compiler/rustc_infer/src/infer/error_reporting/nice_region_error/static_impl_trait.rs

//! Error Reporting for static impl Traits.

use crate::infer::error_reporting::nice_region_error::NiceRegionError;
use crate::infer::lexical_region_resolve::RegionResolutionError;
use crate::infer::{SubregionOrigin, TypeTrace};
use crate::traits::{ObligationCauseCode, UnifyReceiverContext};
use rustc_errors::{struct_span_err, Applicability, DiagnosticBuilder, ErrorReported};
use rustc_hir::def_id::DefId;
use rustc_hir::intravisit::{walk_ty, ErasedMap, NestedVisitorMap, Visitor};
use rustc_hir::{
    self as hir, GenericBound, ImplItem, Item, ItemKind, Lifetime, LifetimeName, Node, TraitItem,
    TyKind,
};
use rustc_middle::ty::{self, AssocItemContainer, RegionKind, Ty, TypeFoldable, TypeVisitor};
use rustc_span::symbol::Ident;
use rustc_span::{MultiSpan, Span};

use std::ops::ControlFlow;

impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
    /// Print the error message for lifetime errors when the return type is a static `impl Trait`,
    /// `dyn Trait` or if a method call on a trait object introduces a static requirement.
    pub(super) fn try_report_static_impl_trait(&self) -> Option<ErrorReported> {
        debug!("try_report_static_impl_trait(error={:?})", self.error);
        let tcx = self.tcx();
        let (var_origin, sub_origin, sub_r, sup_origin, sup_r) = match self.error.as_ref()? {
            RegionResolutionError::SubSupConflict(
                _,
                var_origin,
                sub_origin,
                sub_r,
                sup_origin,
                sup_r,
            ) if **sub_r == RegionKind::ReStatic => {
                (var_origin, sub_origin, sub_r, sup_origin, sup_r)
            }
            RegionResolutionError::ConcreteFailure(
                SubregionOrigin::Subtype(box TypeTrace { cause, .. }),
                sub_r,
                sup_r,
            ) if **sub_r == RegionKind::ReStatic => {
                // This is for an implicit `'static` requirement coming from `impl dyn Trait {}`.
                if let ObligationCauseCode::UnifyReceiver(ctxt) = &cause.code {
                    // This may have a closure and it would cause ICE
                    // through `find_param_with_region` (#78262).
                    let anon_reg_sup = tcx.is_suitable_region(sup_r)?;
                    let fn_returns = tcx.return_type_impl_or_dyn_traits(anon_reg_sup.def_id);
                    if fn_returns.is_empty() {
                        return None;
                    }

