rustc_trait_selection/traits/query/

dropck_outlives.rs

use rustc_data_structures::fx::FxHashSet;
use rustc_infer::traits::query::type_op::DropckOutlives;
use rustc_middle::traits::query::{DropckConstraint, DropckOutlivesResult};
use rustc_middle::ty::{self, EarlyBinder, ParamEnvAnd, Ty, TyCtxt};
use rustc_span::Span;
use tracing::{debug, instrument};

use crate::solve::NextSolverError;
use crate::traits::query::NoSolution;
use crate::traits::query::normalize::QueryNormalizeExt;
use crate::traits::{FromSolverError, Normalized, ObligationCause, ObligationCtxt};

/// This returns true if the type `ty` is "trivial" for
/// dropck-outlives -- that is, if it doesn't require any types to
/// outlive. This is similar but not *quite* the same as the
/// `needs_drop` test in the compiler already -- that is, for every
/// type T for which this function return true, needs-drop would
/// return `false`. But the reverse does not hold: in particular,
/// `needs_drop` returns false for `PhantomData`, but it is not
/// trivial for dropck-outlives.
///
/// Note also that `needs_drop` requires a "global" type (i.e., one
/// with erased regions), but this function does not.
///
// FIXME(@lcnr): remove this module and move this function somewhere else.
pub fn trivial_dropck_outlives<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> bool {
    match ty.kind() {
        // None of these types have a destructor and hence they do not
        // require anything in particular to outlive the dtor's
        // execution.
        ty::Infer(ty::FreshIntTy(_))
        | ty::Infer(ty::FreshFloatTy(_))
        | ty::Bool
        | ty::Int(_)
        | ty::Uint(_)
        | ty::Float(_)
        | ty::Never
        | ty::FnDef(..)
        | ty::FnPtr(..)
        | ty::Char
        | ty::CoroutineWitness(..)
        | ty::RawPtr(_, _)
        | ty::Ref(..)
        | ty::Str
        | ty::Foreign(..)
        | ty::Error(_) => true,

        // `T is PAT` and `[T]` have same properties as T.
        ty::Pat(ty, _) | ty::Slice(ty) => trivial_dropck_outlives(tcx, *ty),
        ty::Array(ty, size) => {
            // Empty array never has a dtor. See issue #110288.
            match size.try_to_target_usize(tcx) {
                Some(0) => true,
                _ => trivial_dropck_outlives(tcx, *ty),
            }
        }

        // (T1..Tn) and closures have same properties as T1..Tn --
        // check if *all* of them are trivial.
        ty::Tuple(tys) => tys.iter().all(|t| trivial_dropck_outlives(tcx, t)),

        ty::Closure(_, args) => trivial_dropck_outlives(tcx, args.as_closure().tupled_upvars_ty()),
        ty::CoroutineClosure(_, args) => {
            trivial_dropck_outlives(tcx, args.as_coroutine_closure().tupled_upvars_ty())
        }

        ty::Adt(def, _) => {
            if def.is_manually_drop() {
                // `ManuallyDrop` never has a dtor.
                true
            } else {
                // Other types might. Moreover, PhantomData doesn't
                // have a dtor, but it is considered to own its
                // content, so it is non-trivial. Unions can have `impl Drop`,
                // and hence are non-trivial as well.
                false
            }
        }

        // The following *might* require a destructor: needs deeper inspection.
        ty::Dynamic(..)
        | ty::Alias(..)
        | ty::Param(_)
        | ty::Placeholder(..)
        | ty::Infer(_)
        | ty::Bound(..)
        | ty::Coroutine(..)
        | ty::UnsafeBinder(_) => false,
    }
}

pub fn compute_dropck_outlives_inner<'tcx>(
    ocx: &ObligationCtxt<'_, 'tcx>,
    goal: ParamEnvAnd<'tcx, DropckOutlives<'tcx>>,
    span: Span,
) -> Result<DropckOutlivesResult<'tcx>, NoSolution> {
    match compute_dropck_outlives_with_errors(ocx, goal, span) {
        Ok(r) => Ok(r),
        Err(_) => Err(NoSolution),
    }
}

