Rollup merge of rust-lang#145041 - lcnr:borrowck-limitations-error, r=BoxyUwU

rework GAT borrowck limitation error

The old one depends on the `ConstraintCategory` of the constraint which meant we did not emit this note if we had to prove the higher ranked trait bound due to e.g. normalization.

This made it annoying brittle and caused MIR borrowck errors to be order dependent, fixes the issue in rust-lang#140737 (comment).

r? types cc ```@amandasystems```

Author: Zalathar

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>