Rollup merge of #118000 - compiler-errors:placeholder-ty-outlives, r=aliemjay

Make regionck care about placeholders in outlives components

Currently, we don't consider a placeholder type `!T` to be a type component when it comes to processing type-outlives obligations. This means that they are essentially treated like unit values with no sub-components, and always outlive any region. This is problematic for `non_lifetime_binders`, and even more problematic for `with_negative_coherence`, since negative coherence uses placeholders as universals.

This PR adds `Component::Placeholder` which acts much like `Component::Param`. This currently causes a regression in some non-lifetime-binders tests because `for<T> T: 'static` doesn't imply itself when processing outlives obligations, so code like this will fail:

```
fn foo() where for<T> T: 'static {
  foo() //~ fails
}
```

Since the where clause doesn't imply itself. This requires making the `MatchAgainstHigherRankedOutlives` relation smarter when it comes to binders.

r? types

compiler/rustc_hir_analysis/src/outlives/utils.rs

@@ -80,6 +80,10 @@ pub(crate) fn insert_outlives_predicate<'tcx>(
                             .or_insert(span);
                     }
 
+                    Component::Placeholder(_) => {
+                        span_bug!(span, "Should not deduce placeholder outlives component");
+                    }
+
                     Component::Alias(alias_ty) => {
                         // This would either arise from something like:
                         //

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -2351,6 +2351,7 @@ pub fn construct_generic_bound_failure(
 
         let labeled_user_string = match bound_kind {
             GenericKind::Param(ref p) => format!("the parameter type `{p}`"),
+            GenericKind::Placeholder(ref p) => format!("the placeholder type `{p:?}`"),
             GenericKind::Alias(ref p) => match p.kind(self.tcx) {
                 ty::AliasKind::Projection | ty::AliasKind::Inherent => {
                     format!("the associated type `{p}`")

compiler/rustc_infer/src/infer/outlives/components.rs

@@ -11,6 +11,7 @@
 pub enum Component<'tcx> {
     Region(ty::Region<'tcx>),
     Param(ty::ParamTy),
+    Placeholder(ty::PlaceholderType),
     UnresolvedInferenceVariable(ty::InferTy),
 
     // Projections like `T::Foo` are tricky because a constraint like
@@ -120,6 +121,10 @@ fn compute_components<'tcx>(
                 out.push(Component::Param(p));
             }
 
+            ty::Placeholder(p) => {
+                out.push(Component::Placeholder(p));
+            }
+
             // For projections, we prefer to generate an obligation like
             // `<P0 as Trait<P1...Pn>>::Foo: 'a`, because this gives the
             // regionck more ways to prove that it holds. However,
@@ -176,7 +181,6 @@ fn compute_components<'tcx>(
             ty::Tuple(..) |       // ...
             ty::FnPtr(_) |        // OutlivesFunction (*)
             ty::Dynamic(..) |     // OutlivesObject, OutlivesFragment (*)
-            ty::Placeholder(..) |
             ty::Bound(..) |
             ty::Error(_) => {
                 // (*) Function pointers and trait objects are both binders.

compiler/rustc_infer/src/infer/outlives/obligations.rs

@@ -243,6 +243,9 @@ fn components_must_outlive(
                 Component::Param(param_ty) => {
                     self.param_ty_must_outlive(origin, region, *param_ty);
                 }
+                Component::Placeholder(placeholder_ty) => {
+                    self.placeholder_ty_must_outlive(origin, region, *placeholder_ty);
+                }
                 Component::Alias(alias_ty) => self.alias_ty_must_outlive(origin, region, *alias_ty),
                 Component::EscapingAlias(subcomponents) => {
                     self.components_must_outlive(origin, &subcomponents, region, category);
@@ -267,10 +270,28 @@ fn param_ty_must_outlive(
         region: ty::Region<'tcx>,
         param_ty: ty::ParamTy,
     ) {
-        let verify_bound = self.verify_bound.param_bound(param_ty);
+        let verify_bound = self.verify_bound.param_or_placeholder_bound(param_ty.to_ty(self.tcx));
         self.delegate.push_verify(origin, GenericKind::Param(param_ty), region, verify_bound);
     }
 