pub fn compute_dropck_outlives_with_errors<'tcx, E>(
    ocx: &ObligationCtxt<'_, 'tcx, E>,
    goal: ParamEnvAnd<'tcx, DropckOutlives<'tcx>>,
    span: Span,
) -> Result<DropckOutlivesResult<'tcx>, Vec<E>>
where
    E: FromSolverError<'tcx, NextSolverError<'tcx>>,
{
    let tcx = ocx.infcx.tcx;
    let ParamEnvAnd { param_env, value: DropckOutlives { dropped_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_inner`. 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 `dropped_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_inner` 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![(dropped_ty, 0)];

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

    let cause = ObligationCause::dummy_with_span(span);
    let mut constraints = DropckConstraint::empty();
    while let Some((ty, depth)) = ty_stack.pop() {
        debug!(
            "{} kinds, {} overflows, {} ty_stack",
            result.kinds.len(),
            result.overflows.len(),
            ty_stack.len()
        );
        dtorck_constraint_for_ty_inner(
            tcx,
            ocx.infcx.typing_env(param_env),
            span,
            depth,
            ty,
            &mut constraints,
        );

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

        // 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(..) {
            let ty = if let Ok(Normalized { value: ty, obligations }) =
                ocx.infcx.at(&cause, param_env).query_normalize(ty)
            {
                ocx.register_obligations(obligations);

                debug!("dropck_outlives: ty from dtorck_types = {:?}", ty);
                ty
            } else {
                // Flush errors b/c `deeply_normalize` doesn't expect pending
                // obligations, and we may have pending obligations from the
                // branch above (from other types).
                let errors = ocx.evaluate_obligations_error_on_ambiguity();
                if !errors.is_empty() {
                    return Err(errors);
                }

                // When query normalization fails, we don't get back an interesting
                // reason that we could use to report an error in borrowck. In order to turn
                // this into a reportable error, we deeply normalize again. We don't expect
                // this to succeed, so delay a bug if it does.
                match ocx.deeply_normalize(&cause, param_env, ty) {
                    Ok(_) => {
                        tcx.dcx().span_delayed_bug(
                            span,
                            format!(
                                "query normalize succeeded of {ty}, \
                                but deep normalize failed",
                            ),
                        );
                        ty
                    }
                    Err(errors) => return Err(errors),
                }
            };

            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::Alias(..) => {
                    result.kinds.push(ty.into());
                }

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

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

/// Returns a set of constraints that needs to be satisfied in
/// order for `ty` to be valid for destruction.
#[instrument(level = "debug", skip(tcx, typing_env, span, constraints))]
pub fn dtorck_constraint_for_ty_inner<'tcx>(
    tcx: TyCtxt<'tcx>,
    typing_env: ty::TypingEnv<'tcx>,
    span: Span,
    depth: usize,
    ty: Ty<'tcx>,
    constraints: &mut DropckConstraint<'tcx>,
) {
    if !tcx.recursion_limit().value_within_limit(depth) {
        constraints.overflows.push(ty);
        return;
    }

    if trivial_dropck_outlives(tcx, ty) {
        return;
    }

    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::CoroutineWitness(..) => {
            // these types never have a destructor
        }

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

        ty::Tuple(tys) => rustc_data_structures::stack::ensure_sufficient_stack(|| {
            for ty in tys.iter() {
                dtorck_constraint_for_ty_inner(tcx, typing_env, span, depth + 1, ty, constraints);
            }
        }),

        ty::Closure(_, args) => rustc_data_structures::stack::ensure_sufficient_stack(|| {
            for ty in args.as_closure().upvar_tys() {
                dtorck_constraint_for_ty_inner(tcx, typing_env, span, depth + 1, ty, constraints);
            }
        }),

        ty::CoroutineClosure(_, args) => {
            rustc_data_structures::stack::ensure_sufficient_stack(|| {
                for ty in args.as_coroutine_closure().upvar_tys() {
                    dtorck_constraint_for_ty_inner(
                        tcx,
                        typing_env,
                        span,
                        depth + 1,
                        ty,
                        constraints,
                    );
                }
            })
        }

