in opaque type handling lift region vars to static if they outlive placeholders

Author: BoxyUwU

compiler/rustc_borrowck/src/handle_placeholders.rs

@@ -344,7 +344,7 @@ pub(crate) fn compute_sccs_applying_placeholder_outlives_constraints<'tcx>(
     }
 }
 
-fn rewrite_placeholder_outlives<'tcx>(
+pub(crate) fn rewrite_placeholder_outlives<'tcx>(
     sccs: &Sccs<RegionVid, ConstraintSccIndex>,
     annotations: &SccAnnotations<'_, '_, RegionTracker>,
     fr_static: RegionVid,

compiler/rustc_borrowck/src/region_infer/opaque_types/member_constraints.rs

@@ -39,7 +39,7 @@ pub(super) fn apply_member_constraints<'tcx>(
     debug!(?member_constraints);
     for scc_a in rcx.constraint_sccs.all_sccs() {
         debug!(?scc_a);
-        // Start by  adding the region values required by outlives constraints. This
+        // Start by adding the region values required by outlives constraints. This
         // matches how we compute the final region values in `fn compute_regions`.
         //
         // We need to do this here to get a lower bound when applying member constraints.
@@ -64,6 +64,7 @@ fn apply_member_constraint<'tcx>(
     // If the member region lives in a higher universe, we currently choose
     // the most conservative option by leaving it unchanged.
     if !rcx.max_placeholder_universe_reached(member).is_root() {
+        debug!("member region reached non root universe, bailing");
         return;
     }
 

compiler/rustc_borrowck/src/region_infer/opaque_types/mod.rs

@@ -253,6 +253,10 @@ fn collect_defining_uses<'tcx>(
                 }
             } else {
                 errors.push(DeferredOpaqueTypeError::InvalidOpaqueTypeArgs(err));
+                debug!(
+                    "collect_defining_uses: InvalidOpaqueTypeArgs for {:?} := {:?}",
+                    non_nll_opaque_type_key, hidden_type
+                );
             }
             continue;
         }
@@ -276,6 +280,7 @@ fn collect_defining_uses<'tcx>(
     defining_uses
 }
 
+#[instrument(level = "debug", skip(rcx, hidden_types, defining_uses, errors))]
 fn compute_definition_site_hidden_types_from_defining_uses<'tcx>(
     rcx: &RegionCtxt<'_, 'tcx>,
     hidden_types: &mut DefinitionSiteHiddenTypes<'tcx>,
@@ -287,13 +292,15 @@ fn compute_definition_site_hidden_types_from_defining_uses<'tcx>(
     let mut decls_modulo_regions: FxIndexMap<OpaqueTypeKey<'tcx>, (OpaqueTypeKey<'tcx>, Span)> =
         FxIndexMap::default();
     for &DefiningUse { opaque_type_key, ref arg_regions, hidden_type } in defining_uses {
+        debug!(?opaque_type_key, ?arg_regions, ?hidden_type);
         // After applying member constraints, we now map all regions in the hidden type
         // to the `arg_regions` of this defining use. In case a region in the hidden type
         // ended up not being equal to any such region, we error.
         let hidden_type =
             match hidden_type.try_fold_with(&mut ToArgRegionsFolder::new(rcx, arg_regions)) {
                 Ok(hidden_type) => hidden_type,
                 Err(r) => {
+                    debug!("UnexpectedHiddenRegion: {:?}", r);
                     errors.push(DeferredOpaqueTypeError::UnexpectedHiddenRegion {
                         hidden_type,
                         opaque_type_key,

