rework GAT borrowck limitation error

compiler/rustc_borrowck/messages.ftl

@@ -90,7 +90,7 @@ borrowck_lifetime_constraints_error =
     lifetime may not live long enough
 
 borrowck_limitations_implies_static =
-    due to current limitations in the borrow checker, this implies a `'static` lifetime
+    due to a current limitation of the type system, this implies a `'static` lifetime
 
 borrowck_move_closure_suggestion =
     consider adding 'move' keyword before the nested closure

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -6,7 +6,9 @@ use rustc_abi::{FieldIdx, VariantIdx};
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_errors::{Applicability, Diag, EmissionGuarantee, MultiSpan, listify};
 use rustc_hir::def::{CtorKind, Namespace};
-use rustc_hir::{self as hir, CoroutineKind, LangItem};
+use rustc_hir::{
+    self as hir, CoroutineKind, GenericBound, LangItem, WhereBoundPredicate, WherePredicateKind,
+};
 use rustc_index::{IndexSlice, IndexVec};
 use rustc_infer::infer::{BoundRegionConversionTime, NllRegionVariableOrigin};
 use rustc_infer::traits::SelectionError;
@@ -658,25 +660,66 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
 
     /// Add a note to region errors and borrow explanations when higher-ranked regions in predicates
     /// implicitly introduce an "outlives `'static`" constraint.
+    ///
+    /// This is very similar to `fn suggest_static_lifetime_for_gat_from_hrtb` which handles this
+    /// note for failed type tests instead of outlives errors.
     fn add_placeholder_from_predicate_note<G: EmissionGuarantee>(
         &self,
-        err: &mut Diag<'_, G>,
+        diag: &mut Diag<'_, G>,
         path: &[OutlivesConstraint<'tcx>],
     ) {
-        let predicate_span = path.iter().find_map(|constraint| {
+        let tcx = self.infcx.tcx;
+        let Some((gat_hir_id, generics)) = path.iter().find_map(|constraint| {
             let outlived = constraint.sub;
             if let Some(origin) = self.regioncx.definitions.get(outlived)
-                && let NllRegionVariableOrigin::Placeholder(_) = origin.origin
-                && let ConstraintCategory::Predicate(span) = constraint.category
+                && let NllRegionVariableOrigin::Placeholder(placeholder) = origin.origin
+                && let Some(id) = placeholder.bound.kind.get_id()
+                && let Some(placeholder_id) = id.as_local()
+                && let gat_hir_id = tcx.local_def_id_to_hir_id(placeholder_id)
+                && let Some(generics_impl) =
+                    tcx.parent_hir_node(tcx.parent_hir_id(gat_hir_id)).generics()
             {
-                Some(span)
+                Some((gat_hir_id, generics_impl))
             } else {
                 None
             }
-        });
+        }) else {
+            return;
+        };
 
-        if let Some(span) = predicate_span {
-            err.span_note(span, "due to current limitations in the borrow checker, this implies a `'static` lifetime");
+        // Look for the where-bound which introduces the placeholder.
+        // As we're using the HIR, we need to handle both `for<'a> T: Trait<'a>`
+        // and `T: for<'a> Trait`<'a>.
+        for pred in generics.predicates {
+            let WherePredicateKind::BoundPredicate(WhereBoundPredicate {
+                bound_generic_params,
+                bounds,
+                ..
+            }) = pred.kind
+            else {
+                continue;
+            };
+            if bound_generic_params
+                .iter()
+                .rfind(|bgp| tcx.local_def_id_to_hir_id(bgp.def_id) == gat_hir_id)
+                .is_some()
+            {
+                diag.span_note(pred.span, fluent::borrowck_limitations_implies_static);
+                return;
+            }
+            for bound in bounds.iter() {
+                if let GenericBound::Trait(bound) = bound {
+                    if bound
+                        .bound_generic_params
+                        .iter()
+                        .rfind(|bgp| tcx.local_def_id_to_hir_id(bgp.def_id) == gat_hir_id)
+                        .is_some()
+                    {
+                        diag.span_note(bound.span, fluent::borrowck_limitations_implies_static);
+                        return;
+                    }
+                }
+            }
         }
     }
 

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -215,7 +215,6 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
         diag: &mut Diag<'_>,
         lower_bound: RegionVid,
     ) {
-        let mut suggestions = vec![];
         let tcx = self.infcx.tcx;
 
         // find generic associated types in the given region 'lower_bound'
@@ -237,9 +236,11 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
             .collect::<Vec<_>>();
         debug!(?gat_id_and_generics);
 
