Add opt_alias_variances and use it in outlives code

compiler-errors · compiler-errors · commit 25911702d81e · 2025-02-05T18:03:26.000Z
diff --git a/compiler/rustc_borrowck/src/type_check/opaque_types.rs b/compiler/rustc_borrowck/src/type_check/opaque_types.rs
@@ -284,7 +284,7 @@ where
             return;
         }
 
-        match ty.kind() {
+        match *ty.kind() {
             ty::Closure(_, args) => {
                 // Skip lifetime parameters of the enclosing item(s)
 
@@ -316,10 +316,12 @@ where
                 args.as_coroutine().resume_ty().visit_with(self);
             }
 
-            ty::Alias(ty::Opaque, ty::AliasTy { def_id, args, .. }) => {
-                // Skip lifetime parameters that are not captures.
-                let variances = self.tcx.variances_of(*def_id);
-
+            ty::Alias(kind, ty::AliasTy { def_id, args, .. })
+                if let Some(variances) = self.tcx.opt_alias_variances(kind, def_id) =>
+            {
+                // Skip lifetime parameters that are not captured, since they do
+                // not need member constraints registered for them; we'll erase
+                // them (and hopefully in the future replace them with placeholders).
                 for (v, s) in std::iter::zip(variances, args.iter()) {
                     if *v != ty::Bivariant {
                         s.visit_with(self);
diff --git a/compiler/rustc_infer/src/infer/outlives/for_liveness.rs b/compiler/rustc_infer/src/infer/outlives/for_liveness.rs
@@ -48,7 +48,7 @@ where
             return ty.super_visit_with(self);
         }
 
-        match ty.kind() {
+        match *ty.kind() {
             // We can prove that an alias is live two ways:
             // 1. All the components are live.
             //
@@ -95,11 +95,9 @@ where
                     assert!(r.type_flags().intersects(ty::TypeFlags::HAS_FREE_REGIONS));
                     r.visit_with(self);
                 } else {
-                    // Skip lifetime parameters that are not captures.
-                    let variances = match kind {
-                        ty::Opaque => Some(self.tcx.variances_of(*def_id)),
-                        _ => None,
-                    };
+                    // Skip lifetime parameters that are not captured, since they do
+                    // not need to be live.
+                    let variances = tcx.opt_alias_variances(kind, def_id);
 
                     for (idx, s) in args.iter().enumerate() {
                         if variances.map(|variances| variances[idx]) != Some(ty::Bivariant) {
diff --git a/compiler/rustc_infer/src/infer/outlives/obligations.rs b/compiler/rustc_infer/src/infer/outlives/obligations.rs
@@ -392,13 +392,19 @@ where
         // the problem is to add `T: 'r`, which isn't true. So, if there are no
         // inference variables, we use a verify constraint instead of adding
         // edges, which winds up enforcing the same condition.
-        let is_opaque = alias_ty.kind(self.tcx) == ty::Opaque;
+        //
+        // Note that this is only strictly necessary for correctness if the
+        // alias has infer regions, since enforcing that the args must outlive
+        // the region will add constraints to the region graph. As mentioned
+        // above, if this is an opaque with no regions, this will just improve
+        // error messages.
+        let kind = alias_ty.kind(self.tcx);
         if approx_env_bounds.is_empty()
             && trait_bounds.is_empty()
-            && (alias_ty.has_infer_regions() || is_opaque)
+            && (alias_ty.has_infer_regions() || kind == ty::Opaque)
         {
             debug!("no declared bounds");
-            let opt_variances = is_opaque.then(|| self.tcx.variances_of(alias_ty.def_id));
+            let opt_variances = self.tcx.opt_alias_variances(kind, alias_ty.def_id);
             self.args_must_outlive(alias_ty.args, origin, region, opt_variances);
             return;
         }
diff --git a/compiler/rustc_infer/src/infer/outlives/verify.rs b/compiler/rustc_infer/src/infer/outlives/verify.rs
@@ -102,12 +102,11 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
 
     #[instrument(level = "debug", skip(self))]
     pub(crate) fn alias_bound(&self, alias_ty: ty::AliasTy<'tcx>) -> VerifyBound<'tcx> {
-        let alias_ty_as_ty = alias_ty.to_ty(self.tcx);
-
         // Search the env for where clauses like `P: 'a`.
         let env_bounds = self.approx_declared_bounds_from_env(alias_ty).into_iter().map(|binder| {
             if let Some(ty::OutlivesPredicate(ty, r)) = binder.no_bound_vars()
-                && ty == alias_ty_as_ty
+                && let ty::Alias(_, alias_ty_from_bound) = *ty.kind()
+                && alias_ty_from_bound == alias_ty
             {
                 // Micro-optimize if this is an exact match (this
                 // occurs often when there are no region variables
@@ -127,7 +126,8 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
         // see the extensive comment in projection_must_outlive
         let recursive_bound = {
             let mut components = smallvec![];
-            compute_alias_components_recursive(self.tcx, alias_ty_as_ty, &mut components);
+            let kind = alias_ty.kind(self.tcx);
+            compute_alias_components_recursive(self.tcx, kind, alias_ty, &mut components);
             self.bound_from_components(&components)
         };
 
diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
@@ -194,6 +194,14 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
         self.variances_of(def_id)
     }
 
