rustc_const_eval/const_eval/

eval_queries.rs

use std::sync::atomic::Ordering::Relaxed;

use either::{Left, Right};
use rustc_abi::{self as abi, BackendRepr};
use rustc_hir::def::DefKind;
use rustc_middle::mir::interpret::{AllocId, ErrorHandled, InterpErrorInfo, ReportedErrorInfo};
use rustc_middle::mir::{self, ConstAlloc, ConstValue};
use rustc_middle::query::TyCtxtAt;
use rustc_middle::ty::layout::{HasTypingEnv, TyAndLayout};
use rustc_middle::ty::print::with_no_trimmed_paths;
use rustc_middle::ty::{self, Ty, TyCtxt};
use rustc_middle::{bug, throw_inval};
use rustc_span::Span;
use rustc_span::def_id::LocalDefId;
use tracing::{debug, instrument, trace};

use super::{CanAccessMutGlobal, CompileTimeInterpCx, CompileTimeMachine};
use crate::const_eval::CheckAlignment;
use crate::interpret::{
    CtfeValidationMode, GlobalId, Immediate, InternError, InternKind, InterpCx, InterpErrorKind,
    InterpResult, MPlaceTy, MemoryKind, OpTy, RefTracking, ReturnContinuation, create_static_alloc,
    intern_const_alloc_recursive, interp_ok, throw_exhaust,
};
use crate::{CTRL_C_RECEIVED, errors};

fn setup_for_eval<'tcx>(
    ecx: &mut CompileTimeInterpCx<'tcx>,
    cid: GlobalId<'tcx>,
    layout: TyAndLayout<'tcx>,
) -> InterpResult<'tcx, (InternKind, MPlaceTy<'tcx>)> {
    let tcx = *ecx.tcx;
    assert!(
        cid.promoted.is_some()
            || matches!(
                ecx.tcx.def_kind(cid.instance.def_id()),
                DefKind::Const { .. }
                    | DefKind::Static { .. }
                    | DefKind::ConstParam
                    | DefKind::AnonConst
                    | DefKind::InlineConst
                    | DefKind::AssocConst { .. }
            ),
        "Unexpected DefKind: {:?}",
        ecx.tcx.def_kind(cid.instance.def_id())
    );
    assert!(layout.is_sized());

    let intern_kind = if cid.promoted.is_some() {
        InternKind::Promoted
    } else {
        match tcx.static_mutability(cid.instance.def_id()) {
            Some(m) => InternKind::Static(m),
            None => InternKind::Constant,
        }
    };

    let return_place = if let InternKind::Static(_) = intern_kind {
        create_static_alloc(ecx, cid.instance.def_id().expect_local(), layout)
    } else {
        ecx.allocate(layout, MemoryKind::Stack)
    };

    return_place.map(|ret| (intern_kind, ret))
}

#[instrument(level = "trace", skip(ecx, body))]
fn eval_body_using_ecx<'tcx, R: InterpretationResult<'tcx>>(
    ecx: &mut CompileTimeInterpCx<'tcx>,
    cid: GlobalId<'tcx>,
    body: &'tcx mir::Body<'tcx>,
) -> InterpResult<'tcx, R> {
    let tcx = *ecx.tcx;
    let layout = ecx.layout_of(body.bound_return_ty().instantiate(tcx, cid.instance.args))?;
    let (intern_kind, ret) = setup_for_eval(ecx, cid, layout)?;

    trace!(
        "eval_body_using_ecx: pushing stack frame for global: {}{}",
        with_no_trimmed_paths!(ecx.tcx.def_path_str(cid.instance.def_id())),
        cid.promoted.map_or_else(String::new, |p| format!("::{p:?}"))
    );

    // This can't use `init_stack_frame` since `body` is not a function,
    // so computing its ABI would fail. It's also not worth it since there are no arguments to pass.
    ecx.push_stack_frame_raw(
        cid.instance,
        body,
        &ret.clone().into(),
        ReturnContinuation::Stop { cleanup: false },
    )?;
    ecx.push_stack_frame_done()?;

