Prefer identity equality over equating types during coercion.

These types are always generic only over their own generic parameters with no inference variables involved.

Author: oli-obk

compiler/rustc_hir_analysis/src/coherence/builtin.rs

@@ -433,14 +433,12 @@ pub fn coerce_unsized_info<'tcx>(
                     // something more accepting, but we use
                     // equality because we want to be able to
                     // perform this check without computing
-                    // variance where possible. (This is because
-                    // we may have to evaluate constraint
+                    // variance or constraining opaque types' hidden types.
+                    // (This is because we may have to evaluate constraint
                     // expressions in the course of execution.)
                     // See e.g., #41936.
-                    if let Ok(ok) = infcx.at(&cause, param_env).eq(DefineOpaqueTypes::No, a, b) {
-                        if ok.obligations.is_empty() {
-                            return None;
-                        }
+                    if a == b {
+                        return None;
                     }
 
                     // Collect up all fields that were significantly changed