+    #[instrument(level = "debug", skip(self))]
+    fn placeholder_ty_must_outlive(
+        &mut self,
+        origin: infer::SubregionOrigin<'tcx>,
+        region: ty::Region<'tcx>,
+        placeholder_ty: ty::PlaceholderType,
+    ) {
+        let verify_bound = self
+            .verify_bound
+            .param_or_placeholder_bound(Ty::new_placeholder(self.tcx, placeholder_ty));
+        self.delegate.push_verify(
+            origin,
+            GenericKind::Placeholder(placeholder_ty),
+            region,
+            verify_bound,
+        );
+    }
+
     #[instrument(level = "debug", skip(self))]
     fn alias_ty_must_outlive(
         &mut self,

compiler/rustc_infer/src/infer/outlives/verify.rs

@@ -37,11 +37,11 @@ pub fn new(
     }
 
     #[instrument(level = "debug", skip(self))]
-    pub fn param_bound(&self, param_ty: ty::ParamTy) -> VerifyBound<'tcx> {
+    pub fn param_or_placeholder_bound(&self, ty: Ty<'tcx>) -> VerifyBound<'tcx> {
         // Start with anything like `T: 'a` we can scrape from the
         // environment. If the environment contains something like
         // `for<'a> T: 'a`, then we know that `T` outlives everything.
-        let declared_bounds_from_env = self.declared_generic_bounds_from_env(param_ty);
+        let declared_bounds_from_env = self.declared_generic_bounds_from_env(ty);
         debug!(?declared_bounds_from_env);
         let mut param_bounds = vec![];
         for declared_bound in declared_bounds_from_env {
@@ -51,7 +51,7 @@ pub fn param_bound(&self, param_ty: ty::ParamTy) -> VerifyBound<'tcx> {
                 param_bounds.push(VerifyBound::OutlivedBy(region));
             } else {
                 // This is `for<'a> T: 'a`. This means that `T` outlives everything! All done here.
-                debug!("found that {param_ty:?} outlives any lifetime, returning empty vector");
+                debug!("found that {ty:?} outlives any lifetime, returning empty vector");
                 return VerifyBound::AllBounds(vec![]);
             }
         }
@@ -168,7 +168,10 @@ fn bound_from_single_component(
     ) -> VerifyBound<'tcx> {
         match *component {
             Component::Region(lt) => VerifyBound::OutlivedBy(lt),
-            Component::Param(param_ty) => self.param_bound(param_ty),
+            Component::Param(param_ty) => self.param_or_placeholder_bound(param_ty.to_ty(self.tcx)),
+            Component::Placeholder(placeholder_ty) => {
+                self.param_or_placeholder_bound(Ty::new_placeholder(self.tcx, placeholder_ty))
+            }
             Component::Alias(alias_ty) => self.alias_bound(alias_ty, visited),
             Component::EscapingAlias(ref components) => {
                 self.bound_from_components(components, visited)
@@ -195,9 +198,9 @@ fn bound_from_single_component(
     /// bounds, but all the bounds it returns can be relied upon.
     fn declared_generic_bounds_from_env(
         &self,
-        param_ty: ty::ParamTy,
+        generic_ty: Ty<'tcx>,
     ) -> Vec<ty::Binder<'tcx, ty::OutlivesPredicate<Ty<'tcx>, ty::Region<'tcx>>>> {
-        let generic_ty = param_ty.to_ty(self.tcx);
+        assert!(matches!(generic_ty.kind(), ty::Param(_) | ty::Placeholder(_)));
         self.declared_generic_bounds_from_env_for_erased_ty(generic_ty)
     }
 

compiler/rustc_infer/src/infer/region_constraints/mod.rs

@@ -147,6 +147,7 @@ pub struct Verify<'tcx> {
 #[derive(Copy, Clone, PartialEq, Eq, Hash, TypeFoldable, TypeVisitable)]
 pub enum GenericKind<'tcx> {
     Param(ty::ParamTy),
+    Placeholder(ty::PlaceholderType),
     Alias(ty::AliasTy<'tcx>),
 }
 