-        // find higher-ranked trait bounds bounded to the generic associated types
+        // Look for the where-bound which introduces the placeholder.
+        // As we're using the HIR, we need to handle both `for<'a> T: Trait<'a>`
+        // and `T: for<'a> Trait`<'a>.
         let mut hrtb_bounds = vec![];
-        gat_id_and_generics.iter().flatten().for_each(|(gat_hir_id, generics)| {
+        gat_id_and_generics.iter().flatten().for_each(|&(gat_hir_id, generics)| {
             for pred in generics.predicates {
                 let BoundPredicate(WhereBoundPredicate { bound_generic_params, bounds, .. }) =
                     pred.kind
@@ -248,17 +249,32 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                 };
                 if bound_generic_params
                     .iter()
-                    .rfind(|bgp| tcx.local_def_id_to_hir_id(bgp.def_id) == *gat_hir_id)
+                    .rfind(|bgp| tcx.local_def_id_to_hir_id(bgp.def_id) == gat_hir_id)
                     .is_some()
                 {
                     for bound in *bounds {
                         hrtb_bounds.push(bound);
                     }
+                } else {
+                    for bound in *bounds {
+                        if let Trait(trait_bound) = bound {
+                            if trait_bound
+                                .bound_generic_params
+                                .iter()
+                                .rfind(|bgp| tcx.local_def_id_to_hir_id(bgp.def_id) == gat_hir_id)
+                                .is_some()
+                            {
+                                hrtb_bounds.push(bound);
+                                return;
+                            }
+                        }
+                    }
                 }
             }
         });
         debug!(?hrtb_bounds);
 
+        let mut suggestions = vec![];
         hrtb_bounds.iter().for_each(|bound| {
             let Trait(PolyTraitRef { trait_ref, span: trait_span, .. }) = bound else {
                 return;

tests/ui/borrowck/implementation-not-general-enough-ice-133252.stderr

@@ -22,12 +22,6 @@ LL |         force_send(async_load(&not_static));
 ...
 LL |     }
    |     - `not_static` dropped here while still borrowed
-   |
-note: due to current limitations in the borrow checker, this implies a `'static` lifetime
-  --> $DIR/implementation-not-general-enough-ice-133252.rs:16:18
-   |
-LL | fn force_send<T: Send>(_: T) {}
-   |                  ^^^^
 
 error: aborting due to 2 previous errors
 

tests/ui/generic-associated-types/bugs/hrtb-implied-1.stderr

@@ -9,11 +9,11 @@ LL |     print_items::<WindowsMut<'_>>(windows);
 LL | }
    | - temporary value is freed at the end of this statement
    |
-note: due to current limitations in the borrow checker, this implies a `'static` lifetime
-  --> $DIR/hrtb-implied-1.rs:26:26
+note: due to a current limitation of the type system, this implies a `'static` lifetime
+  --> $DIR/hrtb-implied-1.rs:26:5
    |
 LL |     for<'a> I::Item<'a>: Debug,
-   |                          ^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error
 

tests/ui/generic-associated-types/bugs/hrtb-implied-2.stderr

@@ -15,7 +15,11 @@ LL |     let _next = iter2.next();
    = note: requirement occurs because of a mutable reference to `Eat<&mut I, F>`
    = note: mutable references are invariant over their type parameter
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
-   = note: due to current limitations in the borrow checker, this implies a `'static` lifetime
+note: due to a current limitation of the type system, this implies a `'static` lifetime
+  --> $DIR/hrtb-implied-2.rs:31:8
+   |
+LL |     F: FnMut(I::Item<'_>),
+   |        ^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error
 

tests/ui/generic-associated-types/bugs/hrtb-implied-3.stderr

@@ -11,11 +11,11 @@ LL |     trivial_bound(iter);
    |     `iter` escapes the function body here
    |     argument requires that `'1` must outlive `'static`
    |
-note: due to current limitations in the borrow checker, this implies a `'static` lifetime
-  --> $DIR/hrtb-implied-3.rs:14:26
+note: due to a current limitation of the type system, this implies a `'static` lifetime
+  --> $DIR/hrtb-implied-3.rs:14:5
    |
 LL |     for<'a> I::Item<'a>: Sized,
-   |                          ^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error
 

tests/ui/generic-associated-types/collectivity-regression.stderr

@@ -7,7 +7,7 @@ LL | |         let _x = x;
 LL | |     };
    | |_____^
    |
-note: due to current limitations in the borrow checker, this implies a `'static` lifetime
+note: due to a current limitation of the type system, this implies a `'static` lifetime
   --> $DIR/collectivity-regression.rs:11:16
    |
 LL |     for<'a> T: Get<Value<'a> = ()>,