    // The main interpreter loop.
    while ecx.step()? {
        if CTRL_C_RECEIVED.load(Relaxed) {
            throw_exhaust!(Interrupted);
        }
    }

    intern_and_validate(ecx, cid, intern_kind, ret)
}

#[instrument(level = "trace", skip(ecx))]
fn eval_trivial_const_using_ecx<'tcx, R: InterpretationResult<'tcx>>(
    ecx: &mut CompileTimeInterpCx<'tcx>,
    cid: GlobalId<'tcx>,
    val: ConstValue,
    ty: Ty<'tcx>,
) -> InterpResult<'tcx, R> {
    let layout = ecx.layout_of(ty)?;
    let (intern_kind, return_place) = setup_for_eval(ecx, cid, layout)?;

    let opty = ecx.const_val_to_op(val, ty, Some(layout))?;
    ecx.copy_op(&opty, &return_place)?;

    intern_and_validate(ecx, cid, intern_kind, return_place)
}

fn intern_and_validate<'tcx, R: InterpretationResult<'tcx>>(
    ecx: &mut CompileTimeInterpCx<'tcx>,
    cid: GlobalId<'tcx>,
    intern_kind: InternKind,
    ret: MPlaceTy<'tcx>,
) -> InterpResult<'tcx, R> {
    // Intern the result
    let intern_result = intern_const_alloc_recursive(ecx, intern_kind, &ret);

    // Since evaluation had no errors, validate the resulting constant.
    const_validate_mplace(ecx, &ret, cid)?;

    // Only report this after validation, as validation produces much better diagnostics.
    // FIXME: ensure validation always reports this and stop making interning care about it.

    match intern_result {
        Ok(()) => {}
        Err(InternError::DanglingPointer) => {
            throw_inval!(AlreadyReported(ReportedErrorInfo::non_const_eval_error(
                ecx.tcx
                    .dcx()
                    .emit_err(errors::DanglingPtrInFinal { span: ecx.tcx.span, kind: intern_kind }),
            )));
        }
        Err(InternError::BadMutablePointer) => {
            throw_inval!(AlreadyReported(ReportedErrorInfo::non_const_eval_error(
                ecx.tcx
                    .dcx()
                    .emit_err(errors::MutablePtrInFinal { span: ecx.tcx.span, kind: intern_kind }),
            )));
        }
        Err(InternError::ConstAllocNotGlobal) => {
            throw_inval!(AlreadyReported(ReportedErrorInfo::non_const_eval_error(
                ecx.tcx.dcx().emit_err(errors::ConstHeapPtrInFinal { span: ecx.tcx.span }),
            )));
        }
        Err(InternError::PartialPointer) => {
            throw_inval!(AlreadyReported(ReportedErrorInfo::non_const_eval_error(
                ecx.tcx
                    .dcx()
                    .emit_err(errors::PartialPtrInFinal { span: ecx.tcx.span, kind: intern_kind }),
            )));
        }
    }

    interp_ok(R::make_result(ret, ecx))
}

/// The `InterpCx` is only meant to be used to do field and index projections into constants for
/// `simd_shuffle` and const patterns in match arms.
///
/// This should *not* be used to do any actual interpretation. In particular, alignment checks are
/// turned off!
///
/// The function containing the `match` that is currently being analyzed may have generic bounds
/// that inform us about the generic bounds of the constant. E.g., using an associated constant
/// of a function's generic parameter will require knowledge about the bounds on the generic
/// parameter. These bounds are passed to `mk_eval_cx` via the `ParamEnv` argument.
pub(crate) fn mk_eval_cx_to_read_const_val<'tcx>(
    tcx: TyCtxt<'tcx>,
    root_span: Span,
    typing_env: ty::TypingEnv<'tcx>,
    can_access_mut_global: CanAccessMutGlobal,
) -> CompileTimeInterpCx<'tcx> {
    debug!("mk_eval_cx: {:?}", typing_env);
    InterpCx::new(
        tcx,
        root_span,
        typing_env,
        CompileTimeMachine::new(can_access_mut_global, CheckAlignment::No),
    )
}

/// Create an interpreter context to inspect the given `ConstValue`.
/// Returns both the context and an `OpTy` that represents the constant.
pub fn mk_eval_cx_for_const_val<'tcx>(
    tcx: TyCtxtAt<'tcx>,
    typing_env: ty::TypingEnv<'tcx>,
    val: mir::ConstValue,
    ty: Ty<'tcx>,
) -> Option<(CompileTimeInterpCx<'tcx>, OpTy<'tcx>)> {
    let ecx = mk_eval_cx_to_read_const_val(tcx.tcx, tcx.span, typing_env, CanAccessMutGlobal::No);
    // FIXME: is it a problem to discard the error here?
    let op = ecx.const_val_to_op(val, ty, None).discard_err()?;
    Some((ecx, op))
}

