Rollup merge of #129028 - compiler-errors:contra, r=lcnr

`impl_trait_overcaptures`: Don't worry about uncaptured contravariant lifetimes if they outlive a captured lifetime

**NOTE:** Review only the first commit carefully. The second one is just moving stuff around, so you can turn whitespace off for that one.

This PR relaxes the `impl_trait_overcaptures` lint to not fire in cases like:

```rust
struct Ctxt<'tcx>(&'tcx ());

impl<'tcx> Ctxt<'tcx> {
    fn compute(&self) -> impl Sized + '_ { }
}
```

Specifically, the lint will not fire if **all** overcaptured regions (i.e. those which will be captured in edition 2024, which are not captured today) **satisfy**:
* The region is contravariant (or bivariant) in the function signature
* The region outlives some other region which is captured by the opaque

### The idea behind this

Why is this OK? My reasoning is that since the region is contravariant in the function signature, we know that it can be shortened arbitrarily at the call site. And specifically, we know it can be shortened to be equal to one of the regions that it outlives -- that's why we need to prove that it outlives some other region that *is* captured.

We could technically relax this further, but there would be (IMO somewhat easy) cases to make this a false negative in real code. For example, if the region is invariant, then we can hit issues like:

```rust
struct Ctxt<'tcx>(&'tcx mut &'tcx mut ());

impl<'tcx> Ctxt<'tcx> {
    fn compute(&self) -> impl Sized + use<'_, 'tcx> { }
    // We use `use<'_, 'tcx>` to show what happens in edition 2024
}

fn test<'a, 'b>(x: &'a Ctxt<'b>, y: &'a Ctxt<'a>) {
    let results = [x.compute(), y.compute()];
    //~^ ERROR lifetime may not live long enough
    // Since both opaques now capture `'tcx`, this enforces that `'a == 'b`.
}
```

### Is this actually totally fine?

There's one case where users might still hit issues, and it's if we turbofish lifetimes directly:

```rust
struct Ctxt<'tcx>(&'tcx ());

impl<'tcx> Ctxt<'tcx> {
    fn compute(&self) -> impl Sized + use<'_, 'tcx> { }
}

fn test<'a, 'b>(x: &'a Ctxt<'b>, y: &'a Ctxt<'a>) {
    let results = [Ctxt::<'b>::compute(x), Ctxt::<'a>::compute(y)];
    //~^ ERROR lifetime may not live long enough
    // Since both opaques now capture `'tcx`, this enforces that `'a == 'b`.
    // Note that we don't shorten `'b` to `'a` since we turbofished it.
}
```

### Well... we should still warn?

I kinda don't care about this case, though I guess we could possibly downgrade the lint to something like `IMPL_TRAIT_OVERCAPTURES_STRICT` instead of suppressing it altogether. Thoughts? If we were to do this, then I'd probably also opt to include the invariant case in `IMPL_TRAIT_OVERCAPTURES_STRICT` and move it out of `IMPL_TRAIT_OVERCAPTURES`.

Author: matthiaskrgr

compiler/rustc_lint/src/impl_trait_overcaptures.rs

@@ -1,19 +1,29 @@
-use rustc_data_structures::fx::FxIndexSet;
+use std::assert_matches::debug_assert_matches;
+use std::cell::LazyCell;
+
+use rustc_data_structures::fx::{FxHashMap, FxIndexMap, FxIndexSet};
 use rustc_data_structures::unord::UnordSet;
 use rustc_errors::{Applicability, LintDiagnostic};
 use rustc_hir as hir;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_infer::infer::outlives::env::OutlivesEnvironment;
+use rustc_infer::infer::TyCtxtInferExt;
 use rustc_macros::LintDiagnostic;
-use rustc_middle::bug;
 use rustc_middle::middle::resolve_bound_vars::ResolvedArg;
+use rustc_middle::ty::relate::{
+    structurally_relate_consts, structurally_relate_tys, Relate, RelateResult, TypeRelation,
+};
 use rustc_middle::ty::{
     self, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable, TypeVisitableExt, TypeVisitor,
 };
+use rustc_middle::{bug, span_bug};
 use rustc_session::lint::FutureIncompatibilityReason;
 use rustc_session::{declare_lint, declare_lint_pass};
 use rustc_span::edition::Edition;
-use rustc_span::Span;
+use rustc_span::{Span, Symbol};
+use rustc_trait_selection::traits::outlives_bounds::InferCtxtExt;
+use rustc_trait_selection::traits::ObligationCtxt;
 
 use crate::{fluent_generated as fluent, LateContext, LateLintPass};
 