@@ -707,6 +708,7 @@ impl<'tcx> fmt::Debug for GenericKind<'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match *self {
             GenericKind::Param(ref p) => write!(f, "{p:?}"),
+            GenericKind::Placeholder(ref p) => write!(f, "{p:?}"),
             GenericKind::Alias(ref p) => write!(f, "{p:?}"),
         }
     }
@@ -716,6 +718,7 @@ impl<'tcx> fmt::Display for GenericKind<'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match *self {
             GenericKind::Param(ref p) => write!(f, "{p}"),
+            GenericKind::Placeholder(ref p) => write!(f, "{p:?}"),
             GenericKind::Alias(ref p) => write!(f, "{p}"),
         }
     }
@@ -725,6 +728,7 @@ impl<'tcx> GenericKind<'tcx> {
     pub fn to_ty(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
         match *self {
             GenericKind::Param(ref p) => p.to_ty(tcx),
+            GenericKind::Placeholder(ref p) => Ty::new_placeholder(tcx, *p),
             GenericKind::Alias(ref p) => p.to_ty(tcx),
         }
     }

compiler/rustc_infer/src/traits/util.rs

@@ -365,6 +365,11 @@ fn elaborate(&mut self, elaboratable: &O) {
                                 Some(ty::ClauseKind::TypeOutlives(ty::OutlivesPredicate(ty, r_min)))
                             }
 
+                            Component::Placeholder(p) => {
+                                let ty = Ty::new_placeholder(tcx, p);
+                                Some(ty::ClauseKind::TypeOutlives(ty::OutlivesPredicate(ty, r_min)))
+                            }
+
                             Component::UnresolvedInferenceVariable(_) => None,
 
                             Component::Alias(alias_ty) => {

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -398,7 +398,8 @@ fn impl_intersection_has_negative_obligation(
     debug!("negative_impl(impl1_def_id={:?}, impl2_def_id={:?})", impl1_def_id, impl2_def_id);
 
     let ref infcx = tcx.infer_ctxt().intercrate(true).with_next_trait_solver(true).build();
-    let universe = infcx.universe();
+    let root_universe = infcx.universe();
+    assert_eq!(root_universe, ty::UniverseIndex::ROOT);
 
     let impl1_header = fresh_impl_header(infcx, impl1_def_id);
     let param_env =
@@ -414,7 +415,12 @@ fn impl_intersection_has_negative_obligation(
         return false;
     };
 
-    plug_infer_with_placeholders(infcx, universe, (impl1_header.impl_args, impl2_header.impl_args));
+    plug_infer_with_placeholders(
+        infcx,
+        root_universe,
+        (impl1_header.impl_args, impl2_header.impl_args),
+    );
+    let param_env = infcx.resolve_vars_if_possible(param_env);
 
     // FIXME(with_negative_coherence): the infcx has constraints from equating
     // the impl headers. We should use these constraints as assumptions, not as

compiler/rustc_trait_selection/src/traits/query/type_op/implied_outlives_bounds.rs

@@ -208,6 +208,9 @@ fn implied_bounds_from_components<'tcx>(
                 Component::Region(r) => Some(OutlivesBound::RegionSubRegion(sub_region, r)),
                 Component::Param(p) => Some(OutlivesBound::RegionSubParam(sub_region, p)),
                 Component::Alias(p) => Some(OutlivesBound::RegionSubAlias(sub_region, p)),
+                Component::Placeholder(_) => {
+                    unimplemented!("Shouldn't expect a placeholder type in implied bounds (yet)")
+                }
                 Component::EscapingAlias(_) =>
                 // If the projection has escaping regions, don't
                 // try to infer any implied bounds even for its

tests/ui/coherence/negative-coherence-check-placeholder-outlives.rs

@@ -0,0 +1,14 @@
+#![feature(negative_impls)]
+#![feature(with_negative_coherence)]
+
+struct Wrap<T>(T);
+
+trait Foo {}
+impl<T: 'static> !Foo for Box<T> {}
+
+trait Bar {}
+impl<T> Bar for T where T: Foo {}
+impl<T> Bar for Box<T> {}
+//~^ ERROR conflicting implementations of trait `Bar` for type `Box<_>`
+
+fn main() {}

tests/ui/coherence/negative-coherence-check-placeholder-outlives.stderr

@@ -0,0 +1,11 @@
+error[E0119]: conflicting implementations of trait `Bar` for type `Box<_>`
+  --> $DIR/negative-coherence-check-placeholder-outlives.rs:11:1
+   |
+LL | impl<T> Bar for T where T: Foo {}
+   | ------------------------------ first implementation here
+LL | impl<T> Bar for Box<T> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Box<_>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0119`.