        ty::Coroutine(def_id, args) => {
            // rust-lang/rust#49918: Locals can be stored across await points in the coroutine,
            // called interior/witness types. Since we do not compute these witnesses until after
            // building MIR, we consider all coroutines to unconditionally require a drop during
            // MIR building. However, considering the coroutine to unconditionally require a drop
            // here may unnecessarily require its upvars' regions to be live when they don't need
            // to be, leading to borrowck errors: <https://github.com/rust-lang/rust/issues/116242>.
            //
            // Here, we implement a more precise approximation for the coroutine's dtorck constraint
            // by considering whether any of the interior types needs drop. Note that this is still
            // an approximation because the coroutine interior has its regions erased, so we must add
            // *all* of the upvars to live types set if we find that *any* interior type needs drop.
            // This is because any of the regions captured in the upvars may be stored in the interior,
            // which then has its regions replaced by a binder (conceptually erasing the regions),
            // so there's no way to enforce that the precise region in the interior type is live
            // since we've lost that information by this point.
            //
            // Note also that this check requires that the coroutine's upvars are use-live, since
            // a region from a type that does not have a destructor that was captured in an upvar
            // may flow into an interior type with a destructor. This is stronger than requiring
            // the upvars are drop-live.
            //
            // For example, if we capture two upvar references `&'1 (), &'2 ()` and have some type
            // in the interior, `for<'r> { NeedsDrop<'r> }`, we have no way to tell whether the
            // region `'r` came from the `'1` or `'2` region, so we require both are live. This
            // could even be unnecessary if `'r` was actually a `'static` region or some region
            // local to the coroutine! That's why it's an approximation.
            let args = args.as_coroutine();

            // Note that we don't care about whether the resume type has any drops since this is
            // redundant; there is no storage for the resume type, so if it is actually stored
            // in the interior, we'll already detect the need for a drop by checking the interior.
            //
            // FIXME(@lcnr): Why do we erase regions in the env here? Seems odd
            let typing_env = tcx.erase_and_anonymize_regions(typing_env);
            let needs_drop = tcx.mir_coroutine_witnesses(def_id).is_some_and(|witness| {
                witness.field_tys.iter().any(|field| field.ty.needs_drop(tcx, typing_env))
            });
            if needs_drop {
                // Pushing types directly to `constraints.outlives` is equivalent
                // to requiring them to be use-live, since if we were instead to
                // recurse on them like we do below, we only end up collecting the
                // types that are relevant for drop-liveness.
                constraints.outlives.extend(args.upvar_tys().iter().map(ty::GenericArg::from));
                constraints.outlives.push(args.resume_ty().into());
            } else {
                // Even if a witness type doesn't need a drop, we still require that
                // the upvars are drop-live. This is only needed if we aren't already
                // counting *all* of the upvars as use-live above, since use-liveness
                // is a *stronger requirement* than drop-liveness. Recursing here
                // unconditionally would just be collecting duplicated types for no
                // reason.
                for ty in args.upvar_tys() {
                    dtorck_constraint_for_ty_inner(
                        tcx,
                        typing_env,
                        span,
                        depth + 1,
                        ty,
                        constraints,
                    );
                }
            }
        }

        ty::Adt(def, args) => {
            let DropckConstraint { 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.iter().map(|t| EarlyBinder::bind(*t).instantiate(tcx, args)));
            constraints
                .outlives
                .extend(outlives.iter().map(|t| EarlyBinder::bind(*t).instantiate(tcx, args)));
            constraints
                .overflows
                .extend(overflows.iter().map(|t| EarlyBinder::bind(*t).instantiate(tcx, args)));
        }

        // 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::Alias(..) | ty::Param(..) => {
            constraints.dtorck_types.push(ty);
        }

        // Can't instantiate binder here.
        ty::UnsafeBinder(_) => {
            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.
            tcx.dcx().span_delayed_bug(span, format!("Unresolved type in dropck: {:?}.", ty));
        }
    }
}