InstCombine Len([_; N]) => const N in MIR

src/test/mir-opt/combine_array_len.rs

@@ -0,0 +1,33 @@
+// 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.
+
+fn norm2(x: [f32; 2]) -> f32 {
+    let a = x[0];
+    let b = x[1];
+    a*a + b*b
+}
+
+fn main() {
+    assert_eq!(norm2([3.0, 4.0]), 5.0*5.0);
+}
+
+// END RUST SOURCE
+
+// START rustc.norm2.InstCombine.before.mir
+//     _5 = Len(_1);
+//     ...
+//     _10 = Len(_1);
+// END rustc.norm2.InstCombine.before.mir
+
+// START rustc.norm2.InstCombine.after.mir
+//     _5 = const 2usize;
+//     ...
+//     _10 = const 2usize;
+// END rustc.norm2.InstCombine.after.mir