tests/ui/traits/non_lifetime_binders/placeholders-dont-outlive-static.bad.stderr

@@ -0,0 +1,26 @@
+warning: the feature `non_lifetime_binders` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/placeholders-dont-outlive-static.rs:6:12
+   |
+LL | #![feature(non_lifetime_binders)]
+   |            ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #108185 <https://github.com/rust-lang/rust/issues/108185> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error[E0310]: the placeholder type `!1_"T"` may not live long enough
+  --> $DIR/placeholders-dont-outlive-static.rs:13:5
+   |
+LL |     foo();
+   |     ^^^^^
+   |     |
+   |     the placeholder type `!1_"T"` must be valid for the static lifetime...
+   |     ...so that the type `T` will meet its required lifetime bounds
+   |
+help: consider adding an explicit lifetime bound
+   |
+LL | fn bad() where !1_"T": 'static {
+   |          +++++++++++++++++++++
+
+error: aborting due to previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0310`.

tests/ui/traits/non_lifetime_binders/placeholders-dont-outlive-static.good.stderr

@@ -0,0 +1,26 @@
+warning: the feature `non_lifetime_binders` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/placeholders-dont-outlive-static.rs:6:12
+   |
+LL | #![feature(non_lifetime_binders)]
+   |            ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #108185 <https://github.com/rust-lang/rust/issues/108185> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error[E0310]: the placeholder type `!1_"T"` may not live long enough
+  --> $DIR/placeholders-dont-outlive-static.rs:19:5
+   |
+LL |     foo();
+   |     ^^^^^
+   |     |
+   |     the placeholder type `!1_"T"` must be valid for the static lifetime...
+   |     ...so that the type `T` will meet its required lifetime bounds
+   |
+help: consider adding an explicit lifetime bound
+   |
+LL | fn good() where for<T> T: 'static, !1_"T": 'static {
+   |                                  +++++++++++++++++
+
+error: aborting due to previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0310`.

tests/ui/traits/non_lifetime_binders/placeholders-dont-outlive-static.rs

@@ -0,0 +1,22 @@
+// revisions: good bad
+
+//[good] known-bug: unknown
+// `for<T> T: 'static` doesn't imply itself when processing outlives obligations
+
+#![feature(non_lifetime_binders)]
+//[bad]~^ WARN the feature `non_lifetime_binders` is incomplete
+
+fn foo() where for<T> T: 'static {}
+
+#[cfg(bad)]
+fn bad() {
+    foo();
+    //[bad]~^ ERROR the placeholder type `!1_"T"` may not live long enough
+}
+
+#[cfg(good)]
+fn good() where for<T> T: 'static {
+    foo();
+}
+
+fn main() {}

tests/ui/traits/non_lifetime_binders/type-match-with-late-bound.rs

@@ -1,10 +1,9 @@
 // edition:2021
-// check-pass
+// known-bug: unknown
 
 // Checks that test_type_match code doesn't ICE when predicates have late-bound types
 
 #![feature(non_lifetime_binders)]
-//~^ WARN is incomplete and may not be safe to use
 
 async fn walk2<'a, T: 'a>(_: T)
 where

tests/ui/traits/non_lifetime_binders/type-match-with-late-bound.stderr

@@ -7,5 +7,20 @@ LL | #![feature(non_lifetime_binders)]
    = note: see issue #108185 <https://github.com/rust-lang/rust/issues/108185> for more information
    = note: `#[warn(incomplete_features)]` on by default
 
-warning: 1 warning emitted
+error[E0309]: the placeholder type `!1_"F"` may not live long enough
+  --> $DIR/type-match-with-late-bound.rs:11:1
+   |
+LL | async fn walk2<'a, T: 'a>(_: T)
+   |                -- the placeholder type `!1_"F"` must be valid for the lifetime `'a` as defined here...
+...
+LL | {}
+   | ^^ ...so that the type `F` will meet its required lifetime bounds
+   |
+help: consider adding an explicit lifetime bound
+   |
+LL |     for<F> F: 'a, !1_"F": 'a
+   |                 ~~~~~~~~~~~~
 
+error: aborting due to previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0309`.