compiler/rustc_trait_selection/src/traits/query/normalize.rs

//! Code for the 'normalization' query. This consists of a wrapper
//! which folds deeply, invoking the underlying
//! `normalize_projection_ty` query when it encounters projections.

use crate::infer::at::At;
use crate::infer::canonical::OriginalQueryValues;
use crate::infer::{InferCtxt, InferOk};
use crate::traits::error_reporting::TypeErrCtxtExt;
use crate::traits::project::{needs_normalization, BoundVarReplacer, PlaceholderReplacer};
use crate::traits::{ObligationCause, PredicateObligation, Reveal};
use rustc_data_structures::sso::SsoHashMap;
use rustc_data_structures::stack::ensure_sufficient_stack;
use rustc_infer::traits::Normalized;
use rustc_middle::ty::fold::{FallibleTypeFolder, TypeFoldable, TypeSuperFoldable};
use rustc_middle::ty::visit::{TypeSuperVisitable, TypeVisitable, TypeVisitableExt};
use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitor};
use rustc_span::DUMMY_SP;

use std::ops::ControlFlow;

use super::NoSolution;

pub use rustc_middle::traits::query::NormalizationResult;

pub trait QueryNormalizeExt<'tcx> {
    /// Normalize a value using the `QueryNormalizer`.
    ///
    /// This normalization should *only* be used when the projection does not
    /// have possible ambiguity or may not be well-formed.
    ///
    /// After codegen, when lifetimes do not matter, it is preferable to instead
    /// use [`TyCtxt::normalize_erasing_regions`], which wraps this procedure.
    fn query_normalize<T>(&self, value: T) -> Result<Normalized<'tcx, T>, NoSolution>
    where
        T: TypeFoldable<TyCtxt<'tcx>>;
}

impl<'cx, 'tcx> QueryNormalizeExt<'tcx> for At<'cx, 'tcx> {
    /// Normalize `value` in the context of the inference context,
    /// yielding a resulting type, or an error if `value` cannot be
    /// normalized. If you don't care about regions, you should prefer
    /// `normalize_erasing_regions`, which is more efficient.
    ///
    /// If the normalization succeeds and is unambiguous, returns back
    /// the normalized value along with various outlives relations (in
    /// the form of obligations that must be discharged).
    ///
    /// N.B., this will *eventually* be the main means of
    /// normalizing, but for now should be used only when we actually
    /// know that normalization will succeed, since error reporting
    /// and other details are still "under development".
    fn query_normalize<T>(&self, value: T) -> Result<Normalized<'tcx, T>, NoSolution>
    where
        T: TypeFoldable<TyCtxt<'tcx>>,
    {
        debug!(
            "normalize::<{}>(value={:?}, param_env={:?}, cause={:?})",
            std::any::type_name::<T>(),
            value,
            self.param_env,
            self.cause,
        );
        if !needs_normalization(&value, self.param_env.reveal()) {
            return Ok(Normalized { value, obligations: vec![] });
        }

        let mut normalizer = QueryNormalizer {
            infcx: self.infcx,
            cause: self.cause,
            param_env: self.param_env,
            obligations: vec![],
            cache: SsoHashMap::new(),
            anon_depth: 0,
            universes: vec![],
        };

        // This is actually a consequence by the way `normalize_erasing_regions` works currently.
        // Because it needs to call the `normalize_generic_arg_after_erasing_regions`, it folds
        // through tys and consts in a `TypeFoldable`. Importantly, it skips binders, leaving us
        // with trying to normalize with escaping bound vars.
        //
        // Here, we just add the universes that we *would* have created had we passed through the binders.
        //
        // We *could* replace escaping bound vars eagerly here, but it doesn't seem really necessary.
        // The rest of the code is already set up to be lazy about replacing bound vars,
        // and only when we actually have to normalize.
        if value.has_escaping_bound_vars() {
            let mut max_visitor =
                MaxEscapingBoundVarVisitor { outer_index: ty::INNERMOST, escaping: 0 };
            value.visit_with(&mut max_visitor);
            if max_visitor.escaping > 0 {
                normalizer.universes.extend((0..max_visitor.escaping).map(|_| None));
            }
        }
        let result = value.try_fold_with(&mut normalizer);
        info!(
            "normalize::<{}>: result={:?} with {} obligations",
            std::any::type_name::<T>(),
            result,
            normalizer.obligations.len(),
        );
        debug!(
            "normalize::<{}>: obligations={:?}",
            std::any::type_name::<T>(),
            normalizer.obligations,
        );
        result.map(|value| Normalized { value, obligations: normalizer.obligations })
    }
}

