make mem-categorization use adjusted type for patterns

Fixes #49631

src/test/ui/rfc-2005-default-binding-mode/borrowck-issue-49631.rs

@@ -0,0 +1,34 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+pub struct Foo {
+}
+
+impl Foo {
+    fn get(&self) -> Option<&Result<String, String>> {
+        None
+    }
+
+    fn mutate(&mut self) { }
+}
+
+fn main() {
+    let mut foo = Foo { };
+
+    // foo.get() returns type Option<&Result<String, String>>, so
+    // using `string` keeps borrow of `foo` alive. Hence calling
+    // `foo.mutate()` should be an error.
+    while let Some(Ok(string)) = foo.get() {
+        foo.mutate();
+        //~^ ERROR cannot borrow `foo` as mutable
+        println!("foo={:?}", *string);
+    }
+}

src/test/ui/rfc-2005-default-binding-mode/borrowck-issue-49631.stderr

@@ -0,0 +1,13 @@
+error[E0502]: cannot borrow `foo` as mutable because it is also borrowed as immutable
+  --> $DIR/borrowck-issue-49631.rs:30:9
+   |
+LL |     while let Some(Ok(string)) = foo.get() {
+   |                                  ---     - immutable borrow ends here
+   |                                  |
+   |                                  immutable borrow occurs here
+LL |         foo.mutate();
+   |         ^^^ mutable borrow occurs here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0502`.