tests/ui/generic-associated-types/extended/lending_iterator.stderr

@@ -12,6 +12,12 @@ error: `Self` does not live long enough
    |
 LL |         <B as FromLendingIterator<A>>::from_iter(self)
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: due to a current limitation of the type system, this implies a `'static` lifetime
+  --> $DIR/lending_iterator.rs:4:21
+   |
+LL |     fn from_iter<T: for<'x> LendingIterator<Item<'x> = A>>(iter: T) -> Self;
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 2 previous errors
 

tests/ui/higher-ranked/trait-bounds/hrtb-just-for-static.stderr

@@ -15,7 +15,7 @@ LL | fn give_some<'a>() {
 LL |     want_hrtb::<&'a u32>()
    |     ^^^^^^^^^^^^^^^^^^^^ requires that `'a` must outlive `'static`
    |
-note: due to current limitations in the borrow checker, this implies a `'static` lifetime
+note: due to a current limitation of the type system, this implies a `'static` lifetime
   --> $DIR/hrtb-just-for-static.rs:9:15
    |
 LL |     where T : for<'a> Foo<&'a isize>

tests/ui/higher-ranked/trait-bounds/hrtb-perfect-forwarding.stderr

@@ -47,7 +47,7 @@ LL | fn foo_hrtb_bar_not<'b, T>(mut t: T)
 LL |     foo_hrtb_bar_not(&mut t);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^ requires that `'b` must outlive `'static`
    |
-note: due to current limitations in the borrow checker, this implies a `'static` lifetime
+note: due to a current limitation of the type system, this implies a `'static` lifetime
   --> $DIR/hrtb-perfect-forwarding.rs:37:8
    |
 LL |     T: for<'a> Foo<&'a isize> + Bar<&'b isize>,

tests/ui/implied-bounds/normalization-placeholder-leak.fail.stderr

@@ -30,6 +30,12 @@ LL |     fn test_lifetime<'lt, T: Trait>(_: Foo<&'lt u8>) {}
    |     |                |
    |     |                lifetime `'lt` defined here
    |     requires that `'lt` must outlive `'static`
+   |
+note: due to a current limitation of the type system, this implies a `'static` lifetime
+  --> $DIR/normalization-placeholder-leak.rs:19:5
+   |
+LL |     for<'x> T::Ty<'x>: Sized;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: lifetime may not live long enough
   --> $DIR/normalization-placeholder-leak.rs:38:5
@@ -39,6 +45,12 @@ LL |     fn test_alias<'lt, T: AnotherTrait>(_: Foo<T::Ty2::<'lt>>) {}
    |     |             |
    |     |             lifetime `'lt` defined here
    |     requires that `'lt` must outlive `'static`
+   |
+note: due to a current limitation of the type system, this implies a `'static` lifetime
+  --> $DIR/normalization-placeholder-leak.rs:19:5
+   |
+LL |     for<'x> T::Ty<'x>: Sized;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 6 previous errors
 

tests/ui/issues/issue-26217.stderr

@@ -6,11 +6,11 @@ LL | fn bar<'a>() {
 LL |     foo::<&'a i32>();
    |     ^^^^^^^^^^^^^^ requires that `'a` must outlive `'static`
    |
-note: due to current limitations in the borrow checker, this implies a `'static` lifetime
-  --> $DIR/issue-26217.rs:1:30
+note: due to a current limitation of the type system, this implies a `'static` lifetime
+  --> $DIR/issue-26217.rs:1:19
    |
 LL | fn foo<T>() where for<'a> T: 'a {}
-   |                              ^^
+   |                   ^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error
 

tests/ui/lifetimes/issue-105507.fixed

@@ -25,8 +25,8 @@ impl<T> ProjectedMyTrait for T
     where
         T: Project,
         for<'a> T::Projected<'a>: MyTrait,
-        //~^ NOTE due to current limitations in the borrow checker, this implies a `'static` lifetime
-        //~| NOTE due to current limitations in the borrow checker, this implies a `'static` lifetime
+        //~^ NOTE due to a current limitation of the type system, this implies a `'static` lifetime
+        //~| NOTE due to a current limitation of the type system, this implies a `'static` lifetime
 {}
 
 fn require_trait<T: MyTrait>(_: T) {}

tests/ui/lifetimes/issue-105507.rs

@@ -25,8 +25,8 @@ impl<T> ProjectedMyTrait for T
     where
         T: Project,
         for<'a> T::Projected<'a>: MyTrait,
-        //~^ NOTE due to current limitations in the borrow checker, this implies a `'static` lifetime
-        //~| NOTE due to current limitations in the borrow checker, this implies a `'static` lifetime
+        //~^ NOTE due to a current limitation of the type system, this implies a `'static` lifetime
+        //~| NOTE due to a current limitation of the type system, this implies a `'static` lifetime
 {}
 