compiler/rustc_borrowck/src/region_infer/opaque_types/region_ctxt.rs

@@ -11,7 +11,9 @@ use crate::constraints::ConstraintSccIndex;
 use crate::handle_placeholders::{SccAnnotations, region_definitions};
 use crate::region_infer::reverse_sccs::ReverseSccGraph;
 use crate::region_infer::values::RegionValues;
-use crate::region_infer::{ConstraintSccs, RegionDefinition, RegionTracker, Representative};
+use crate::region_infer::{
+    ConstraintSccs, OutlivesConstraintSet, RegionDefinition, RegionTracker, Representative,
+};
 use crate::type_check::MirTypeckRegionConstraints;
 use crate::type_check::free_region_relations::UniversalRegionRelations;
 use crate::universal_regions::UniversalRegions;
@@ -39,16 +41,36 @@ impl<'a, 'tcx> RegionCtxt<'a, 'tcx> {
         location_map: Rc<DenseLocationMap>,
         constraints: &MirTypeckRegionConstraints<'tcx>,
     ) -> RegionCtxt<'a, 'tcx> {
+        let mut outlives_constraints = constraints.outlives_constraints.clone();
         let universal_regions = &universal_region_relations.universal_regions;
         let (definitions, _has_placeholders) = region_definitions(infcx, universal_regions);
+
+        let compute_sccs =
+            |outlives_constraints: &OutlivesConstraintSet<'tcx>,
+             annotations: &mut SccAnnotations<'_, 'tcx, RegionTracker>| {
+                ConstraintSccs::new_with_annotation(
+                    &outlives_constraints
+                        .graph(definitions.len())
+                        .region_graph(outlives_constraints, universal_regions.fr_static),
+                    annotations,
+                )
+            };
+
         let mut scc_annotations = SccAnnotations::init(&definitions);
-        let constraint_sccs = ConstraintSccs::new_with_annotation(
-            &constraints
-                .outlives_constraints
-                .graph(definitions.len())
-                .region_graph(&constraints.outlives_constraints, universal_regions.fr_static),
-            &mut scc_annotations,
+        let mut constraint_sccs = compute_sccs(&outlives_constraints, &mut scc_annotations);
+
+        let added_constraints = crate::handle_placeholders::rewrite_placeholder_outlives(
+            &constraint_sccs,
+            &scc_annotations,
+            universal_regions.fr_static,
+            &mut outlives_constraints,
         );
+
+        if added_constraints {
+            scc_annotations = SccAnnotations::init(&definitions);
+            constraint_sccs = compute_sccs(&outlives_constraints, &mut scc_annotations);
+        }
+
         let scc_annotations = scc_annotations.scc_to_annotation;
 
         // Unlike the `RegionInferenceContext`, we only care about free regions

tests/ui/impl-trait/member-constraints/placeholders_lift_to_static.rs

@@ -0,0 +1,33 @@
+//@ check-pass
+
+// We have some `RPIT` with an item bound of `for<'a> Outlives<'a>`. We
+// infer a hidden type of `&'?x i32` where `'?x` is required to outlive
+// some placeholder `'!a` due to the `for<'a> Outlives<'a>` item bound.
+//
+// We previously did not write constraints of the form `'?x: '!a` into
+// `'?x: 'static`. This caused member constraints to bail and not consider
+// `'?x` to be constrained to an arg region.
+
+pub trait Outlives<'a> {}
+impl<'a, T: 'a> Outlives<'a> for T {}
+
+pub fn foo() -> impl for<'a> Outlives<'a> {
+    let x: &'static i32 = &1;
+    x
+}
+
+// This *didn't* regress but feels like it's "the same thing" so
+// test it anyway
+pub fn bar() -> impl Sized {
+    let x: &'static i32 = &1;
+    hr_outlives(x)
+}
+
+fn hr_outlives<T>(v: T) -> T
+where
+    for<'a> T: 'a
+{
+    v
+}
+
+fn main() {}

tests/ui/impl-trait/nested-rpit-hrtb.rs

@@ -48,7 +48,6 @@ fn one_hrtb_mention_fn_trait_param_uses<'b>() -> impl for<'a> Bar<'a, Assoc = im
 // This should resolve.
 fn one_hrtb_mention_fn_outlives_uses<'b>() -> impl for<'a> Bar<'a, Assoc = impl Sized + 'b> {}
 //~^ ERROR implementation of `Bar` is not general enough
-//~| ERROR lifetime may not live long enough
 
