internal constraints are better than placeholder outlives

Author: lcnr

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -1726,9 +1726,10 @@ impl<'tcx> RegionInferenceContext<'tcx> {
                 // `BoringNoLocation` constraints can point to user-written code, but are less
                 // specific, and are not used for relations that would make sense to blame.
                 ConstraintCategory::BoringNoLocation => 6,
-                // Do not blame internal constraints.
-                ConstraintCategory::OutlivesUnnameablePlaceholder(_) => 7,
-                ConstraintCategory::Internal => 8,
+                // Do not blame internal constraints if we can avoid it. Never blame
+                // the `'region: 'static` constraints introduced by placeholder outlives.
+                ConstraintCategory::Internal => 7,
+                ConstraintCategory::OutlivesUnnameablePlaceholder(_) => 8,
             };
 
             debug!("constraint {constraint:?} category: {category:?}, interest: {interest:?}");

tests/ui/higher-ranked/do-not-blame-outlives-static-ice.rs

@@ -0,0 +1,12 @@
+//@ compile-flags: -Zdeduplicate-diagnostics=yes
+
+// Regression test for #146467.
+trait Trait { type Assoc; }
+
+impl Trait for fn(&()) { type Assoc = (); }
+
+fn f(_: for<'a> fn(<fn(&'a ()) as Trait>::Assoc)) {}
+//~^ ERROR implementation of `Trait` is not general enough
+//~| ERROR higher-ranked subtype error
+
+fn main() {}

tests/ui/higher-ranked/do-not-blame-outlives-static-ice.stderr

@@ -0,0 +1,17 @@
+error: implementation of `Trait` is not general enough
+  --> $DIR/do-not-blame-outlives-static-ice.rs:7:9
+   |
+LL | fn f(_: for<'a> fn(<fn(&'a ()) as Trait>::Assoc)) {}
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Trait` is not general enough
+   |
+   = note: `for<'a> fn(&'a ())` must implement `Trait`, for any lifetime `'0`...
+   = note: ...but `Trait` is actually implemented for the type `for<'a> fn(&'a ())`
+
+error: higher-ranked subtype error
+  --> $DIR/do-not-blame-outlives-static-ice.rs:7:1
+   |
+LL | fn f(_: for<'a> fn(<fn(&'a ()) as Trait>::Assoc)) {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+