/// This function converts an interpreter value into a MIR constant.
///
/// The `for_diagnostics` flag turns the usual rules for returning `ConstValue::Scalar` into a
/// best-effort attempt. This is not okay for use in const-eval sine it breaks invariants rustc
/// relies on, but it is okay for diagnostics which will just give up gracefully when they
/// encounter an `Indirect` they cannot handle.
#[instrument(skip(ecx), level = "debug")]
pub(super) fn op_to_const<'tcx>(
    ecx: &CompileTimeInterpCx<'tcx>,
    op: &OpTy<'tcx>,
    for_diagnostics: bool,
) -> ConstValue {
    // Handle ZST consistently and early.
    if op.layout.is_zst() {
        return ConstValue::ZeroSized;
    }

    // All scalar types should be stored as `ConstValue::Scalar`. This is needed to make
    // `ConstValue::try_to_scalar` efficient; we want that to work for *all* constants of scalar
    // type (it's used throughout the compiler and having it work just on literals is not enough)
    // and we want it to be fast (i.e., don't go to an `Allocation` and reconstruct the `Scalar`
    // from its byte-serialized form).
    let force_as_immediate = match op.layout.backend_repr {
        BackendRepr::Scalar(abi::Scalar::Initialized { .. }) => true,
        // We don't *force* `ConstValue::Slice` for `ScalarPair`. This has the advantage that if the
        // input `op` is a place, then turning it into a `ConstValue` and back into a `OpTy` will
        // not have to generate any duplicate allocations (we preserve the original `AllocId` in
        // `ConstValue::Indirect`). It means accessing the contents of a slice can be slow (since
        // they can be stored as `ConstValue::Indirect`), but that's not relevant since we barely
        // ever have to do this. (`try_get_slice_bytes_for_diagnostics` exists to provide this
        // functionality.)
        _ => false,
    };
    let immediate = if force_as_immediate {
        match ecx.read_immediate(op).report_err() {
            Ok(imm) => Right(imm),
            Err(err) => {
                if for_diagnostics {
                    // This discard the error, but for diagnostics that's okay.
                    op.as_mplace_or_imm()
                } else {
                    panic!("normalization works on validated constants: {err:?}")
                }
            }
        }
    } else {
        op.as_mplace_or_imm()
    };

    debug!(?immediate);

    match immediate {
        Left(ref mplace) => {
            let (prov, offset) =
                mplace.ptr().into_pointer_or_addr().unwrap().prov_and_relative_offset();
            let alloc_id = prov.alloc_id();
            ConstValue::Indirect { alloc_id, offset }
        }
        // see comment on `let force_as_immediate` above
        Right(imm) => match *imm {
            Immediate::Scalar(x) => ConstValue::Scalar(x),
            Immediate::ScalarPair(a, b) => {
                debug!("ScalarPair(a: {:?}, b: {:?})", a, b);
                // This codepath solely exists for `valtree_to_const_value` to not need to generate
                // a `ConstValue::Indirect` for wide references, so it is tightly restricted to just
                // that case.
                let pointee_ty = imm.layout.ty.builtin_deref(false).unwrap(); // `false` = no raw ptrs
                debug_assert!(
                    matches!(
                        ecx.tcx.struct_tail_for_codegen(pointee_ty, ecx.typing_env()).kind(),
                        ty::Str | ty::Slice(..),
                    ),
                    "`ConstValue::Slice` is for slice-tailed types only, but got {}",
                    imm.layout.ty,
                );
                let msg = "`op_to_const` on an immediate scalar pair must only be used on slice references to the beginning of an actual allocation";
                let ptr = a.to_pointer(ecx).expect(msg);
                let (prov, offset) =
                    ptr.into_pointer_or_addr().expect(msg).prov_and_relative_offset();
                let alloc_id = prov.alloc_id();
                assert!(offset == abi::Size::ZERO, "{}", msg);
                let meta = b.to_target_usize(ecx).expect(msg);
                ConstValue::Slice { alloc_id, meta }
            }
            Immediate::Uninit => bug!("`Uninit` is not a valid value for {}", op.layout.ty),
        },
    }
}

#[instrument(skip(tcx), level = "debug", ret)]
pub(crate) fn turn_into_const_value<'tcx>(
    tcx: TyCtxt<'tcx>,
    constant: ConstAlloc<'tcx>,
    key: ty::PseudoCanonicalInput<'tcx, GlobalId<'tcx>>,
) -> ConstValue {
    let cid = key.value;
    let def_id = cid.instance.def.def_id();
    let is_static = tcx.is_static(def_id);
    // This is just accessing an already computed constant, so no need to check alignment here.
    let ecx = mk_eval_cx_to_read_const_val(
        tcx,
        tcx.def_span(key.value.instance.def_id()),
        key.typing_env,
        CanAccessMutGlobal::from(is_static),
    );

    let mplace = ecx.raw_const_to_mplace(constant).expect(
        "can only fail if layout computation failed, \
        which should have given a good error before ever invoking this function",
    );
    assert!(
        !is_static || cid.promoted.is_some(),
        "the `eval_to_const_value_raw` query should not be used for statics, use `eval_to_allocation` instead"
    );