                    let param = self.find_param_with_region(sup_r, sub_r)?;
                    let lifetime = if sup_r.has_name() {
                        format!("lifetime `{}`", sup_r)
                    } else {
                        "an anonymous lifetime `'_`".to_string()
                    };
                    let mut err = struct_span_err!(
                        tcx.sess,
                        cause.span,
                        E0772,
                        "{} has {} but calling `{}` introduces an implicit `'static` lifetime \
                         requirement",
                        param
                            .param
                            .pat
                            .simple_ident()
                            .map(|s| format!("`{}`", s))
                            .unwrap_or_else(|| "`fn` parameter".to_string()),
                        lifetime,
                        ctxt.assoc_item.ident,
                    );
                    err.span_label(param.param_ty_span, &format!("this data with {}...", lifetime));
                    err.span_label(
                        cause.span,
                        &format!(
                            "...is captured and required to live as long as `'static` here \
                             because of an implicit lifetime bound on the {}",
                            match ctxt.assoc_item.container {
                                AssocItemContainer::TraitContainer(id) =>
                                    format!("`impl` of `{}`", tcx.def_path_str(id)),
                                AssocItemContainer::ImplContainer(_) =>
                                    "inherent `impl`".to_string(),
                            },
                        ),
                    );
                    if self.find_impl_on_dyn_trait(&mut err, param.param_ty, &ctxt) {
                        err.emit();
                        return Some(ErrorReported);
                    } else {
                        err.cancel();
                    }
                }
                return None;
            }
            _ => return None,
        };
        debug!(
            "try_report_static_impl_trait(var={:?}, sub={:?} {:?} sup={:?} {:?})",
            var_origin, sub_origin, sub_r, sup_origin, sup_r
        );
        let anon_reg_sup = tcx.is_suitable_region(sup_r)?;
        debug!("try_report_static_impl_trait: anon_reg_sup={:?}", anon_reg_sup);
        let sp = var_origin.span();
        let return_sp = sub_origin.span();
        let param = self.find_param_with_region(sup_r, sub_r)?;
        let (lifetime_name, lifetime) = if sup_r.has_name() {
            (sup_r.to_string(), format!("lifetime `{}`", sup_r))
        } else {
            ("'_".to_owned(), "an anonymous lifetime `'_`".to_string())
        };
        let param_name = param
            .param
            .pat
            .simple_ident()
            .map(|s| format!("`{}`", s))
            .unwrap_or_else(|| "`fn` parameter".to_string());
        let mut err = struct_span_err!(
            tcx.sess,
            sp,
            E0759,
            "{} has {} but it needs to satisfy a `'static` lifetime requirement",
            param_name,
            lifetime,
        );
        err.span_label(param.param_ty_span, &format!("this data with {}...", lifetime));
        debug!("try_report_static_impl_trait: param_info={:?}", param);

        // We try to make the output have fewer overlapping spans if possible.
        if (sp == sup_origin.span() || !return_sp.overlaps(sup_origin.span()))
            && sup_origin.span() != return_sp
        {
            // FIXME: account for `async fn` like in `async-await/issues/issue-62097.rs`

            // Customize the spans and labels depending on their relative order so
            // that split sentences flow correctly.
            if sup_origin.span().overlaps(return_sp) && sp == sup_origin.span() {
                // Avoid the following:
                //
                // error: cannot infer an appropriate lifetime
                //   --> $DIR/must_outlive_least_region_or_bound.rs:18:50
                //    |
                // LL | fn foo(x: &i32) -> Box<dyn Debug> { Box::new(x) }
                //    |           ----                      ---------^-
                //
                // and instead show:
                //
                // error: cannot infer an appropriate lifetime
                //   --> $DIR/must_outlive_least_region_or_bound.rs:18:50
                //    |
                // LL | fn foo(x: &i32) -> Box<dyn Debug> { Box::new(x) }
                //    |           ----                               ^
                err.span_label(
                    sup_origin.span(),
                    "...is captured here, requiring it to live as long as `'static`",
                );
            } else {
                err.span_label(sup_origin.span(), "...is captured here...");
                if return_sp < sup_origin.span() {
                    err.span_note(
                        return_sp,
                        "...and is required to live as long as `'static` here",
                    );
                } else {
                    err.span_label(
                        return_sp,
                        "...and is required to live as long as `'static` here",
                    );
                }
            }
        } else {
            err.span_label(
                return_sp,
                "...is captured and required to live as long as `'static` here",
            );
        }

        let fn_returns = tcx.return_type_impl_or_dyn_traits(anon_reg_sup.def_id);

