DXR is a code search and navigation tool aimed at making sense of large projects. It supports full-text and regex searches as well as structural queries.

Git (4fb54ed484)

VCS Links

Line Code
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326
use rustc_data_structures::fx::FxHashSet;
use rustc_hir::def_id::DefId;
use rustc_infer::infer::canonical::{Canonical, QueryResponse};
use rustc_infer::infer::TyCtxtInferExt;
use rustc_infer::traits::TraitEngineExt as _;
use rustc_middle::ty::query::Providers;
use rustc_middle::ty::subst::{InternalSubsts, Subst};
use rustc_middle::ty::{self, ParamEnvAnd, Ty, TyCtxt};
use rustc_span::source_map::{Span, DUMMY_SP};
use rustc_trait_selection::traits::query::dropck_outlives::trivial_dropck_outlives;
use rustc_trait_selection::traits::query::dropck_outlives::{
    DropckOutlivesResult, DtorckConstraint,
};
use rustc_trait_selection::traits::query::normalize::AtExt;
use rustc_trait_selection::traits::query::{CanonicalTyGoal, NoSolution};
use rustc_trait_selection::traits::{
    Normalized, ObligationCause, TraitEngine, TraitEngineExt as _,
};

crate fn provide(p: &mut Providers<'_>) {
    *p = Providers { dropck_outlives, adt_dtorck_constraint, ..*p };
}

fn dropck_outlives<'tcx>(
    tcx: TyCtxt<'tcx>,
    canonical_goal: CanonicalTyGoal<'tcx>,
) -> Result<&'tcx Canonical<'tcx, QueryResponse<'tcx, DropckOutlivesResult<'tcx>>>, NoSolution> {
    debug!("dropck_outlives(goal={:#?})", canonical_goal);

    tcx.infer_ctxt().enter_with_canonical(
        DUMMY_SP,
        &canonical_goal,
        |ref infcx, goal, canonical_inference_vars| {
            let tcx = infcx.tcx;
            let ParamEnvAnd { param_env, value: for_ty } = goal;

            let mut result = DropckOutlivesResult { kinds: vec![], overflows: vec![] };

            // A stack of types left to process. Each round, we pop
            // something from the stack and invoke
            // `dtorck_constraint_for_ty`. This may produce new types that
            // have to be pushed on the stack. This continues until we have explored
            // all the reachable types from the type `for_ty`.
            //
            // Example: Imagine that we have the following code:
            //
            // ```rust
            // struct A {
            //     value: B,
            //     children: Vec<A>,
            // }
            //
            // struct B {
            //     value: u32
            // }
            //
            // fn f() {
            //   let a: A = ...;
            //   ..
            // } // here, `a` is dropped
            // ```
            //
            // at the point where `a` is dropped, we need to figure out
            // which types inside of `a` contain region data that may be
            // accessed by any destructors in `a`. We begin by pushing `A`
            // onto the stack, as that is the type of `a`. We will then
            // invoke `dtorck_constraint_for_ty` which will expand `A`
            // into the types of its fields `(B, Vec<A>)`. These will get
            // pushed onto the stack. Eventually, expanding `Vec<A>` will
            // lead to us trying to push `A` a second time -- to prevent
            // infinite recursion, we notice that `A` was already pushed
            // once and stop.
            let mut ty_stack = vec![(for_ty, 0)];

            // Set used to detect infinite recursion.
            let mut ty_set = FxHashSet::default();

            let mut fulfill_cx = TraitEngine::new(infcx.tcx);

            let cause = ObligationCause::dummy();
            let mut constraints = DtorckConstraint::empty();
            while let Some((ty, depth)) = ty_stack.pop() {
                info!(
                    "{} kinds, {} overflows, {} ty_stack",
                    result.kinds.len(),
                    result.overflows.len(),
                    ty_stack.len()
                );
                dtorck_constraint_for_ty(tcx, DUMMY_SP, for_ty, depth, ty, &mut constraints)?;

