Auto merge of #88708 - Aaron1011:aggregate-usage, r=oli-obk

Add `ConstraintCategory::Usage` for handling aggregate construction

In some cases, we emit borrowcheck diagnostics pointing
at a particular field expression in a struct expression
(e.g. `MyStruct { field: my_expr }`). However, this
behavior currently relies on us choosing the
`ConstraintCategory::Boring` with the 'correct' span.
When adding additional variants to `ConstraintCategory`,
(or changing existing usages away from `ConstraintCategory::Boring`),
the current behavior can easily get broken, since a non-boring
constraint will get chosen over a boring one.

To make the diagnostic output less fragile, this commit
adds a `ConstraintCategory::Usage` variant. We use this variant
for the temporary assignments created for each field of
an aggregate we are constructing.

Using this new variant, we can emit a message mentioning
"this usage", emphasizing the fact that the error message
is related to the specific use site (in the struct expression).

This is preparation for additional work on improving NLL error messages
(see #57374)

2 files changed, 14 insertions, 0 deletions

diff --git a/compiler/rustc_borrowck/src/diagnostics/region_errors.rs b/compiler/rustc_borrowck/src/diagnostics/region_errors.rs
index 57d2a3c5ce9..0761d63c665 100644
--- a/compiler/rustc_borrowck/src/diagnostics/region_errors.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/region_errors.rs
@@ -40,6 +40,7 @@ impl ConstraintDescription for ConstraintCategory {
             ConstraintCategory::CopyBound => "copying this value ",
             ConstraintCategory::OpaqueType => "opaque type ",
             ConstraintCategory::ClosureUpvar(_) => "closure capture ",
+            ConstraintCategory::Usage => "this usage ",
             ConstraintCategory::Boring
             | ConstraintCategory::BoringNoLocation
             | ConstraintCategory::Internal => "",
diff --git a/compiler/rustc_borrowck/src/type_check/mod.rs b/compiler/rustc_borrowck/src/type_check/mod.rs
index 41c004ea596..5ccf3806025 100644
--- a/compiler/rustc_borrowck/src/type_check/mod.rs
+++ b/compiler/rustc_borrowck/src/type_check/mod.rs
@@ -1388,11 +1388,24 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                             ConstraintCategory::Return(ReturnConstraint::Normal)
                         }
                     }
+                    Some(l)
+                        if matches!(
+                            body.local_decls[l].local_info,
+                            Some(box LocalInfo::AggregateTemp)
+                        ) =>
+                    {
+                        ConstraintCategory::Usage
+                    }
                     Some(l) if !body.local_decls[l].is_user_variable() => {
                         ConstraintCategory::Boring
                     }
                     _ => ConstraintCategory::Assignment,
                 };
+                debug!(
+                    "assignment category: {:?} {:?}",
+                    category,
+                    place.as_local().map(|l| &body.local_decls[l])
+                );
 
                 let place_ty = place.ty(body, tcx).ty;
                 let place_ty = self.normalize(place_ty, location);