    // Turn this into a proper constant.
    op_to_const(&ecx, &mplace.into(), /* for diagnostics */ false)
}

#[instrument(skip(tcx), level = "debug")]
pub fn eval_to_const_value_raw_provider<'tcx>(
    tcx: TyCtxt<'tcx>,
    key: ty::PseudoCanonicalInput<'tcx, GlobalId<'tcx>>,
) -> ::rustc_middle::mir::interpret::EvalToConstValueResult<'tcx> {
    if let Some((value, _ty)) = tcx.trivial_const(key.value.instance.def_id()) {
        return Ok(value);
    }
    tcx.eval_to_allocation_raw(key).map(|val| turn_into_const_value(tcx, val, key))
}

#[instrument(skip(tcx), level = "debug")]
pub fn eval_static_initializer_provider<'tcx>(
    tcx: TyCtxt<'tcx>,
    def_id: LocalDefId,
) -> ::rustc_middle::mir::interpret::EvalStaticInitializerRawResult<'tcx> {
    assert!(tcx.is_static(def_id.to_def_id()));

    let instance = ty::Instance::mono(tcx, def_id.to_def_id());
    let cid = rustc_middle::mir::interpret::GlobalId { instance, promoted: None };
    eval_in_interpreter(tcx, cid, ty::TypingEnv::fully_monomorphized())
}

pub trait InterpretationResult<'tcx> {
    /// This function takes the place where the result of the evaluation is stored
    /// and prepares it for returning it in the appropriate format needed by the specific
    /// evaluation query.
    fn make_result(
        mplace: MPlaceTy<'tcx>,
        ecx: &mut InterpCx<'tcx, CompileTimeMachine<'tcx>>,
    ) -> Self;
}

impl<'tcx> InterpretationResult<'tcx> for ConstAlloc<'tcx> {
    fn make_result(
        mplace: MPlaceTy<'tcx>,
        _ecx: &mut InterpCx<'tcx, CompileTimeMachine<'tcx>>,
    ) -> Self {
        ConstAlloc { alloc_id: mplace.ptr().provenance.unwrap().alloc_id(), ty: mplace.layout.ty }
    }
}

#[instrument(skip(tcx), level = "debug")]
pub fn eval_to_allocation_raw_provider<'tcx>(
    tcx: TyCtxt<'tcx>,
    key: ty::PseudoCanonicalInput<'tcx, GlobalId<'tcx>>,
) -> ::rustc_middle::mir::interpret::EvalToAllocationRawResult<'tcx> {
    // This shouldn't be used for statics, since statics are conceptually places,
    // not values -- so what we do here could break pointer identity.
    assert!(key.value.promoted.is_some() || !tcx.is_static(key.value.instance.def_id()));

    if cfg!(debug_assertions) {
        match key.typing_env.typing_mode() {
            ty::TypingMode::PostAnalysis => {}
            ty::TypingMode::Coherence
            | ty::TypingMode::Analysis { .. }
            | ty::TypingMode::Borrowck { .. }
            | ty::TypingMode::PostBorrowckAnalysis { .. } => {
                bug!(
                    "Const eval should always happens in PostAnalysis mode. See the comment in `InterpCx::new` for more details."
                )
            }
        }

        // Make sure we format the instance even if we do not print it.
        // This serves as a regression test against an ICE on printing.
        // The next two lines concatenated contain some discussion:
        // https://rust-lang.zulipchat.com/#narrow/stream/146212-t-compiler.2Fconst-eval/
        // subject/anon_const_instance_printing/near/135980032
        let instance = with_no_trimmed_paths!(key.value.instance.to_string());
        trace!("const eval: {:?} ({})", key, instance);
    }

    eval_in_interpreter(tcx, key.value, key.typing_env)
}

fn eval_in_interpreter<'tcx, R: InterpretationResult<'tcx>>(
    tcx: TyCtxt<'tcx>,
    cid: GlobalId<'tcx>,
    typing_env: ty::TypingEnv<'tcx>,
) -> Result<R, ErrorHandled> {
    let def = cid.instance.def.def_id();
    // `type const` don't have bodys
    debug_assert!(!tcx.is_type_const(def), "CTFE tried to evaluate type-const: {:?}", def);

