diff --git a/src/librustc/middle/borrowck/gather_loans/restrictions.rs b/src/librustc/middle/borrowck/gather_loans/restrictions.rs
index d1e9a2c0a7475..a686084a4a2bc 100644
--- a/src/librustc/middle/borrowck/gather_loans/restrictions.rs
+++ b/src/librustc/middle/borrowck/gather_loans/restrictions.rs
@@ -70,18 +70,19 @@ impl<'a> RestrictionsContext<'a> {
             mc::cat_arg(local_id) => {
                 // R-Variable, locally declared
                 let lp = Rc::new(LpVar(local_id));
-                SafeIf(lp.clone(), vec!(lp))
+                SafeIf(lp.clone(), vec![lp])
             }
 
             mc::cat_upvar(upvar_id, _) => {
                 // R-Variable, captured into closure
                 let lp = Rc::new(LpUpvar(upvar_id));
-                SafeIf(lp.clone(), vec!(lp))
+                SafeIf(lp.clone(), vec![lp])
             }
 
-            mc::cat_copied_upvar(..) => {
-                // FIXME(#2152) allow mutation of upvars
-                Safe
+            mc::cat_copied_upvar(mc::CopiedUpvar { upvar_id, .. }) => {
+                // R-Variable, copied/moved into closure
+                let lp = Rc::new(LpVar(upvar_id));
+                SafeIf(lp.clone(), vec![lp])
             }
 
             mc::cat_downcast(cmt_base) => {
diff --git a/src/test/compile-fail/cannot-mutate-captured-non-mut-var.rs b/src/test/compile-fail/cannot-mutate-captured-non-mut-var.rs
index 6bc436d3c18ce..fcb09c200000b 100644
--- a/src/test/compile-fail/cannot-mutate-captured-non-mut-var.rs
+++ b/src/test/compile-fail/cannot-mutate-captured-non-mut-var.rs
@@ -12,4 +12,8 @@ fn main() {
     let x = 1i;
     proc() { x = 2; };
     //~^ ERROR: cannot assign to immutable captured outer variable in a proc `x`
+
+    let s = std::io::stdin();
+    proc() { s.lines(); };
+    //~^ ERROR: cannot borrow immutable captured outer variable in a proc `s` as mutable
 }
diff --git a/src/test/run-pass/issue-16671.rs b/src/test/run-pass/issue-16671.rs
new file mode 100644
index 0000000000000..a0d384418f972
--- /dev/null
+++ b/src/test/run-pass/issue-16671.rs
@@ -0,0 +1,24 @@
+// Copyright 2014 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.
+
+#![forbid(warnings)]
+
+// Pretty printing tests complain about `use std::predule::*`
+#![allow(unused_imports)]
+
+// A var moved into a proc, that has a mutable loan path should
+// not trigger a misleading unused_mut warning.
+
+pub fn main() {
+    let mut stdin = std::io::stdin();
+    spawn(proc() {
+        let _ = stdin.lines();
+    });
+}