// Visitor to find the maximum escaping bound var
struct MaxEscapingBoundVarVisitor {
    // The index which would count as escaping
    outer_index: ty::DebruijnIndex,
    escaping: usize,
}

impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for MaxEscapingBoundVarVisitor {
    fn visit_binder<T: TypeVisitable<TyCtxt<'tcx>>>(
        &mut self,
        t: &ty::Binder<'tcx, T>,
    ) -> ControlFlow<Self::BreakTy> {
        self.outer_index.shift_in(1);
        let result = t.super_visit_with(self);
        self.outer_index.shift_out(1);
        result
    }

    #[inline]
    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
        if t.outer_exclusive_binder() > self.outer_index {
            self.escaping = self
                .escaping
                .max(t.outer_exclusive_binder().as_usize() - self.outer_index.as_usize());
        }
        ControlFlow::Continue(())
    }

    #[inline]
    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
        match *r {
            ty::ReLateBound(debruijn, _) if debruijn > self.outer_index => {
                self.escaping =
                    self.escaping.max(debruijn.as_usize() - self.outer_index.as_usize());
            }
            _ => {}
        }
        ControlFlow::Continue(())
    }

    fn visit_const(&mut self, ct: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
        match ct.kind() {
            ty::ConstKind::Bound(debruijn, _) if debruijn >= self.outer_index => {
                self.escaping =
                    self.escaping.max(debruijn.as_usize() - self.outer_index.as_usize());
                ControlFlow::Continue(())
            }
            _ => ct.super_visit_with(self),
        }
    }
}

struct QueryNormalizer<'cx, 'tcx> {
    infcx: &'cx InferCtxt<'tcx>,
    cause: &'cx ObligationCause<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    obligations: Vec<PredicateObligation<'tcx>>,
    cache: SsoHashMap<Ty<'tcx>, Ty<'tcx>>,
    anon_depth: usize,
    universes: Vec<Option<ty::UniverseIndex>>,
}

impl<'cx, 'tcx> FallibleTypeFolder<TyCtxt<'tcx>> for QueryNormalizer<'cx, 'tcx> {
    type Error = NoSolution;

    fn interner(&self) -> TyCtxt<'tcx> {
        self.infcx.tcx
    }

    fn try_fold_binder<T: TypeFoldable<TyCtxt<'tcx>>>(
        &mut self,
        t: ty::Binder<'tcx, T>,
    ) -> Result<ty::Binder<'tcx, T>, Self::Error> {
        self.universes.push(None);
        let t = t.try_super_fold_with(self);
        self.universes.pop();
        t
    }

    #[instrument(level = "debug", skip(self))]
    fn try_fold_ty(&mut self, ty: Ty<'tcx>) -> Result<Ty<'tcx>, Self::Error> {
        if !needs_normalization(&ty, self.param_env.reveal()) {
            return Ok(ty);
        }

        if let Some(ty) = self.cache.get(&ty) {
            return Ok(*ty);
        }

        let (kind, data) = match *ty.kind() {
            ty::Alias(kind, data) => (kind, data),
            _ => {
                let res = ty.try_super_fold_with(self)?;
                self.cache.insert(ty, res);
                return Ok(res);
            }
        };

        // See note in `rustc_trait_selection::traits::project` about why we
        // wait to fold the substs.

        // Wrap this in a closure so we don't accidentally return from the outer function
        let res = match kind {
            // This is really important. While we *can* handle this, this has
            // severe performance implications for large opaque types with
            // late-bound regions. See `issue-88862` benchmark.
            ty::Opaque if !data.substs.has_escaping_bound_vars() => {
                // Only normalize `impl Trait` outside of type inference, usually in codegen.
                match self.param_env.reveal() {
                    Reveal::UserFacing => ty.try_super_fold_with(self)?,

                    Reveal::All => {
                        let substs = data.substs.try_fold_with(self)?;
                        let recursion_limit = self.interner().recursion_limit();
                        if !recursion_limit.value_within_limit(self.anon_depth) {
                            // A closure or generator may have itself as in its upvars.
                            // This should be checked handled by the recursion check for opaque
                            // types, but we may end up here before that check can happen.
                            // In that case, we delay a bug to mark the trip, and continue without
                            // revealing the opaque.
                            self.infcx
                                .err_ctxt()
                                .build_overflow_error(&ty, self.cause.span, true)
                                .delay_as_bug();
                            return ty.try_super_fold_with(self);
                        }