    let is_static = tcx.is_static(def);
    let mut ecx = InterpCx::new(
        tcx,
        tcx.def_span(def),
        typing_env,
        // Statics (and promoteds inside statics) may access mutable global memory, because unlike consts
        // they do not have to behave "as if" they were evaluated at runtime.
        // For consts however we want to ensure they behave "as if" they were evaluated at runtime,
        // so we have to reject reading mutable global memory.
        CompileTimeMachine::new(CanAccessMutGlobal::from(is_static), CheckAlignment::Error),
    );

    let result = if let Some((value, ty)) = tcx.trivial_const(def) {
        eval_trivial_const_using_ecx(&mut ecx, cid, value, ty)
    } else {
        ecx.load_mir(cid.instance.def, cid.promoted)
            .and_then(|body| eval_body_using_ecx(&mut ecx, cid, body))
    };
    result.report_err().map_err(|error| report_eval_error(&ecx, cid, error))
}

#[inline(always)]
fn const_validate_mplace<'tcx>(
    ecx: &mut InterpCx<'tcx, CompileTimeMachine<'tcx>>,
    mplace: &MPlaceTy<'tcx>,
    cid: GlobalId<'tcx>,
) -> Result<(), ErrorHandled> {
    let alloc_id = mplace.ptr().provenance.unwrap().alloc_id();
    let mut ref_tracking = RefTracking::new(mplace.clone(), mplace.layout.ty);
    let mut inner = false;
    while let Some((mplace, path)) = ref_tracking.next() {
        let mode = match ecx.tcx.static_mutability(cid.instance.def_id()) {
            _ if cid.promoted.is_some() => CtfeValidationMode::Promoted,
            Some(mutbl) => CtfeValidationMode::Static { mutbl }, // a `static`
            None => {
                // This is a normal `const` (not promoted).
                // The outermost allocation is always only copied, so having `UnsafeCell` in there
                // is okay despite them being in immutable memory.
                CtfeValidationMode::Const { allow_immutable_unsafe_cell: !inner }
            }
        };
        ecx.const_validate_operand(&mplace.into(), path, &mut ref_tracking, mode)
            .report_err()
            // Instead of just reporting the `InterpError` via the usual machinery, we give a more targeted
            // error about the validation failure.
            .map_err(|error| report_validation_error(&ecx, cid, error, alloc_id))?;
        inner = true;
    }

    Ok(())
}

#[inline(never)]
fn report_eval_error<'tcx>(
    ecx: &InterpCx<'tcx, CompileTimeMachine<'tcx>>,
    cid: GlobalId<'tcx>,
    error: InterpErrorInfo<'tcx>,
) -> ErrorHandled {
    let (error, backtrace) = error.into_parts();
    backtrace.print_backtrace();

    super::report(ecx, error, |diag, span, frames| {
        let num_frames = frames.len();
        diag.span_label(
            span,
            format!(
                "evaluation of `{instance}` failed {where_}",
                instance = with_no_trimmed_paths!(cid.instance.to_string()),
                where_ = if num_frames == 0 { "here" } else { "inside this call" },
            ),
        );
        for frame in frames {
            diag.subdiagnostic(frame);
        }
    })
}

#[inline(never)]
fn report_validation_error<'tcx>(
    ecx: &InterpCx<'tcx, CompileTimeMachine<'tcx>>,
    cid: GlobalId<'tcx>,
    error: InterpErrorInfo<'tcx>,
    alloc_id: AllocId,
) -> ErrorHandled {
    if !matches!(error.kind(), InterpErrorKind::UndefinedBehavior(_)) {
        // Some other error happened during validation, e.g. an unsupported operation.
        return report_eval_error(ecx, cid, error);
    }

    let (error, backtrace) = error.into_parts();
    backtrace.print_backtrace();

    let bytes = ecx.print_alloc_bytes_for_diagnostics(alloc_id);
    let info = ecx.get_alloc_info(alloc_id);
    let raw_bytes =
        errors::RawBytesNote { size: info.size.bytes(), align: info.align.bytes(), bytes };

    crate::const_eval::report(ecx, error, move |diag, span, frames| {
        diag.span_label(span, "it is undefined behavior to use this value");
        diag.note("the rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.");
        assert!(frames.is_empty()); // we just report validation errors for the final const here
        diag.subdiagnostic(raw_bytes);
    })
}