use clippy_utils::consts::{Constant, mir_to_const};
use clippy_utils::diagnostics::span_lint;
use rustc_hir::{Item, ItemKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty::util::IntTypeExt;
use rustc_middle::ty::{self, IntTy, UintTy};
use rustc_session::declare_lint_pass;

declare_clippy_lint! {
    /// ### What it does
    /// Checks for C-like enumerations that are
    /// `repr(isize/usize)` and have values that don't fit into an `i32`.
    ///
    /// ### Why is this bad?
    /// This will truncate the variant value on 32 bit
    /// architectures, but works fine on 64 bit.
    ///
    /// ### Example
    /// ```no_run
    /// # #[cfg(target_pointer_width = "64")]
    /// #[repr(usize)]
    /// enum NonPortable {
    ///     X = 0x1_0000_0000,
    ///     Y = 0,
    /// }
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub ENUM_CLIKE_UNPORTABLE_VARIANT,
    correctness,
    "C-like enums that are `repr(isize/usize)` and have values that don't fit into an `i32`"
}

declare_lint_pass!(UnportableVariant => [ENUM_CLIKE_UNPORTABLE_VARIANT]);

impl<'tcx> LateLintPass<'tcx> for UnportableVariant {
    #[expect(clippy::cast_possible_wrap)]
    fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'_>) {
        if cx.tcx.data_layout.pointer_size().bits() != 64 {
            return;
        }
        if let ItemKind::Enum(_, _, def) = &item.kind {
            for var in def.variants {
                if let Some(anon_const) = &var.disr_expr {
                    let def_id = cx.tcx.hir_body_owner_def_id(anon_const.body);
                    let mut ty = cx.tcx.type_of(def_id.to_def_id()).instantiate_identity();
                    let constant = cx.tcx.const_eval_poly(def_id.to_def_id()).ok();
                    if let Some(Constant::Int(val)) = constant.and_then(|c| mir_to_const(cx.tcx, c, ty)) {
                        if let ty::Adt(adt, _) = ty.kind()
                            && adt.is_enum()
                        {
                            ty = adt.repr().discr_type().to_ty(cx.tcx);
                        }
                        match ty.kind() {
                            ty::Int(IntTy::Isize) => {
                                let val = ((val as i128) << 64) >> 64;
                                if i32::try_from(val).is_ok() {
                                    continue;
                                }
                            },
                            ty::Uint(UintTy::Usize) if val > u128::from(u32::MAX) => {},
                            _ => continue,
                        }
                        span_lint(
                            cx,
                            ENUM_CLIKE_UNPORTABLE_VARIANT,
                            var.span,
                            "C-like enum variant discriminant is not portable to 32-bit targets",
                        );
                    }
                }
            }
        }
    }
}