                        let generic_ty = self.interner().type_of(data.def_id);
                        let concrete_ty = generic_ty.subst(self.interner(), substs);
                        self.anon_depth += 1;
                        if concrete_ty == ty {
                            bug!(
                                "infinite recursion generic_ty: {:#?}, substs: {:#?}, \
                                 concrete_ty: {:#?}, ty: {:#?}",
                                generic_ty,
                                substs,
                                concrete_ty,
                                ty
                            );
                        }
                        let folded_ty = ensure_sufficient_stack(|| self.try_fold_ty(concrete_ty));
                        self.anon_depth -= 1;
                        folded_ty?
                    }
                }
            }

            ty::Opaque => ty.try_super_fold_with(self)?,

            ty::Projection | ty::Inherent => {
                // See note in `rustc_trait_selection::traits::project`

                let infcx = self.infcx;
                let tcx = infcx.tcx;
                // Just an optimization: When we don't have escaping bound vars,
                // we don't need to replace them with placeholders.
                let (data, maps) = if data.has_escaping_bound_vars() {
                    let (data, mapped_regions, mapped_types, mapped_consts) =
                        BoundVarReplacer::replace_bound_vars(infcx, &mut self.universes, data);
                    (data, Some((mapped_regions, mapped_types, mapped_consts)))
                } else {
                    (data, None)
                };
                let data = data.try_fold_with(self)?;

                let mut orig_values = OriginalQueryValues::default();
                // HACK(matthewjasper) `'static` is special-cased in selection,
                // so we cannot canonicalize it.
                let c_data = infcx
                    .canonicalize_query_keep_static(self.param_env.and(data), &mut orig_values);
                debug!("QueryNormalizer: c_data = {:#?}", c_data);
                debug!("QueryNormalizer: orig_values = {:#?}", orig_values);
                let result = match kind {
                    ty::Projection => tcx.normalize_projection_ty(c_data),
                    ty::Inherent => tcx.normalize_inherent_projection_ty(c_data),
                    _ => unreachable!(),
                }?;
                // We don't expect ambiguity.
                if result.is_ambiguous() {
                    // Rustdoc normalizes possibly not well-formed types, so only
                    // treat this as a bug if we're not in rustdoc.
                    if !tcx.sess.opts.actually_rustdoc {
                        tcx.sess.delay_span_bug(
                            DUMMY_SP,
                            format!("unexpected ambiguity: {:?} {:?}", c_data, result),
                        );
                    }
                    return Err(NoSolution);
                }
                let InferOk { value: result, obligations } = infcx
                    .instantiate_query_response_and_region_obligations(
                        self.cause,
                        self.param_env,
                        &orig_values,
                        result,
                    )?;
                debug!("QueryNormalizer: result = {:#?}", result);
                debug!("QueryNormalizer: obligations = {:#?}", obligations);
                self.obligations.extend(obligations);
                let res = if let Some((mapped_regions, mapped_types, mapped_consts)) = maps {
                    PlaceholderReplacer::replace_placeholders(
                        infcx,
                        mapped_regions,
                        mapped_types,
                        mapped_consts,
                        &self.universes,
                        result.normalized_ty,
                    )
                } else {
                    result.normalized_ty
                };
                // `tcx.normalize_projection_ty` may normalize to a type that still has
                // unevaluated consts, so keep normalizing here if that's the case.
                if res != ty && res.has_type_flags(ty::TypeFlags::HAS_CT_PROJECTION) {
                    res.try_super_fold_with(self)?
                } else {
                    res
                }
            }
        };

        self.cache.insert(ty, res);
        Ok(res)
    }

    fn try_fold_const(
        &mut self,
        constant: ty::Const<'tcx>,
    ) -> Result<ty::Const<'tcx>, Self::Error> {
        if !needs_normalization(&constant, self.param_env.reveal()) {
            return Ok(constant);
        }

        let constant = constant.try_super_fold_with(self)?;
        debug!(?constant, ?self.param_env);
        Ok(crate::traits::project::with_replaced_escaping_bound_vars(
            self.infcx,
            &mut self.universes,
            constant,
            |constant| constant.eval(self.infcx.tcx, self.param_env),
        ))
    }

    #[inline]
    fn try_fold_predicate(
        &mut self,
        p: ty::Predicate<'tcx>,
    ) -> Result<ty::Predicate<'tcx>, Self::Error> {
        if p.allow_normalization() && needs_normalization(&p, self.param_env.reveal()) {
            p.try_super_fold_with(self)
        } else {
            Ok(p)
        }
    }
}