        let mut override_error_code = None;
        if let SubregionOrigin::Subtype(box TypeTrace { cause, .. }) = &sup_origin {
            if let ObligationCauseCode::UnifyReceiver(ctxt) = &cause.code {
                // Handle case of `impl Foo for dyn Bar { fn qux(&self) {} }` introducing a
                // `'static` lifetime when called as a method on a binding: `bar.qux()`.
                if self.find_impl_on_dyn_trait(&mut err, param.param_ty, &ctxt) {
                    override_error_code = Some(ctxt.assoc_item.ident);
                }
            }
        }
        if let SubregionOrigin::Subtype(box TypeTrace { cause, .. }) = &sub_origin {
            if let ObligationCauseCode::ItemObligation(item_def_id) = cause.code {
                // Same case of `impl Foo for dyn Bar { fn qux(&self) {} }` introducing a `'static`
                // lifetime as above, but called using a fully-qualified path to the method:
                // `Foo::qux(bar)`.
                let mut v = TraitObjectVisitor(vec![]);
                v.visit_ty(param.param_ty);
                if let Some((ident, self_ty)) =
                    self.get_impl_ident_and_self_ty_from_trait(item_def_id, &v.0[..])
                {
                    if self.suggest_constrain_dyn_trait_in_impl(&mut err, &v.0[..], ident, self_ty)
                    {
                        override_error_code = Some(ident);
                    }
                }
            }
        }
        if let (Some(ident), true) = (override_error_code, fn_returns.is_empty()) {
            // Provide a more targeted error code and description.
            err.code(rustc_errors::error_code!(E0772));
            err.set_primary_message(&format!(
                "{} has {} but calling `{}` introduces an implicit `'static` lifetime \
                requirement",
                param_name, lifetime, ident,
            ));
        }

        debug!("try_report_static_impl_trait: fn_return={:?}", fn_returns);
        // FIXME: account for the need of parens in `&(dyn Trait + '_)`
        let consider = "consider changing the";
        let declare = "to declare that the";
        let arg = match param.param.pat.simple_ident() {
            Some(simple_ident) => format!("argument `{}`", simple_ident),
            None => "the argument".to_string(),
        };
        let explicit = format!("you can add an explicit `{}` lifetime bound", lifetime_name);
        let explicit_static = format!("explicit `'static` bound to the lifetime of {}", arg);
        let captures = format!("captures data from {}", arg);
        let add_static_bound = "alternatively, add an explicit `'static` bound to this reference";
        let plus_lt = format!(" + {}", lifetime_name);
        for fn_return in fn_returns {
            if fn_return.span.desugaring_kind().is_some() {
                // Skip `async` desugaring `impl Future`.
                continue;
            }
            match fn_return.kind {
                TyKind::OpaqueDef(item_id, _) => {
                    let item = tcx.hir().item(item_id.id);
                    let opaque = if let ItemKind::OpaqueTy(opaque) = &item.kind {
                        opaque
                    } else {
                        err.emit();
                        return Some(ErrorReported);
                    };

                    if let Some(span) = opaque
                        .bounds
                        .iter()
                        .filter_map(|arg| match arg {
                            GenericBound::Outlives(Lifetime {
                                name: LifetimeName::Static,
                                span,
                                ..
                            }) => Some(*span),
                            _ => None,
                        })
                        .next()
                    {
                        err.span_suggestion_verbose(
                            span,
                            &format!("{} `impl Trait`'s {}", consider, explicit_static),
                            lifetime_name.clone(),
                            Applicability::MaybeIncorrect,
                        );
                        err.span_suggestion_verbose(
                            param.param_ty_span,
                            add_static_bound,
                            param.param_ty.to_string(),
                            Applicability::MaybeIncorrect,
                        );
                    } else if opaque
                        .bounds
                        .iter()
                        .filter_map(|arg| match arg {
                            GenericBound::Outlives(Lifetime { name, span, .. })
                                if name.ident().to_string() == lifetime_name =>
                            {
                                Some(*span)
                            }
                            _ => None,
                        })
                        .next()
                        .is_some()
                    {
                    } else {
                        err.span_suggestion_verbose(
                            fn_return.span.shrink_to_hi(),
                            &format!(
                                "{declare} `impl Trait` {captures}, {explicit}",
                                declare = declare,
                                captures = captures,
                                explicit = explicit,
                            ),
                            plus_lt.clone(),
                            Applicability::MaybeIncorrect,
                        );
                    }
                }
                TyKind::TraitObject(_, lt) => match lt.name {
                    LifetimeName::ImplicitObjectLifetimeDefault => {
                        err.span_suggestion_verbose(
                            fn_return.span.shrink_to_hi(),
                            &format!(
                                "{declare} trait object {captures}, {explicit}",
                                declare = declare,
                                captures = captures,
                                explicit = explicit,
                            ),
                            plus_lt.clone(),
                            Applicability::MaybeIncorrect,
                        );
                    }
                    name if name.ident().to_string() != lifetime_name => {
                        // With this check we avoid suggesting redundant bounds. This
                        // would happen if there are nested impl/dyn traits and only
                        // one of them has the bound we'd suggest already there, like
                        // in `impl Foo<X = dyn Bar> + '_`.
                        err.span_suggestion_verbose(
                            lt.span,
                            &format!("{} trait object's {}", consider, explicit_static),
                            lifetime_name.clone(),
                            Applicability::MaybeIncorrect,
                        );
                        err.span_suggestion_verbose(
                            param.param_ty_span,
                            add_static_bound,
                            param.param_ty.to_string(),
                            Applicability::MaybeIncorrect,
                        );
                    }
                    _ => {}
                },
                _ => {}
            }
        }
        err.emit();
        Some(ErrorReported)
    }

