Extend the nullable pointer optimization to captured vars of closures

src/librustc_trans/trans/adt.rs

@@ -440,6 +440,22 @@ fn find_discr_field_candidate<'tcx>(tcx: &ty::ctxt<'tcx>,
             None
         },
 
+        // Perhaps one of the upvars of this struct is non-zero
+        // Let's recurse and find out!
+        ty::ty_closure(def_id, substs) => {
+            let typer = NormalizingClosureTyper::new(tcx);
+            let upvars = typer.closure_upvars(def_id, substs).unwrap();
+            let upvar_types = upvars.iter().map(|u| u.ty).collect::<Vec<_>>();
+
+            for (j, &ty) in upvar_types.iter().enumerate() {
+                if let Some(mut fpath) = find_discr_field_candidate(tcx, ty, path.clone()) {
+                    fpath.push(j);
+                    return Some(fpath);
+                }
+            }
+            None
+        },
+
         // Can we use one of the fields in this tuple?
         ty::ty_tup(ref tys) => {
             for (j, &ty) in tys.iter().enumerate() {

src/test/run-pass/nullable-pointer-opt-closures.rs

@@ -0,0 +1,43 @@
+// Copyright 2015 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.
+
+
+use std::mem;
+
+pub fn main() {
+    // By Ref Capture
+    let a = 10i32;
+    let b = Some(|| println!("{}", a));
+    // When we capture by reference we can use any of the
+    // captures as the discriminant since they're all
+    // behind a pointer.
+    assert_eq!(mem::size_of_val(&b), mem::size_of::<usize>());
+
+    // By Value Capture
+    let a = Box::new(12i32);
+    let b = Some(move || println!("{}", a));
+    // We captured `a` by value and since it's a `Box` we can use it
+    // as the discriminant.
+    assert_eq!(mem::size_of_val(&b), mem::size_of::<Box<i32>>());
+
+    // By Value Capture - Transitive case
+    let a = "Hello".to_string(); // String -> Vec -> Unique -> NonZero
+    let b = Some(move || println!("{}", a));
+    // We captured `a` by value and since down the chain it contains
+    // a `NonZero` field, we can use it as the discriminant.
+    assert_eq!(mem::size_of_val(&b), mem::size_of::<String>());
+
+    // By Value - No Optimization
+    let a = 14i32;
+    let b = Some(move || println!("{}", a));
+    // We captured `a` by value but we can't use it as the discriminant
+    // thus we end up with an extra field for the discriminant
+    assert_eq!(mem::size_of_val(&b), mem::size_of::<(i32, i32)>());
+}