 fn require_trait<T: MyTrait>(_: T) {}

tests/ui/lifetimes/issue-105507.stderr

@@ -4,7 +4,7 @@ error: `T` does not live long enough
 LL |     require_trait(wrap);
    |     ^^^^^^^^^^^^^^^^^^^
    |
-note: due to current limitations in the borrow checker, this implies a `'static` lifetime
+note: due to a current limitation of the type system, this implies a `'static` lifetime
   --> $DIR/issue-105507.rs:27:35
    |
 LL |         for<'a> T::Projected<'a>: MyTrait,
@@ -20,7 +20,7 @@ error: `U` does not live long enough
 LL |     require_trait(wrap1);
    |     ^^^^^^^^^^^^^^^^^^^^
    |
-note: due to current limitations in the borrow checker, this implies a `'static` lifetime
+note: due to a current limitation of the type system, this implies a `'static` lifetime
   --> $DIR/issue-105507.rs:27:35
    |
 LL |         for<'a> T::Projected<'a>: MyTrait,

tests/ui/mismatched_types/closure-arg-type-mismatch.stderr

@@ -57,7 +57,7 @@ LL |     baz(f);
    = note: requirement occurs because of a mutable pointer to `&u32`
    = note: mutable pointers are invariant over their type parameter
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
-note: due to current limitations in the borrow checker, this implies a `'static` lifetime
+note: due to a current limitation of the type system, this implies a `'static` lifetime
   --> $DIR/closure-arg-type-mismatch.rs:8:11
    |
 LL | fn baz<F: Fn(*mut &u32)>(_: F) {}

tests/ui/nll/local-outlives-static-via-hrtb.stderr

@@ -12,11 +12,11 @@ LL |     assert_static_via_hrtb_with_assoc_type(&&local);
 LL | }
    | - `local` dropped here while still borrowed
    |
-note: due to current limitations in the borrow checker, this implies a `'static` lifetime
-  --> $DIR/local-outlives-static-via-hrtb.rs:15:53
+note: due to a current limitation of the type system, this implies a `'static` lifetime
+  --> $DIR/local-outlives-static-via-hrtb.rs:15:42
    |
 LL | fn assert_static_via_hrtb<G>(_: G) where for<'a> G: Outlives<'a> {}
-   |                                                     ^^^^^^^^^^^^
+   |                                          ^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0597]: `local` does not live long enough
   --> $DIR/local-outlives-static-via-hrtb.rs:25:45
@@ -32,11 +32,11 @@ LL |     assert_static_via_hrtb_with_assoc_type(&&local);
 LL | }
    | - `local` dropped here while still borrowed
    |
-note: due to current limitations in the borrow checker, this implies a `'static` lifetime
-  --> $DIR/local-outlives-static-via-hrtb.rs:19:20
+note: due to a current limitation of the type system, this implies a `'static` lifetime
+  --> $DIR/local-outlives-static-via-hrtb.rs:19:5
    |
 LL |     for<'a> &'a T: Reference<AssociatedType = &'a ()>,
-   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 2 previous errors
 

tests/ui/nll/polonius/location-insensitive-scopes-issue-117146.nll.stderr

@@ -13,11 +13,11 @@ LL |     let b = |_| &a;
 LL | }
    | - `a` dropped here while still borrowed
    |
-note: due to current limitations in the borrow checker, this implies a `'static` lifetime
-  --> $DIR/location-insensitive-scopes-issue-117146.rs:20:22
+note: due to a current limitation of the type system, this implies a `'static` lifetime
+  --> $DIR/location-insensitive-scopes-issue-117146.rs:20:11
    |
 LL | fn bad<F: Fn(&()) -> &()>(_: F) {}
-   |                      ^^^
+   |           ^^^^^^^^^^^^^^
 
 error: implementation of `Fn` is not general enough
   --> $DIR/location-insensitive-scopes-issue-117146.rs:13:5

tests/ui/nll/polonius/location-insensitive-scopes-issue-117146.polonius.stderr

@@ -13,11 +13,11 @@ LL |     let b = |_| &a;
 LL | }
    | - `a` dropped here while still borrowed
    |
-note: due to current limitations in the borrow checker, this implies a `'static` lifetime
-  --> $DIR/location-insensitive-scopes-issue-117146.rs:20:22
+note: due to a current limitation of the type system, this implies a `'static` lifetime
+  --> $DIR/location-insensitive-scopes-issue-117146.rs:20:11
    |
 LL | fn bad<F: Fn(&()) -> &()>(_: F) {}
-   |                      ^^^
+   |           ^^^^^^^^^^^^^^
 
 error: implementation of `Fn` is not general enough
   --> $DIR/location-insensitive-scopes-issue-117146.rs:13:5