@@ -119,42 +129,87 @@ impl<'tcx> LateLintPass<'tcx> for ImplTraitOvercaptures {
     }
 }
 
+#[derive(PartialEq, Eq, Hash, Debug, Copy, Clone)]
+enum ParamKind {
+    // Early-bound var.
+    Early(Symbol, u32),
+    // Late-bound var on function, not within a binder. We can capture these.
+    Free(DefId, Symbol),
+    // Late-bound var in a binder. We can't capture these yet.
+    Late,
+}
+
 fn check_fn(tcx: TyCtxt<'_>, parent_def_id: LocalDefId) {
     let sig = tcx.fn_sig(parent_def_id).instantiate_identity();
 
-    let mut in_scope_parameters = FxIndexSet::default();
+    let mut in_scope_parameters = FxIndexMap::default();
     // Populate the in_scope_parameters list first with all of the generics in scope
     let mut current_def_id = Some(parent_def_id.to_def_id());
     while let Some(def_id) = current_def_id {
         let generics = tcx.generics_of(def_id);
         for param in &generics.own_params {
-            in_scope_parameters.insert(param.def_id);
+            in_scope_parameters.insert(param.def_id, ParamKind::Early(param.name, param.index));
         }
         current_def_id = generics.parent;
     }
 
+    for bound_var in sig.bound_vars() {
+        let ty::BoundVariableKind::Region(ty::BoundRegionKind::BrNamed(def_id, name)) = bound_var
+        else {
+            span_bug!(tcx.def_span(parent_def_id), "unexpected non-lifetime binder on fn sig");
+        };
+
+        in_scope_parameters.insert(def_id, ParamKind::Free(def_id, name));
+    }
+
+    let sig = tcx.liberate_late_bound_regions(parent_def_id.to_def_id(), sig);
+
     // Then visit the signature to walk through all the binders (incl. the late-bound
     // vars on the function itself, which we need to count too).
     sig.visit_with(&mut VisitOpaqueTypes {
         tcx,
         parent_def_id,
         in_scope_parameters,
         seen: Default::default(),
+        // Lazily compute these two, since they're likely a bit expensive.
+        variances: LazyCell::new(|| {
+            let mut functional_variances = FunctionalVariances {
+                tcx: tcx,
+                variances: FxHashMap::default(),
+                ambient_variance: ty::Covariant,
+                generics: tcx.generics_of(parent_def_id),
+            };
+            functional_variances.relate(sig, sig).unwrap();
+            functional_variances.variances
+        }),
+        outlives_env: LazyCell::new(|| {
+            let param_env = tcx.param_env(parent_def_id);
+            let infcx = tcx.infer_ctxt().build();
+            let ocx = ObligationCtxt::new(&infcx);
+            let assumed_wf_tys = ocx.assumed_wf_types(param_env, parent_def_id).unwrap_or_default();
+            let implied_bounds =
+                infcx.implied_bounds_tys_compat(param_env, parent_def_id, &assumed_wf_tys, false);
+            OutlivesEnvironment::with_bounds(param_env, implied_bounds)
+        }),
     });
 }
 
-struct VisitOpaqueTypes<'tcx> {
+struct VisitOpaqueTypes<'tcx, VarFn, OutlivesFn> {
     tcx: TyCtxt<'tcx>,
     parent_def_id: LocalDefId,
-    in_scope_parameters: FxIndexSet<DefId>,
+    in_scope_parameters: FxIndexMap<DefId, ParamKind>,
+    variances: LazyCell<FxHashMap<DefId, ty::Variance>, VarFn>,
+    outlives_env: LazyCell<OutlivesEnvironment<'tcx>, OutlivesFn>,
     seen: FxIndexSet<LocalDefId>,
 }
 
-impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for VisitOpaqueTypes<'tcx> {
-    fn visit_binder<T: TypeVisitable<TyCtxt<'tcx>>>(
-        &mut self,
-        t: &ty::Binder<'tcx, T>,
-    ) -> Self::Result {
+impl<'tcx, VarFn, OutlivesFn> TypeVisitor<TyCtxt<'tcx>>
+    for VisitOpaqueTypes<'tcx, VarFn, OutlivesFn>
+where
+    VarFn: FnOnce() -> FxHashMap<DefId, ty::Variance>,
+    OutlivesFn: FnOnce() -> OutlivesEnvironment<'tcx>,
+{
+    fn visit_binder<T: TypeVisitable<TyCtxt<'tcx>>>(&mut self, t: &ty::Binder<'tcx, T>) {
         // When we get into a binder, we need to add its own bound vars to the scope.
         let mut added = vec![];
         for arg in t.bound_vars() {
@@ -163,8 +218,8 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for VisitOpaqueTypes<'tcx> {
                 ty::BoundVariableKind::Region(ty::BoundRegionKind::BrNamed(def_id, ..))
                 | ty::BoundVariableKind::Ty(ty::BoundTyKind::Param(def_id, _)) => {
                     added.push(def_id);
-                    let unique = self.in_scope_parameters.insert(def_id);
-                    assert!(unique);
+                    let unique = self.in_scope_parameters.insert(def_id, ParamKind::Late);
+                    assert_eq!(unique, None);
                 }
                 _ => {
                     self.tcx.dcx().span_delayed_bug(
@@ -184,7 +239,7 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for VisitOpaqueTypes<'tcx> {
         }
     }
 
