from_unchecked(id))
            }
            TypeOrConstParamData::ConstParamData(_) => {
                GenericParamId::ConstParamId(ConstParamId::from_unchecked(id))
            }
        };
        GenericParamDef { index, id }
    };
    let own_params_for_generic_params = |parent, params: &GenericParams| {
        let mut result = Vec::with_capacity(params.len());
        let mut type_and_consts = params.iter_type_or_consts();
        let mut index = 0;
        if let Some(self_param) = params.trait_self_param() {
            result.push(mk_ty(parent, 0, self_param, &params[self_param]));
            type_and_consts.next();
            index += 1;
        }
        result.extend(params.iter_lt().map(|(local_id, _data)| {
            let lt = mk_lt(parent, index, local_id);
            index += 1;
            lt
        }));
        result.extend(type_and_consts.map(|(local_id, data)| {
            let ty = mk_ty(parent, index, local_id, data);
            index += 1;
            ty
        }));
        result
    };

    let db = interner.db;
    let (parent, own_params) = match (def.try_into(), def) {
        (Ok(def), _) => (
            parent_generic_def(db, def),
            own_params_for_generic_params(def, &db.generic_params(def)),
        ),
        (_, SolverDefId::InternedOpaqueTyId(id)) => {
            match db.lookup_intern_impl_trait_id(id) {
                crate::ImplTraitId::ReturnTypeImplTrait(function_id, _) => {
                    // The opaque type itself does not have generics - only the parent function
                    (Some(GenericDefId::FunctionId(function_id)), vec![])
                }
                crate::ImplTraitId::TypeAliasImplTrait(type_alias_id, _) => {
                    (Some(type_alias_id.into()), Vec::new())
                }
            }
        }
        (_, SolverDefId::BuiltinDeriveImplId(id)) => {
            return crate::builtin_derive::generics_of(interner, id);
        }
        _ => panic!("No generics for {def:?}"),
    };
    let parent_generics = parent.map(|def| Box::new(generics(interner, def.into())));

    Generics {
        parent,
        parent_count: parent_generics.map_or(0, |g| g.parent_count + g.own_params.len()),
        own_params,
    }
}

#[derive(Debug)]
pub struct Generics {
    pub parent: Option<GenericDefId>,
    pub parent_count: usize,
    pub own_params: Vec<GenericParamDef>,
}

impl Generics {
    pub(crate) fn push_param(&mut self, id: GenericParamId) {
        let index = self.count() as u32;
        self.own_params.push(GenericParamDef { index, id });
    }
}

#[derive(Debug)]
pub struct GenericParamDef {
    index: u32,
    pub(crate) id: GenericParamId,
}

impl GenericParamDef {
    /// Returns the index of the param on the self generics only
    /// (i.e. not including parent generics)
    pub fn index(&self) -> u32 {
        self.index
    }
}

impl<'db> rustc_type_ir::inherent::GenericsOf<DbInterner<'db>> for Generics {
    fn count(&self) -> usize {
        self.parent_count + self.own_params.len()
    }
}