Don't do expected type fudging if the inputs don't need inference guidance

Author: compiler-errors

Diff for: compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -211,34 +211,41 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // We use this to guide coercion inference; it's output is "fudged" which means
         // any remaining type variables are assigned to new, unrelated variables. This
         // is because the inference guidance here is only speculative.
-        let expected_input_tys: Option<Vec<_>> = expectation
-            .only_has_type(self)
-            .and_then(|expected_output| {
-                self.fudge_inference_if_ok(|| {
-                    let ocx = ObligationCtxt::new(self);
-
-                    // Attempt to apply a subtyping relationship between the formal
-                    // return type (likely containing type variables if the function
-                    // is polymorphic) and the expected return type.
-                    // No argument expectations are produced if unification fails.
-                    let origin = self.misc(call_span);
-                    ocx.sup(&origin, self.param_env, expected_output, formal_output)?;
-                    if !ocx.select_where_possible().is_empty() {
-                        return Err(TypeError::Mismatch);
-                    }
+        //
+        // We only do this if the formals have non-region infer vars, since this is only
+        // meant to guide inference.
+        let expected_input_tys: Option<Vec<_>> = if formal_input_tys.has_non_region_infer() {
+            expectation
+                .only_has_type(self)
+                .and_then(|expected_output| {
+                    self.fudge_inference_if_ok(|| {
+                        let ocx = ObligationCtxt::new(self);
+
+                        // Attempt to apply a subtyping relationship between the formal
+                        // return type (likely containing type variables if the function
+                        // is polymorphic) and the expected return type.
+                        // No argument expectations are produced if unification fails.
+                        let origin = self.misc(call_span);
+                        ocx.sup(&origin, self.param_env, expected_output, formal_output)?;
+                        if !ocx.select_where_possible().is_empty() {
+                            return Err(TypeError::Mismatch);
+                        }
 
-                    // Record all the argument types, with the args
-                    // produced from the above subtyping unification.
-                    Ok(Some(
-                        formal_input_tys
-                            .iter()
-                            .map(|&ty| self.resolve_vars_if_possible(ty))
-                            .collect(),
-                    ))
+                        // Record all the argument types, with the args
+                        // produced from the above subtyping unification.
+                        Ok(Some(
+                            formal_input_tys
+                                .iter()
+                                .map(|&ty| self.resolve_vars_if_possible(ty))
+                                .collect(),
+                        ))
+                    })
+                    .ok()
                 })
-                .ok()
-            })
-            .unwrap_or_default();
+                .unwrap_or_default()
+        } else {
+            None
+        };
 
         let mut err_code = E0061;