    fn get_impl_ident_and_self_ty_from_trait(
        &self,
        def_id: DefId,
        trait_objects: &[DefId],
    ) -> Option<(Ident, &'tcx hir::Ty<'tcx>)> {
        let tcx = self.tcx();
        match tcx.hir().get_if_local(def_id) {
            Some(Node::ImplItem(ImplItem { ident, hir_id, .. })) => {
                match tcx.hir().find(tcx.hir().get_parent_item(*hir_id)) {
                    Some(Node::Item(Item { kind: ItemKind::Impl { self_ty, .. }, .. })) => {
                        Some((*ident, self_ty))
                    }
                    _ => None,
                }
            }
            Some(Node::TraitItem(TraitItem { ident, hir_id, .. })) => {
                let parent_id = tcx.hir().get_parent_item(*hir_id);
                match tcx.hir().find(parent_id) {
                    Some(Node::Item(Item { kind: ItemKind::Trait(..), .. })) => {
                        // The method being called is defined in the `trait`, but the `'static`
                        // obligation comes from the `impl`. Find that `impl` so that we can point
                        // at it in the suggestion.
                        let trait_did = tcx.hir().local_def_id(parent_id).to_def_id();
                        match tcx
                            .hir()
                            .trait_impls(trait_did)
                            .iter()
                            .filter_map(|impl_node| {
                                let impl_did = tcx.hir().local_def_id(*impl_node);
                                match tcx.hir().get_if_local(impl_did.to_def_id()) {
                                    Some(Node::Item(Item {
                                        kind: ItemKind::Impl { self_ty, .. },
                                        ..
                                    })) if trait_objects.iter().all(|did| {
                                        // FIXME: we should check `self_ty` against the receiver
                                        // type in the `UnifyReceiver` context, but for now, use
                                        // this imperfect proxy. This will fail if there are
                                        // multiple `impl`s for the same trait like
                                        // `impl Foo for Box<dyn Bar>` and `impl Foo for dyn Bar`.
                                        // In that case, only the first one will get suggestions.
                                        let mut hir_v = HirTraitObjectVisitor(vec![], *did);
                                        hir_v.visit_ty(self_ty);
                                        !hir_v.0.is_empty()
                                    }) =>
                                    {
                                        Some(self_ty)
                                    }
                                    _ => None,
                                }
                            })
                            .next()
                        {
                            Some(self_ty) => Some((*ident, self_ty)),
                            _ => None,
                        }
                    }
                    _ => None,
                }
            }
            _ => None,
        }
    }