+    fn opt_alias_variances(
+        self,
+        kind: impl Into<ty::AliasTermKind>,
+        def_id: DefId,
+    ) -> Option<&'tcx [ty::Variance]> {
+        self.opt_alias_variances(kind, def_id)
+    }
+
     fn type_of(self, def_id: DefId) -> ty::EarlyBinder<'tcx, Ty<'tcx>> {
         self.type_of(def_id)
     }
diff --git a/compiler/rustc_middle/src/ty/util.rs b/compiler/rustc_middle/src/ty/util.rs
@@ -951,6 +951,29 @@ impl<'tcx> TyCtxt<'tcx> {
 
         ty
     }
+
+    // Computes the variances for an alias (opaque or RPITIT) that represent
+    // its (un)captured regions.
+    pub fn opt_alias_variances(
+        self,
+        kind: impl Into<ty::AliasTermKind>,
+        def_id: DefId,
+    ) -> Option<&'tcx [ty::Variance]> {
+        match kind.into() {
+            ty::AliasTermKind::ProjectionTy => {
+                if self.is_impl_trait_in_trait(def_id) {
+                    Some(self.variances_of(def_id))
+                } else {
+                    None
+                }
+            }
+            ty::AliasTermKind::OpaqueTy => Some(self.variances_of(def_id)),
+            ty::AliasTermKind::InherentTy
+            | ty::AliasTermKind::WeakTy
+            | ty::AliasTermKind::UnevaluatedConst
+            | ty::AliasTermKind::ProjectionConst => None,
+        }
+    }
 }
 
 struct OpaqueTypeExpander<'tcx> {
diff --git a/compiler/rustc_type_ir/src/interner.rs b/compiler/rustc_type_ir/src/interner.rs
@@ -141,6 +141,12 @@ pub trait Interner:
     type VariancesOf: Copy + Debug + SliceLike<Item = ty::Variance>;
     fn variances_of(self, def_id: Self::DefId) -> Self::VariancesOf;
 
+    fn opt_alias_variances(
+        self,
+        kind: impl Into<ty::AliasTermKind>,
+        def_id: Self::DefId,
+    ) -> Option<Self::VariancesOf>;
+
     fn type_of(self, def_id: Self::DefId) -> ty::EarlyBinder<Self, Self::Ty>;
 
     type AdtDef: AdtDef<Self>;
diff --git a/compiler/rustc_type_ir/src/outlives.rs b/compiler/rustc_type_ir/src/outlives.rs
@@ -148,7 +148,7 @@ impl<I: Interner> TypeVisitor<I> for OutlivesCollector<'_, I> {
             // trait-ref. Therefore, if we see any higher-ranked regions,
             // we simply fallback to the most restrictive rule, which
             // requires that `Pi: 'a` for all `i`.
-            ty::Alias(_, alias_ty) => {
+            ty::Alias(kind, alias_ty) => {
                 if !alias_ty.has_escaping_bound_vars() {
                     // best case: no escaping regions, so push the
                     // projection and skip the subtree (thus generating no
@@ -162,7 +162,7 @@ impl<I: Interner> TypeVisitor<I> for OutlivesCollector<'_, I> {
                     // OutlivesProjectionComponents. Continue walking
                     // through and constrain Pi.
                     let mut subcomponents = smallvec![];
-                    compute_alias_components_recursive(self.cx, ty, &mut subcomponents);
+                    compute_alias_components_recursive(self.cx, kind, alias_ty, &mut subcomponents);
                     self.out.push(Component::EscapingAlias(subcomponents.into_iter().collect()));
                 }
             }
@@ -217,21 +217,17 @@ impl<I: Interner> TypeVisitor<I> for OutlivesCollector<'_, I> {
     }
 }
 
-/// Collect [Component]s for *all* the args of `parent`.
+/// Collect [Component]s for *all* the args of `alias_ty`.
 ///
-/// This should not be used to get the components of `parent` itself.
+/// This should not be used to get the components of `alias_ty` itself.
 /// Use [push_outlives_components] instead.
 pub fn compute_alias_components_recursive<I: Interner>(
     cx: I,
-    alias_ty: I::Ty,
+    kind: ty::AliasTyKind,
+    alias_ty: ty::AliasTy<I>,
     out: &mut SmallVec<[Component<I>; 4]>,
 ) {
-    let ty::Alias(kind, alias_ty) = alias_ty.kind() else {
-        unreachable!("can only call `compute_alias_components_recursive` on an alias type")
-    };
-
-    let opt_variances =
-        if kind == ty::Opaque { Some(cx.variances_of(alias_ty.def_id)) } else { None };
+    let opt_variances = cx.opt_alias_variances(kind, alias_ty.def_id);
 
     let mut visitor = OutlivesCollector { cx, out, visited: Default::default() };
 
diff --git a/compiler/rustc_type_ir/src/predicate.rs b/compiler/rustc_type_ir/src/predicate.rs
@@ -469,6 +469,17 @@ impl AliasTermKind {
     }
 }
 