 // This should resolve.
 fn two_htrb_trait_param() -> impl for<'a> Foo<'a, Assoc = impl for<'b> Qux<'b>> {}

tests/ui/impl-trait/nested-rpit-hrtb.stderr

@@ -1,5 +1,5 @@
 error[E0261]: use of undeclared lifetime name `'b`
-  --> $DIR/nested-rpit-hrtb.rs:57:77
+  --> $DIR/nested-rpit-hrtb.rs:56:77
    |
 LL | fn two_htrb_outlives() -> impl for<'a> Foo<'a, Assoc = impl for<'b> Sized + 'b> {}
    |                                                                             ^^ undeclared lifetime
@@ -15,7 +15,7 @@ LL | fn two_htrb_outlives<'b>() -> impl for<'a> Foo<'a, Assoc = impl for<'b> Siz
    |                     ++++
 
 error[E0261]: use of undeclared lifetime name `'b`
-  --> $DIR/nested-rpit-hrtb.rs:65:82
+  --> $DIR/nested-rpit-hrtb.rs:64:82
    |
 LL | fn two_htrb_outlives_uses() -> impl for<'a> Bar<'a, Assoc = impl for<'b> Sized + 'b> {}
    |                                                                                  ^^ undeclared lifetime
@@ -87,12 +87,6 @@ LL | fn one_hrtb_mention_fn_trait_param_uses<'b>() -> impl for<'a> Bar<'a, Assoc
            but trait `Qux<'_>` is implemented for `()`
    = help: for that trait implementation, expected `()`, found `&'a ()`
 
-error: lifetime may not live long enough
-  --> $DIR/nested-rpit-hrtb.rs:49:93
-   |
-LL | fn one_hrtb_mention_fn_outlives_uses<'b>() -> impl for<'a> Bar<'a, Assoc = impl Sized + 'b> {}
-   |                                      -- lifetime `'b` defined here                          ^^ opaque type requires that `'b` must outlive `'static`
-
 error: implementation of `Bar` is not general enough
   --> $DIR/nested-rpit-hrtb.rs:49:93
    |
@@ -103,7 +97,7 @@ LL | fn one_hrtb_mention_fn_outlives_uses<'b>() -> impl for<'a> Bar<'a, Assoc =
    = note: ...but it actually implements `Bar<'0>`, for some specific lifetime `'0`
 
 error[E0277]: the trait bound `for<'a, 'b> &'a (): Qux<'b>` is not satisfied
-  --> $DIR/nested-rpit-hrtb.rs:61:64
+  --> $DIR/nested-rpit-hrtb.rs:60:64
    |
 LL | fn two_htrb_trait_param_uses() -> impl for<'a> Bar<'a, Assoc = impl for<'b> Qux<'b>> {}
    |                                                                ^^^^^^^^^^^^^^^^^^^^ the trait `for<'a, 'b> Qux<'b>` is not implemented for `&'a ()`
@@ -112,7 +106,7 @@ LL | fn two_htrb_trait_param_uses() -> impl for<'a> Bar<'a, Assoc = impl for<'b>
            but trait `Qux<'_>` is implemented for `()`
    = help: for that trait implementation, expected `()`, found `&'a ()`
 
-error: aborting due to 10 previous errors
+error: aborting due to 9 previous errors
 
 Some errors have detailed explanations: E0261, E0277, E0657.
 For more information about an error, try `rustc --explain E0261`.

tests/ui/nll/ice-106874.stderr

@@ -17,6 +17,20 @@ LL |     A(B(C::new(D::new(move |st| f(st)))))
    = note: ...but it actually implements `FnOnce<(&'1 mut V,)>`, for some specific lifetime `'1`
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
+error: higher-ranked subtype error
+  --> $DIR/ice-106874.rs:8:5
+   |
+LL |     A(B(C::new(D::new(move |st| f(st)))))
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: higher-ranked subtype error
+  --> $DIR/ice-106874.rs:8:5
+   |
+LL |     A(B(C::new(D::new(move |st| f(st)))))
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
 error: implementation of `FnOnce` is not general enough
   --> $DIR/ice-106874.rs:8:7
    |
@@ -75,17 +89,5 @@ LL |     A(B(C::new(D::new(move |st| f(st)))))
    = note: ...but it actually implements `Fn<(&'2 mut V,)>`, for some specific lifetime `'2`
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
-error: higher-ranked subtype error
-  --> $DIR/ice-106874.rs:8:7
-   |
-LL |     A(B(C::new(D::new(move |st| f(st)))))
-   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
-error: higher-ranked subtype error
-  --> $DIR/ice-106874.rs:8:41
-   |
-LL |     A(B(C::new(D::new(move |st| f(st)))))
-   |                                         ^
-
 error: aborting due to 10 previous errors