    /// When we call a method coming from an `impl Foo for dyn Bar`, `dyn Bar` introduces a default
    /// `'static` obligation. Suggest relaxing that implicit bound.
    fn find_impl_on_dyn_trait(
        &self,
        err: &mut DiagnosticBuilder<'_>,
        ty: Ty<'_>,
        ctxt: &UnifyReceiverContext<'tcx>,
    ) -> bool {
        let tcx = self.tcx();

        // Find the method being called.
        let instance = match ty::Instance::resolve(
            tcx,
            ctxt.param_env,
            ctxt.assoc_item.def_id,
            self.infcx.resolve_vars_if_possible(ctxt.substs),
        ) {
            Ok(Some(instance)) => instance,
            _ => return false,
        };

        let mut v = TraitObjectVisitor(vec![]);
        v.visit_ty(ty);

        // Get the `Ident` of the method being called and the corresponding `impl` (to point at
        // `Bar` in `impl Foo for dyn Bar {}` and the definition of the method being called).
        let (ident, self_ty) =
            match self.get_impl_ident_and_self_ty_from_trait(instance.def_id(), &v.0[..]) {
                Some((ident, self_ty)) => (ident, self_ty),
                None => return false,
            };

        // Find the trait object types in the argument, so we point at *only* the trait object.
        self.suggest_constrain_dyn_trait_in_impl(err, &v.0[..], ident, self_ty)
    }

    fn suggest_constrain_dyn_trait_in_impl(
        &self,
        err: &mut DiagnosticBuilder<'_>,
        found_dids: &[DefId],
        ident: Ident,
        self_ty: &hir::Ty<'_>,
    ) -> bool {
        let mut suggested = false;
        for found_did in found_dids {
            let mut hir_v = HirTraitObjectVisitor(vec![], *found_did);
            hir_v.visit_ty(&self_ty);
            for span in &hir_v.0 {
                let mut multi_span: MultiSpan = vec![*span].into();
                multi_span.push_span_label(
                    *span,
                    "this has an implicit `'static` lifetime requirement".to_string(),
                );
                multi_span.push_span_label(
                    ident.span,
                    "calling this method introduces the `impl`'s 'static` requirement".to_string(),
                );
                err.span_note(multi_span, "the used `impl` has a `'static` requirement");
                err.span_suggestion_verbose(
                    span.shrink_to_hi(),
                    "consider relaxing the implicit `'static` requirement",
                    " + '_".to_string(),
                    Applicability::MaybeIncorrect,
                );
                suggested = true;
            }
        }
        suggested
    }
}

/// Collect all the trait objects in a type that could have received an implicit `'static` lifetime.
struct TraitObjectVisitor(Vec<DefId>);

impl TypeVisitor<'_> for TraitObjectVisitor {
    fn visit_ty(&mut self, t: Ty<'_>) -> ControlFlow<Self::BreakTy> {
        match t.kind() {
            ty::Dynamic(preds, RegionKind::ReStatic) => {
                if let Some(def_id) = preds.principal_def_id() {
                    self.0.push(def_id);
                }
                ControlFlow::CONTINUE
            }
            _ => t.super_visit_with(self),
        }
    }
}

/// Collect all `hir::Ty<'_>` `Span`s for trait objects with an implicit lifetime.
struct HirTraitObjectVisitor(Vec<Span>, DefId);

impl<'tcx> Visitor<'tcx> for HirTraitObjectVisitor {
    type Map = ErasedMap<'tcx>;

    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
        NestedVisitorMap::None
    }

    fn visit_ty(&mut self, t: &'tcx hir::Ty<'tcx>) {
        if let TyKind::TraitObject(
            poly_trait_refs,
            Lifetime { name: LifetimeName::ImplicitObjectLifetimeDefault, .. },
        ) = t.kind
        {
            for ptr in poly_trait_refs {
                if Some(self.1) == ptr.trait_ref.trait_def_id() {
                    self.0.push(ptr.span);
                }
            }
        }
        walk_ty(self, t);
    }
}