+impl From<ty::AliasTyKind> for AliasTermKind {
+    fn from(value: ty::AliasTyKind) -> Self {
+        match value {
+            ty::Projection => AliasTermKind::ProjectionTy,
+            ty::Opaque => AliasTermKind::OpaqueTy,
+            ty::Weak => AliasTermKind::WeakTy,
+            ty::Inherent => AliasTermKind::InherentTy,
+        }
+    }
+}
+
 /// Represents the unprojected term of a projection goal.
 ///
 /// * For a projection, this would be `<Ty as Trait<...>>::N<...>`.
diff --git a/compiler/rustc_type_ir/src/relate.rs b/compiler/rustc_type_ir/src/relate.rs
@@ -236,28 +236,14 @@ impl<I: Interner> Relate<I> for ty::AliasTy<I> {
                 ExpectedFound::new(a, b)
             }))
         } else {
-            let args = match a.kind(relation.cx()) {
-                ty::Opaque => relate_args_with_variances(
-                    relation,
-                    a.def_id,
-                    relation.cx().variances_of(a.def_id),
-                    a.args,
-                    b.args,
+            let cx = relation.cx();
+            let args = if let Some(variances) = cx.opt_alias_variances(a.kind(cx), a.def_id) {
+                relate_args_with_variances(
+                    relation, a.def_id, variances, a.args, b.args,
                     false, // do not fetch `type_of(a_def_id)`, as it will cause a cycle
-                )?,
-                ty::Projection if relation.cx().is_impl_trait_in_trait(a.def_id) => {
-                    relate_args_with_variances(
-                        relation,
-                        a.def_id,
-                        relation.cx().variances_of(a.def_id),
-                        a.args,
-                        b.args,
-                        false, // do not fetch `type_of(a_def_id)`, as it will cause a cycle
-                    )?
-                }
-                ty::Projection | ty::Weak | ty::Inherent => {
-                    relate_args_invariantly(relation, a.args, b.args)?
-                }
+                )?
+            } else {
+                relate_args_invariantly(relation, a.args, b.args)?
             };
             Ok(ty::AliasTy::new_from_args(relation.cx(), a.def_id, args))
         }
diff --git a/tests/ui/impl-trait/precise-capturing/rpitit-outlives-2.rs b/tests/ui/impl-trait/precise-capturing/rpitit-outlives-2.rs
@@ -0,0 +1,19 @@
+//@ check-pass
+
+// Ensure that we skip uncaptured args from RPITITs when comptuing outlives.
+
+#![feature(precise_capturing_in_traits)]
+
+struct Invariant<T>(*mut T);
+
+trait Foo {
+    fn hello<'s: 's>(&'s self) -> Invariant<impl Sized + use<Self>>;
+}
+
+fn outlives_static(_: impl Sized + 'static) {}
+
+fn hello<'s, T: Foo + 'static>(x: &'s T) {
+    outlives_static(x.hello());
+}
+
+fn main() {}
diff --git a/tests/ui/impl-trait/precise-capturing/rpitit-outlives.rs b/tests/ui/impl-trait/precise-capturing/rpitit-outlives.rs
@@ -0,0 +1,18 @@
+//@ check-pass
+
+// Ensure that we skip uncaptured args from RPITITs when collecting the regions
+// to enforce member constraints in opaque type inference.
+
+#![feature(precise_capturing_in_traits)]
+
+struct Invariant<T>(*mut T);
+
+trait Foo {
+    fn hello<'s: 's>(&'s self) -> Invariant<impl Sized + use<Self>>;
+}
+
+fn hello<'s, T: Foo>(x: &'s T) -> Invariant<impl Sized> {
+    x.hello()
+}
+
+fn main() {}

Original file line number	Diff line number	Diff line change
`@@ -194,6 +194,14 @@ impl<'tcx> Interner for TyCtxt<'tcx> {`
`194`	`194`	`self.variances_of(def_id)`
`195`	`195`	`}`
`196`	`196`
	`197`	`+ fn opt_alias_variances(`
	`198`	`+ self,`
	`199`	`+ kind: impl Into<ty::AliasTermKind>,`
	`200`	`+ def_id: DefId,`
	`201`	`+ ) -> Option<&'tcx [ty::Variance]> {`
	`202`	`+ self.opt_alias_variances(kind, def_id)`
	`203`	`+ }`
	`204`	`+`
`197`	`205`	`fn type_of(self, def_id: DefId) -> ty::EarlyBinder<'tcx, Ty<'tcx>> {`
`198`	`206`	`self.type_of(def_id)`
`199`	`207`	`}`