-    fn visit_ty(&mut self, t: Ty<'tcx>) -> Self::Result {
+    fn visit_ty(&mut self, t: Ty<'tcx>) {
         if !t.has_aliases() {
             return;
         }
@@ -207,89 +262,126 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for VisitOpaqueTypes<'tcx> {
             && let hir::OpaqueTyOrigin::FnReturn(parent_def_id) = opaque.origin
             && parent_def_id == self.parent_def_id
         {
-            // Compute the set of args that are captured by the opaque...
-            let mut captured = FxIndexSet::default();
-            let variances = self.tcx.variances_of(opaque_def_id);
-            let mut current_def_id = Some(opaque_def_id.to_def_id());
-            while let Some(def_id) = current_def_id {
-                let generics = self.tcx.generics_of(def_id);
-                for param in &generics.own_params {
-                    // A param is captured if it's invariant.
-                    if variances[param.index as usize] != ty::Invariant {
-                        continue;
-                    }
-                    // We need to turn all `ty::Param`/`ConstKind::Param` and
-                    // `ReEarlyParam`/`ReBound` into def ids.
-                    captured.insert(extract_def_id_from_arg(
-                        self.tcx,
-                        generics,
-                        opaque_ty.args[param.index as usize],
-                    ));
-                }
-                current_def_id = generics.parent;
-            }
-
-            // Compute the set of in scope params that are not captured. Get their spans,
-            // since that's all we really care about them for emitting the diagnostic.
-            let uncaptured_spans: Vec<_> = self
-                .in_scope_parameters
-                .iter()
-                .filter(|def_id| !captured.contains(*def_id))
-                .map(|def_id| self.tcx.def_span(def_id))
-                .collect();
-
             let opaque_span = self.tcx.def_span(opaque_def_id);
             let new_capture_rules =
                 opaque_span.at_least_rust_2024() || self.tcx.features().lifetime_capture_rules_2024;
-
-            // If we have uncaptured args, and if the opaque doesn't already have
-            // `use<>` syntax on it, and we're < edition 2024, then warn the user.
             if !new_capture_rules
                 && !opaque.bounds.iter().any(|bound| matches!(bound, hir::GenericBound::Use(..)))
-                && !uncaptured_spans.is_empty()
             {
-                let suggestion = if let Ok(snippet) =
-                    self.tcx.sess.source_map().span_to_snippet(opaque_span)
-                    && snippet.starts_with("impl ")
-                {
-                    let (lifetimes, others): (Vec<_>, Vec<_>) = captured
-                        .into_iter()
-                        .partition(|def_id| self.tcx.def_kind(*def_id) == DefKind::LifetimeParam);
-                    // Take all lifetime params first, then all others (ty/ct).
-                    let generics: Vec<_> = lifetimes
-                        .into_iter()
-                        .chain(others)
-                        .map(|def_id| self.tcx.item_name(def_id).to_string())
-                        .collect();
-                    // Make sure that we're not trying to name any APITs
-                    if generics.iter().all(|name| !name.starts_with("impl ")) {
-                        Some((
-                            format!(" + use<{}>", generics.join(", ")),
-                            opaque_span.shrink_to_hi(),
-                        ))
+                // Compute the set of args that are captured by the opaque...
+                let mut captured = FxIndexSet::default();
+                let mut captured_regions = FxIndexSet::default();
+                let variances = self.tcx.variances_of(opaque_def_id);
+                let mut current_def_id = Some(opaque_def_id.to_def_id());
+                while let Some(def_id) = current_def_id {
+                    let generics = self.tcx.generics_of(def_id);
+                    for param in &generics.own_params {
+                        // A param is captured if it's invariant.
+                        if variances[param.index as usize] != ty::Invariant {
+                            continue;
+                        }
+
+                        let arg = opaque_ty.args[param.index as usize];
+                        // We need to turn all `ty::Param`/`ConstKind::Param` and
+                        // `ReEarlyParam`/`ReBound` into def ids.
+                        captured.insert(extract_def_id_from_arg(self.tcx, generics, arg));
+
+                        captured_regions.extend(arg.as_region());
+                    }
+                    current_def_id = generics.parent;
+                }
+
+                // Compute the set of in scope params that are not captured.
+                let mut uncaptured_args: FxIndexSet<_> = self
+                    .in_scope_parameters
+                    .iter()
+                    .filter(|&(def_id, _)| !captured.contains(def_id))
+                    .collect();
+                // Remove the set of lifetimes that are in-scope that outlive some other captured
