rust/compiler/rustc_const_eval/src/const_eval/mod.rs

// Not in interpret to make sure we do not use private implementation details

use rustc_hir::Mutability;
use rustc_middle::mir;
use rustc_middle::mir::interpret::{EvalToValTreeResult, GlobalId};
use rustc_middle::ty::{self, TyCtxt};
use rustc_span::{source_map::DUMMY_SP, symbol::Symbol};

use crate::interpret::{
    intern_const_alloc_recursive, ConstValue, InternKind, InterpCx, InterpResult, MemPlaceMeta,
    Scalar,
};

mod error;
mod eval_queries;
mod fn_queries;
mod machine;
mod valtrees;

pub use error::*;
pub use eval_queries::*;
pub use fn_queries::*;
pub use machine::*;
pub(crate) use valtrees::{const_to_valtree_inner, valtree_to_const_value};

pub(crate) fn const_caller_location(
    tcx: TyCtxt<'_>,
    (file, line, col): (Symbol, u32, u32),
) -> ConstValue<'_> {
    trace!("const_caller_location: {}:{}:{}", file, line, col);
    let mut ecx = mk_eval_cx(tcx, DUMMY_SP, ty::ParamEnv::reveal_all(), false);

    let loc_place = ecx.alloc_caller_location(file, line, col);
    if intern_const_alloc_recursive(&mut ecx, InternKind::Constant, &loc_place).is_err() {
        bug!("intern_const_alloc_recursive should not error in this case")
    }
    ConstValue::Scalar(Scalar::from_maybe_pointer(loc_place.ptr, &tcx))
}

// We forbid type-level constants that contain more than `VALTREE_MAX_NODES` nodes.
const VALTREE_MAX_NODES: usize = 100000;

pub(crate) enum ValTreeCreationError {
    NodesOverflow,
    NonSupportedType,
    Other,
}
pub(crate) type ValTreeCreationResult<'tcx> = Result<ty::ValTree<'tcx>, ValTreeCreationError>;

/// Evaluates a constant and turns it into a type-level constant value.
pub(crate) fn eval_to_valtree<'tcx>(
    tcx: TyCtxt<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    cid: GlobalId<'tcx>,
) -> EvalToValTreeResult<'tcx> {
    let const_alloc = tcx.eval_to_allocation_raw(param_env.and(cid))?;

    // FIXME Need to provide a span to `eval_to_valtree`
    let ecx = mk_eval_cx(
        tcx, DUMMY_SP, param_env,
        // It is absolutely crucial for soundness that
        // we do not read from static items or other mutable memory.
        false,
    );
    let place = ecx.raw_const_to_mplace(const_alloc).unwrap();
    debug!(?place);

    let mut num_nodes = 0;
    let valtree_result = const_to_valtree_inner(&ecx, &place, &mut num_nodes);

    match valtree_result {
        Ok(valtree) => Ok(Some(valtree)),
        Err(err) => {
            let did = cid.instance.def_id();
            let s = cid.display(tcx);
            match err {
                ValTreeCreationError::NodesOverflow => {
                    let msg = format!("maximum number of nodes exceeded in constant {}", &s);
                    let mut diag = match tcx.hir().span_if_local(did) {
                        Some(span) => tcx.sess.struct_span_err(span, &msg),
                        None => tcx.sess.struct_err(&msg),
                    };
                    diag.emit();

                    Ok(None)
                }
                ValTreeCreationError::NonSupportedType | ValTreeCreationError::Other => Ok(None),
            }
        }
    }
}

#[instrument(skip(tcx), level = "debug")]
pub(crate) fn try_destructure_mir_constant<'tcx>(
    tcx: TyCtxt<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    val: mir::ConstantKind<'tcx>,
) -> InterpResult<'tcx, mir::DestructuredMirConstant<'tcx>> {
    trace!("destructure_mir_constant: {:?}", val);
    let ecx = mk_eval_cx(tcx, DUMMY_SP, param_env, false);
    let op = ecx.mir_const_to_op(&val, None)?;

    // We go to `usize` as we cannot allocate anything bigger anyway.
    let (field_count, variant, down) = match val.ty().kind() {
        ty::Array(_, len) => (len.eval_usize(tcx, param_env) as usize, None, op),
        ty::Adt(def, _) if def.variants().is_empty() => {
            throw_ub!(Unreachable)
        }
        ty::Adt(def, _) => {
            let variant = ecx.read_discriminant(&op)?.1;
            let down = ecx.operand_downcast(&op, variant)?;
            (def.variants()[variant].fields.len(), Some(variant), down)
        }
        ty::Tuple(substs) => (substs.len(), None, op),
        _ => bug!("cannot destructure mir constant {:?}", val),
    };

    let fields_iter = (0..field_count)
        .map(|i| {
            let field_op = ecx.operand_field(&down, i)?;
            let val = op_to_const(&ecx, &field_op);
            Ok(mir::ConstantKind::Val(val, field_op.layout.ty))
        })
        .collect::<InterpResult<'tcx, Vec<_>>>()?;
    let fields = tcx.arena.alloc_from_iter(fields_iter);

    Ok(mir::DestructuredMirConstant { variant, fields })
}

#[instrument(skip(tcx), level = "debug")]
pub(crate) fn deref_mir_constant<'tcx>(
    tcx: TyCtxt<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    val: mir::ConstantKind<'tcx>,
) -> mir::ConstantKind<'tcx> {
    let ecx = mk_eval_cx(tcx, DUMMY_SP, param_env, false);
    let op = ecx.mir_const_to_op(&val, None).unwrap();
    let mplace = ecx.deref_operand(&op).unwrap();
    if let Some(alloc_id) = mplace.ptr.provenance {
        assert_eq!(
            tcx.global_alloc(alloc_id).unwrap_memory().0.0.mutability,
            Mutability::Not,
            "deref_mir_constant cannot be used with mutable allocations as \
            that could allow pattern matching to observe mutable statics",
        );
    }

    let ty = match mplace.meta {
        MemPlaceMeta::None => mplace.layout.ty,
        // In case of unsized types, figure out the real type behind.
        MemPlaceMeta::Meta(scalar) => match mplace.layout.ty.kind() {
            ty::Str => bug!("there's no sized equivalent of a `str`"),
            ty::Slice(elem_ty) => tcx.mk_array(*elem_ty, scalar.to_machine_usize(&tcx).unwrap()),
            _ => bug!(
                "type {} should not have metadata, but had {:?}",
                mplace.layout.ty,
                mplace.meta
            ),
        },
    };

    mir::ConstantKind::Val(op_to_const(&ecx, &mplace.into()), ty)
}