                // "outlives" represent types/regions that may be touched
                // by a destructor.
                result.kinds.extend(constraints.outlives.drain(..));
                result.overflows.extend(constraints.overflows.drain(..));

                // If we have even one overflow, we should stop trying to evaluate further --
                // chances are, the subsequent overflows for this evaluation won't provide useful
                // information and will just decrease the speed at which we can emit these errors
                // (since we'll be printing for just that much longer for the often enormous types
                // that result here).
                if !result.overflows.is_empty() {
                    break;
                }

                // dtorck types are "types that will get dropped but which
                // do not themselves define a destructor", more or less. We have
                // to push them onto the stack to be expanded.
                for ty in constraints.dtorck_types.drain(..) {
                    match infcx.at(&cause, param_env).normalize(&ty) {
                        Ok(Normalized { value: ty, obligations }) => {
                            fulfill_cx.register_predicate_obligations(infcx, obligations);

                            debug!("dropck_outlives: ty from dtorck_types = {:?}", ty);

                            match ty.kind {
                                // All parameters live for the duration of the
                                // function.
                                ty::Param(..) => {}

                                // A projection that we couldn't resolve - it
                                // might have a destructor.
                                ty::Projection(..) | ty::Opaque(..) => {
                                    result.kinds.push(ty.into());
                                }

                                _ => {
                                    if ty_set.insert(ty) {
                                        ty_stack.push((ty, depth + 1));
                                    }
                                }
                            }
                        }

                        // We don't actually expect to fail to normalize.
                        // That implies a WF error somewhere else.
                        Err(NoSolution) => {
                            return Err(NoSolution);
                        }
                    }
                }
            }

            debug!("dropck_outlives: result = {:#?}", result);

            infcx.make_canonicalized_query_response(
                canonical_inference_vars,
                result,
                &mut *fulfill_cx,
            )
        },
    )
}

/// Returns a set of constraints that needs to be satisfied in
/// order for `ty` to be valid for destruction.
fn dtorck_constraint_for_ty<'tcx>(
    tcx: TyCtxt<'tcx>,
    span: Span,
    for_ty: Ty<'tcx>,
    depth: usize,
    ty: Ty<'tcx>,
    constraints: &mut DtorckConstraint<'tcx>,
) -> Result<(), NoSolution> {
    debug!("dtorck_constraint_for_ty({:?}, {:?}, {:?}, {:?})", span, for_ty, depth, ty);

    if !tcx.sess.recursion_limit().value_within_limit(depth) {
        constraints.overflows.push(ty);
        return Ok(());
    }

    if trivial_dropck_outlives(tcx, ty) {
        return Ok(());
    }

    match ty.kind {
        ty::Bool
        | ty::Char
        | ty::Int(_)
        | ty::Uint(_)
        | ty::Float(_)
        | ty::Str
        | ty::Never
        | ty::Foreign(..)
        | ty::RawPtr(..)
        | ty::Ref(..)
        | ty::FnDef(..)
        | ty::FnPtr(_)
        | ty::GeneratorWitness(..) => {
            // these types never have a destructor
        }

        ty::Array(ety, _) | ty::Slice(ety) => {
            // single-element containers, behave like their element
            rustc_data_structures::stack::ensure_sufficient_stack(|| {
                dtorck_constraint_for_ty(tcx, span, for_ty, depth + 1, ety, constraints)
            })?;
        }

        ty::Tuple(tys) => rustc_data_structures::stack::ensure_sufficient_stack(|| {
            for ty in tys.iter() {
                dtorck_constraint_for_ty(
                    tcx,
                    span,
                    for_ty,
                    depth + 1,
                    ty.expect_ty(),
                    constraints,
                )?;
            }
            Ok::<_, NoSolution>(())
        })?,

        ty::Closure(_, substs) => rustc_data_structures::stack::ensure_sufficient_stack(|| {
            for ty in substs.as_closure().upvar_tys() {
                dtorck_constraint_for_ty(tcx, span, for_ty, depth + 1, ty, constraints)?;
            }
            Ok::<_, NoSolution>(())
        })?,

