//! lint on C-like enums that are `repr(isize/usize)` and have values that
//! don't fit into an `i32`

use rustc::lint::*;
use rustc::middle::const_val::ConstVal;
use rustc_const_math::*;
use rustc::hir::*;
use rustc::ty;
use rustc::traits::Reveal;
use rustc::ty::subst::Substs;
use utils::span_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.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// #[repr(usize)]
/// enum NonPortable {
///     X = 0x1_0000_0000,
///     Y = 0
/// }
/// ```
declare_lint! {
    pub ENUM_CLIKE_UNPORTABLE_VARIANT,
    Warn,
    "C-like enums that are `repr(isize/usize)` and have values that don't fit into an `i32`"
}

pub struct UnportableVariant;

impl LintPass for UnportableVariant {
    fn get_lints(&self) -> LintArray {
        lint_array!(ENUM_CLIKE_UNPORTABLE_VARIANT)
    }
}

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for UnportableVariant {
    #[allow(cast_possible_truncation, cast_sign_loss)]
    fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx Item) {
        if let ItemEnum(ref def, _) = item.node {
            for var in &def.variants {
                let variant = &var.node;
                if let Some(body_id) = variant.disr_expr {
                    let expr = &cx.tcx.hir.body(body_id).value;
                    let did = cx.tcx.hir.body_owner_def_id(body_id);
                    let param_env = ty::ParamEnv::empty(Reveal::UserFacing);
                    let substs = Substs::identity_for_item(cx.tcx.global_tcx(), did);
                    let bad = match cx.tcx.at(expr.span).const_eval(
                        param_env.and((did, substs)),
                    ) {
                        Ok(ConstVal::Integral(Usize(Us64(i)))) => i as u32 as u64 != i,
                        Ok(ConstVal::Integral(Isize(Is64(i)))) => i as i32 as i64 != i,
                        _ => false,
                    };
                    if bad {
                        span_lint(
                            cx,
                            ENUM_CLIKE_UNPORTABLE_VARIANT,
                            var.span,
                            "Clike enum variant discriminant is not portable to 32-bit targets",
                        );
                    }
                }
            }
        }
    }
}