+                // lifetime and are contravariant (i.e. covariant in argument position).
+                uncaptured_args.retain(|&(def_id, kind)| {
+                    let Some(ty::Bivariant | ty::Contravariant) = self.variances.get(def_id) else {
+                        // Keep all covariant/invariant args. Also if variance is `None`,
+                        // then that means it's either not a lifetime, or it didn't show up
+                        // anywhere in the signature.
+                        return true;
+                    };
+                    // We only computed variance of lifetimes...
+                    debug_assert_matches!(self.tcx.def_kind(def_id), DefKind::LifetimeParam);
+                    let uncaptured = match *kind {
+                        ParamKind::Early(name, index) => ty::Region::new_early_param(
+                            self.tcx,
+                            ty::EarlyParamRegion { name, index },
+                        ),
+                        ParamKind::Free(def_id, name) => ty::Region::new_late_param(
+                            self.tcx,
+                            self.parent_def_id.to_def_id(),
+                            ty::BoundRegionKind::BrNamed(def_id, name),
+                        ),
+                        // Totally ignore late bound args from binders.
+                        ParamKind::Late => return true,
+                    };
+                    // Does this region outlive any captured region?
+                    !captured_regions.iter().any(|r| {
+                        self.outlives_env
+                            .free_region_map()
+                            .sub_free_regions(self.tcx, *r, uncaptured)
+                    })
+                });
+
+                // If we have uncaptured args, and if the opaque doesn't already have
+                // `use<>` syntax on it, and we're < edition 2024, then warn the user.
+                if !uncaptured_args.is_empty() {
+                    let suggestion = if let Ok(snippet) =
+                        self.tcx.sess.source_map().span_to_snippet(opaque_span)
+                        && snippet.starts_with("impl ")
+                    {
+                        let (lifetimes, others): (Vec<_>, Vec<_>) =
+                            captured.into_iter().partition(|def_id| {
+                                self.tcx.def_kind(*def_id) == DefKind::LifetimeParam
+                            });
+                        // Take all lifetime params first, then all others (ty/ct).
+                        let generics: Vec<_> = lifetimes
+                            .into_iter()
+                            .chain(others)
+                            .map(|def_id| self.tcx.item_name(def_id).to_string())
+                            .collect();
+                        // Make sure that we're not trying to name any APITs
+                        if generics.iter().all(|name| !name.starts_with("impl ")) {
+                            Some((
+                                format!(" + use<{}>", generics.join(", ")),
+                                opaque_span.shrink_to_hi(),
+                            ))
+                        } else {
+                            None
+                        }
                     } else {
                         None
-                    }
-                } else {
-                    None
-                };
-
-                self.tcx.emit_node_span_lint(
-                    IMPL_TRAIT_OVERCAPTURES,
-                    self.tcx.local_def_id_to_hir_id(opaque_def_id),
-                    opaque_span,
-                    ImplTraitOvercapturesLint {
-                        self_ty: t,
-                        num_captured: uncaptured_spans.len(),
-                        uncaptured_spans,
-                        suggestion,
-                    },
-                );
+                    };
+
+                    let uncaptured_spans: Vec<_> = uncaptured_args
+                        .into_iter()
+                        .map(|(def_id, _)| self.tcx.def_span(def_id))
+                        .collect();
+
+                    self.tcx.emit_node_span_lint(
+                        IMPL_TRAIT_OVERCAPTURES,
+                        self.tcx.local_def_id_to_hir_id(opaque_def_id),
+                        opaque_span,
+                        ImplTraitOvercapturesLint {
+                            self_ty: t,
+                            num_captured: uncaptured_spans.len(),
+                            uncaptured_spans,
+                            suggestion,
+                        },
+                    );
+                }
             }
+
             // Otherwise, if we are edition 2024, have `use<>` syntax, and
             // have no uncaptured args, then we should warn to the user that
             // it's redundant to capture all args explicitly.
-            else if new_capture_rules
+            if new_capture_rules
                 && let Some((captured_args, capturing_span)) =
                     opaque.bounds.iter().find_map(|bound| match *bound {
                         hir::GenericBound::Use(a, s) => Some((a, s)),