        ty::Generator(_, substs, _movability) => {
            // rust-lang/rust#49918: types can be constructed, stored
            // in the interior, and sit idle when generator yields
            // (and is subsequently dropped).
            //
            // It would be nice to descend into interior of a
            // generator to determine what effects dropping it might
            // have (by looking at any drop effects associated with
            // its interior).
            //
            // However, the interior's representation uses things like
            // GeneratorWitness that explicitly assume they are not
            // traversed in such a manner. So instead, we will
            // simplify things for now by treating all generators as
            // if they were like trait objects, where its upvars must
            // all be alive for the generator's (potential)
            // destructor.
            //
            // In particular, skipping over `_interior` is safe
            // because any side-effects from dropping `_interior` can
            // only take place through references with lifetimes
            // derived from lifetimes attached to the upvars and resume
            // argument, and we *do* incorporate those here.

            constraints.outlives.extend(
                substs
                    .as_generator()
                    .upvar_tys()
                    .map(|t| -> ty::subst::GenericArg<'tcx> { t.into() }),
            );
            constraints.outlives.push(substs.as_generator().resume_ty().into());
        }

        ty::Adt(def, substs) => {
            let DtorckConstraint { dtorck_types, outlives, overflows } =
                tcx.at(span).adt_dtorck_constraint(def.did)?;
            // FIXME: we can try to recursively `dtorck_constraint_on_ty`
            // there, but that needs some way to handle cycles.
            constraints.dtorck_types.extend(dtorck_types.subst(tcx, substs));
            constraints.outlives.extend(outlives.subst(tcx, substs));
            constraints.overflows.extend(overflows.subst(tcx, substs));
        }

        // Objects must be alive in order for their destructor
        // to be called.
        ty::Dynamic(..) => {
            constraints.outlives.push(ty.into());
        }

        // Types that can't be resolved. Pass them forward.
        ty::Projection(..) | ty::Opaque(..) | ty::Param(..) => {
            constraints.dtorck_types.push(ty);
        }

        ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error => {
            // By the time this code runs, all type variables ought to
            // be fully resolved.
            return Err(NoSolution);
        }
    }

    Ok(())
}

/// Calculates the dtorck constraint for a type.
crate fn adt_dtorck_constraint(
    tcx: TyCtxt<'_>,
    def_id: DefId,
) -> Result<DtorckConstraint<'_>, NoSolution> {
    let def = tcx.adt_def(def_id);
    let span = tcx.def_span(def_id);
    debug!("dtorck_constraint: {:?}", def);

    if def.is_phantom_data() {
        // The first generic parameter here is guaranteed to be a type because it's
        // `PhantomData`.
        let substs = InternalSubsts::identity_for_item(tcx, def_id);
        assert_eq!(substs.len(), 1);
        let result = DtorckConstraint {
            outlives: vec![],
            dtorck_types: vec![substs.type_at(0)],
            overflows: vec![],
        };
        debug!("dtorck_constraint: {:?} => {:?}", def, result);
        return Ok(result);
    }

    let mut result = DtorckConstraint::empty();
    for field in def.all_fields() {
        let fty = tcx.type_of(field.did);
        dtorck_constraint_for_ty(tcx, span, fty, 0, fty, &mut result)?;
    }
    result.outlives.extend(tcx.destructor_constraints(def));
    dedup_dtorck_constraint(&mut result);

    debug!("dtorck_constraint: {:?} => {:?}", def, result);

    Ok(result)
}

fn dedup_dtorck_constraint(c: &mut DtorckConstraint<'_>) {
    let mut outlives = FxHashSet::default();
    let mut dtorck_types = FxHashSet::default();

    c.outlives.retain(|&val| outlives.replace(val).is_none());
    c.dtorck_types.retain(|&val| dtorck_types.replace(val).is_none());
}