rust/compiler/rustc_trait_selection/src/traits/const_evaluatable.rs

use rustc_hir::def::DefKind;
use rustc_infer::infer::InferCtxt;
use rustc_middle::mir::interpret::ErrorHandled;
use rustc_middle::ty::subst::SubstsRef;
use rustc_middle::ty::{self, TypeFoldable};
use rustc_session::lint;
use rustc_span::def_id::DefId;
use rustc_span::Span;

pub fn is_const_evaluatable<'cx, 'tcx>(
    infcx: &InferCtxt<'cx, 'tcx>,
    def: ty::WithOptConstParam<DefId>,
    substs: SubstsRef<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    span: Span,
) -> Result<(), ErrorHandled> {
    let future_compat_lint = || {
        if let Some(local_def_id) = def.did.as_local() {
            infcx.tcx.struct_span_lint_hir(
                lint::builtin::CONST_EVALUATABLE_UNCHECKED,
                infcx.tcx.hir().local_def_id_to_hir_id(local_def_id),
                span,
                |err| {
                    err.build("cannot use constants which depend on generic parameters in types")
                        .emit();
                },
            );
        }
    };

    // FIXME: We should only try to evaluate a given constant here if it is fully concrete
    // as we don't want to allow things like `[u8; std::mem::size_of::<*mut T>()]`.
    //
    // We previously did not check this, so we only emit a future compat warning if
    // const evaluation succeeds and the given constant is still polymorphic for now
    // and hopefully soon change this to an error.
    //
    // See #74595 for more details about this.
    let concrete = infcx.const_eval_resolve(param_env, def, substs, None, Some(span));

    let def_kind = infcx.tcx.def_kind(def.did);
    match def_kind {
        DefKind::AnonConst => {
            let mir_body = if let Some(def) = def.as_const_arg() {
                infcx.tcx.optimized_mir_of_const_arg(def)
            } else {
                infcx.tcx.optimized_mir(def.did)
            };
            if mir_body.is_polymorphic && concrete.is_ok() {
                future_compat_lint();
            }
        }
        _ => {
            if substs.has_param_types_or_consts() && concrete.is_ok() {
                future_compat_lint();
            }
        }
    }

    concrete.map(drop)
}