@@ -327,7 +419,7 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for VisitOpaqueTypes<'tcx> {
                 if self
                     .in_scope_parameters
                     .iter()
-                    .all(|def_id| explicitly_captured.contains(def_id))
+                    .all(|(def_id, _)| explicitly_captured.contains(def_id))
                 {
                     self.tcx.emit_node_span_lint(
                         IMPL_TRAIT_REDUNDANT_CAPTURES,
@@ -396,7 +488,11 @@ fn extract_def_id_from_arg<'tcx>(
             ty::ReBound(
                 _,
                 ty::BoundRegion { kind: ty::BoundRegionKind::BrNamed(def_id, ..), .. },
-            ) => def_id,
+            )
+            | ty::ReLateParam(ty::LateParamRegion {
+                scope: _,
+                bound_region: ty::BoundRegionKind::BrNamed(def_id, ..),
+            }) => def_id,
             _ => unreachable!(),
         },
         ty::GenericArgKind::Type(ty) => {
@@ -413,3 +509,106 @@ fn extract_def_id_from_arg<'tcx>(
         }
     }
 }
+
+/// Computes the variances of regions that appear in the type, but considering
+/// late-bound regions too, which don't have their variance computed usually.
+///
+/// Like generalization, this is a unary operation implemented on top of the binary
+/// relation infrastructure, mostly because it's much easier to have the relation
+/// track the variance for you, rather than having to do it yourself.
+struct FunctionalVariances<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    variances: FxHashMap<DefId, ty::Variance>,
+    ambient_variance: ty::Variance,
+    generics: &'tcx ty::Generics,
+}
+
+impl<'tcx> TypeRelation<TyCtxt<'tcx>> for FunctionalVariances<'tcx> {
+    fn cx(&self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn relate_with_variance<T: ty::relate::Relate<TyCtxt<'tcx>>>(
+        &mut self,
+        variance: rustc_type_ir::Variance,
+        _: ty::VarianceDiagInfo<TyCtxt<'tcx>>,
+        a: T,
+        b: T,
+    ) -> RelateResult<'tcx, T> {
+        let old_variance = self.ambient_variance;
+        self.ambient_variance = self.ambient_variance.xform(variance);
+        self.relate(a, b).unwrap();
+        self.ambient_variance = old_variance;
+        Ok(a)
+    }
+
+    fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
+        structurally_relate_tys(self, a, b).unwrap();
+        Ok(a)
+    }
+
+    fn regions(
+        &mut self,
+        a: ty::Region<'tcx>,
+        _: ty::Region<'tcx>,
+    ) -> RelateResult<'tcx, ty::Region<'tcx>> {
+        let def_id = match *a {
+            ty::ReEarlyParam(ebr) => self.generics.region_param(ebr, self.tcx).def_id,
+            ty::ReBound(
+                _,
+                ty::BoundRegion { kind: ty::BoundRegionKind::BrNamed(def_id, ..), .. },
+            )
+            | ty::ReLateParam(ty::LateParamRegion {
+                scope: _,
+                bound_region: ty::BoundRegionKind::BrNamed(def_id, ..),
+            }) => def_id,
+            _ => {
+                return Ok(a);
+            }
+        };
+
+        if let Some(variance) = self.variances.get_mut(&def_id) {
+            *variance = unify(*variance, self.ambient_variance);
+        } else {
+            self.variances.insert(def_id, self.ambient_variance);
+        }
+
+        Ok(a)
+    }
+
+    fn consts(
+        &mut self,
+        a: ty::Const<'tcx>,
+        b: ty::Const<'tcx>,
+    ) -> RelateResult<'tcx, ty::Const<'tcx>> {
+        structurally_relate_consts(self, a, b).unwrap();
+        Ok(a)
+    }
+
+    fn binders<T>(
+        &mut self,
+        a: ty::Binder<'tcx, T>,
+        b: ty::Binder<'tcx, T>,
+    ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
+    where
+        T: Relate<TyCtxt<'tcx>>,
+    {
+        self.relate(a.skip_binder(), b.skip_binder()).unwrap();
+        Ok(a)
+    }
+}
+
+/// What is the variance that satisfies the two variances?
+fn unify(a: ty::Variance, b: ty::Variance) -> ty::Variance {
+    match (a, b) {
+        // Bivariance is lattice bottom.
+        (ty::Bivariant, other) | (other, ty::Bivariant) => other,
+        // Invariant is lattice top.
+        (ty::Invariant, _) | (_, ty::Invariant) => ty::Invariant,
+        // If type is required to be covariant and contravariant, then it's invariant.
+        (ty::Contravariant, ty::Covariant) | (ty::Covariant, ty::Contravariant) => ty::Invariant,
+        // Otherwise, co + co = co, contra + contra = contra.
+        (ty::Contravariant, ty::Contravariant) => ty::Contravariant,
+        (ty::Covariant, ty::Covariant) => ty::Covariant,
+    }
+}

compiler/rustc_lint/src/lib.rs

@@ -30,6 +30,7 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(array_windows)]
+#![feature(assert_matches)]
 #![feature(box_patterns)]
 #![feature(control_flow_enum)]
 #![feature(extract_if)]

tests/ui/impl-trait/precise-capturing/overcaptures-2024-but-fine.rs

@@ -0,0 +1,15 @@
+//@ check-pass
+
+#![deny(impl_trait_overcaptures)]
+
+struct Ctxt<'tcx>(&'tcx ());
+
+// In `compute`, we don't care that we're "overcapturing" `'tcx`
+// in edition 2024, because it can be shortened at the call site
+// and we know it outlives `'_`.
+
+impl<'tcx> Ctxt<'tcx> {
+    fn compute(&self) -> impl Sized + '_ {}
+}
+
+fn main() {}