Try normalizing types without RevealAll in ParamEnv in MIR validation

compiler/rustc_const_eval/src/transform/validate.rs

@@ -181,16 +181,23 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         if (src, dest).has_opaque_types() {
             return true;
         }
-        // Normalize projections and things like that.
-        let param_env = self.param_env.with_reveal_all_normalized(self.tcx);
-        let src = self.tcx.normalize_erasing_regions(param_env, src);
-        let dest = self.tcx.normalize_erasing_regions(param_env, dest);
 
+        // Normalize projections and things like that.
         // Type-changing assignments can happen when subtyping is used. While
         // all normal lifetimes are erased, higher-ranked types with their
         // late-bound lifetimes are still around and can lead to type
         // differences. So we compare ignoring lifetimes.
-        equal_up_to_regions(self.tcx, param_env, src, dest)
+
+        // First, try with reveal_all. This might not work in some cases, as the predicates
+        // can be cleared in reveal_all mode. We try the reveal first anyways as it is used
+        // by some other passes like inlining as well.
+        let param_env = self.param_env.with_reveal_all_normalized(self.tcx);
+        if equal_up_to_regions(self.tcx, param_env, src, dest) {
+            return true;
+        }
+
+        // If this fails, we can try it without the reveal.
+        equal_up_to_regions(self.tcx, self.param_env, src, dest)
     }
 }
 

src/test/ui/mir/issue-99866.rs

@@ -0,0 +1,25 @@
+// check-pass
+pub trait Backend {
+    type DescriptorSetLayout;
+}
+
+pub struct Back;
+
+impl Backend for Back {
+    type DescriptorSetLayout = u32;
+}
+
+pub struct HalSetLayouts {
+    vertex_layout: <Back as Backend>::DescriptorSetLayout,
+}
+
+impl HalSetLayouts {
+    pub fn iter<DSL>(self) -> DSL
+    where
+        Back: Backend<DescriptorSetLayout = DSL>,
+    {
+        self.vertex_layout
+    }
+}
+
+fn main() {}